XYLENE POWER LTD.

FNR SAFETY

By Charles Rhodes, P.Eng., Ph.D.

INTRODUCTION:
Elsewhere on this website liquid sodium cooled Fast Neutron Reactors (FNRs) are identified as being the only presently available sustainable, dependable and economic technology for meeting mankind's clean power requirements. Until such time as the material problems related to molten salt breeder reactors and/or hydrogen isotope fusion systems have been solved mankind must live with the safety issues related to liquid sodium cooled FNRs.

This web page focuses on public safety aspects of liquid sodium cooled FNRs. FNRs must be designed so that they can be economically and safely assembled, operated and maintained at urban sites. Each FNR Nuclear Power Plant (NPP) consists of an isolated nuclear island that is connected via radial low pressure non-radioactive heat transport loops to up to eight nearby steam turbogenerator systems. At all times at least 8% of the total heat transport capacity must remain functional to ensure safe rejection of FNR fission product decay heat.

Natural circulation in The heat transport sytem will move fission product decay heat from the liquid sodium pool to the NaK-salt heat exchangers and/or the NaK-HTF heat exchangers. However, a reliable source of standby electricity is required to black start circulation of the nitrate salt and/or synthetic Heat Transfer Fluid (HTF) so that electricity generation can commence. If possible one of the FNR's eight turbogenerators should be kept running at all times to provide continuous control and house power for the entire nuclear power plant (NPP).

This web page attempts to identify the major FNR related safety issues. Some of these issues are shared with thermal reactors. Other issues, such as sodium fire suppression, are unique to sodium cooled FNRs.
 

REFERENCE:
Overview of Generation IV Reactor Safety Maters
 

INHERENT SAFETY:
The reactivity of a FNR is a strong function of the FNR's fuel geometry. In normal operation a liquid sodium pool type FNR, as discussed herein, is inherently safe because:
a) The fuel geometry is stable;
b) There is a negative temperature coefficient of reactivity below the boi‌ling point of sodium;
c) The reactivity is kept close to zero at the operating temperature setpoint by passive thermal expansion and contraction;
d) The fuel tubes remain deeply immersed in liquid sodium, even in a severe earthquake, so there is no danger of liquid sodium wave action triggering sodium void instability;
e) Air and water exclusion by a argon filled double wall enclosure prevents a sodium fire;
f) There is a wide temperature difference (> 400 degrees C) between the maximum fuel tube outside surface temperature and the sodium coolant boiling point, which prevents sodium void instability;
g) There is no high pressure fluid containment within the nuclear island;
h) The liquid sodium pool has a large thermal mass that safely absorbs thermal power transients;
i) In normal operation the steady state thermal power output is proportional to the NaK coolant circulation rate. The minimum thermal power output is the flux of fission product decay heat;
j) There is reliable reactor cold shutdown and rapid heat removal under all credible serious threat circumstances;
k) Any external event causing a physical shock large enough to significantly change the fuel geometry also trips two independent reactor emergency cold shutdown systems;
l) Any abnormal rapid rise in fissile fuel temperature causes a temporary injection of negative reactivity that prevents a thermal power spike during an emergency shutdown.
m) Radio isotopes, except Na-24, are contained in sealed fuel tubes.
n) In the event of a fuel tube leak most radio isotopes are chemically bound by the liquid Na coolant;
o) Inert gas radio isotopes and Cs-137 are confined by the inner enclosure sheet stainless steel wall covering;
p) In the event of an accident involving fuel tube melting, but not caused by penetration of the enclosure or dome by a missile, the double wall domed enclosure will protect the public by fully containing airborne radio isotopes.
q) The dome has a movable patch to temporarily cover a localized hole in the dome resulting from a missile attack;
r) There are eight independent turbogenerator halls, each with a dedicated heat transport system. At least one such system should always be kept fully operational to ensure continuing capacity for removal of fission product decay heat.
 

PUBLIC SAFETY MATTERS THAT MUST BE ADDRESSED BY A SODIUM COOLED FNR DESIGN:
A sodium cooled FNR contains large amounts of both chemical and nuclear potential energy. To achieve public safety and to minimize liability insurance premiums it is essential that the FNR design prevents unplanned release of this potential energy. Major public safety matters that FNR engineers must focus on to provide walk-away safety include:

A) The FNR elevation must be sufficient to ensure no future flooding of the sodium pool by water

B) The FNR enclosure consists of an external strucural wall and an internal thermal wall.

C) Both walls are gas and liquid tight to prevent passage of radio isotopes.

D) The space between the external structual wall and the internal thermal wall is accessible for service by suitably protected personnel.

E) The external structural wall is mostly concrete 1 m thick to stop gamma radiation.

F) The internal thermal wall mostly consists of metal sheathed NaF, NaCl and rigid fiberfrax insulation.

G) The FNR external structural enclosure has an external coating and roofing to exclude precipitation.

H) The FNR external structural enclosure must be sufficiently robust to withstand direct interactions with major tornados and hurricanes that can rapidly lower the atmospheric pressure outside the enclosure by as much as 3 psi = 20,000 Pa.

I) The FNR must have two independent cool shutdown systems.

J) A breach of either the external structural wall or the internal thermal wall must automatically trigger a reactor cool shutdown.

K) A breach of both the external structural wall and the internal thermal wall must trigger an automatic sodium fire suppression system as detailed on the web page titled: FNR Fire Suppression.

L) A NaK leak or NaK fire must trigger automatic drain down to a dump tank of the NaK in a particular heat transfer circuit. Note that the sodium fire suppression system requires continuing operation of part of the NaK heat transport capacity.

M) Each heat exchange gallery must have reliable autonatic NaK fire detection and suppresion with Na2CO3.

N) Each heat exchange gallery must have a reliable automatic means of HTF fire detection and suppression.

O) The thermal insulating wall and ceiling over the sodium pool:
i) Must continuously contain an argon gas and sodium vapor mix at 460 deg C to 500 deg C;
ii) Must be interior metal sheathed to prevent sodium vapor penetration of the fiberfrax insulation;
iii) Must maintain a positive argon pressure inside the fibrefrax insulation to keep sodium vapor and air out;
iv) Must be exterior metal sheathed and sealed to prevent argon gas (and possible radioisotope) leakage into the service access space.

P) There must be sufficient on-site cool argon gas storage in bladder type reservoirs to allow safe pool space transition from 500 degrees C to 20 degrees C without admitting air to the pool space.

Q) The inside surface of the external structural wall must have an airtight coating so as to reliably contain airborne radio isotopes.

R) There must be an airconditioning apparatus mounted in the dome roof access space that rejects heat to the outside that leaks from the sodium pool space through the themal wall and into the surrounding service access space, without releasing air borne radio isotopes.

S) The nuclear island must withstand earthquakes as discussed onthe web page titled: FNR Earthquake Protection

T) The FNR must have triple nested sodium pool walls filled with NaF that ensure continuing partial immersion of of the intermediate heat exchange bundles in liquid sodium in spite of an inner or middle wall leak.

U) The liquid sodium depth above the FNR fuel assembly must be sufficient to protect the FNR fuel assembly from shear forces due to translational earthquake movement. The earthquake energy must be dissipate by liquid sodium surface wave motion)

V) The FNR must provide at least two stages of radioactivity isolation between the sodium pool and the heat transfer fluid that flows away from the nuclear island.

W) The nuclear power plant must provide an alternate paths for rejection of fission product decay heat via either turbogenerators, condensers and cooling towers or via atmospheric pressure evaporation of locally stored water.

X) The fuel assembly must be of robust construction such that after initial setup it reamins dimensionally stable.

Y) The FNR fuel assembly reactivity must decrease with increasing temperature over the temperature range 20 deg C to 800 deg C.

Z) The FNR fuel assembly reactivity must decrease as gravity acts on any movable fuel bundles.

AA) The fuel assembly reactivity must decrease if an overhead object with a density greater than liquid sodium falls onto the fuel assembly.

BB) To prevent FNR core fuel centerline melting the FNR heat transport system must be designed such that in normal reactor operation the return NaK temperature always remains above the minimum safe fuel assembly inlet temperaure. Beware of the large TCE of sodium causing thermal stratification in the sodium pool that might lead to a sudden drop in sodium inlet temperture during an earthquake.

CC) To prevent fuel centerline melting the sodium pool must have sufficient thermal mass to ensure that the inlet temperature to the fuel assembly remains above its specified minimum, in spite ot transient control problems in the heat transport systems.

DD) If the fissile fuel gets too hot axial fuel disassembly should force a reduction in reactivity.

EE) Provide for fuel sustainable FNR operation with simple refueling and efficient central fuel recycling.
 

A) EXCLUSION OF WATER BY SITE SELECTION:
a) The FNR must be sited on a hill of sufficient height with respect to the surrounding land and water table that the FNR's sodium pool cannot be flooded by water. This is a non-negotiable site selection requirement;
b) No pressurized city water pipes are permitted within the nuclear island;
c) The FNR enclosure should have two redundant sump pumps in addition to having its gravity drain which must be above the local water table.
 

B) ENCLOSURE ISSUES:
ENCLOSURE DESCRIPTION:
The structural enclosure consists of a concrete cylinder with 1 m thick walls, 31 m in outside diameter that stabilizes an interior light weight gas tight ceramic fiber insulated octagonal wall, 25 m ID, 27 m OD. Within the enclosed space is a 20 m ID sodium pool.

The concrete cylindrical wall is stabilized by 1 m thick 8 m long radial concrete shear walls. The shear walls are connected together by 1 m thick external concrete walls that provide additional protection against low angle missile and aircraft impacts.

The cylindrical wall is further stabilized by four stairwells which effectively convert the 49 m oD into a 49 m X 49 m square.

The top of the enclosure is a steel dome roof, about 33 m in diameter, that at its middle is about 8 m higher than at its edges. This dome supports a gas tight ceiling that is about 14.5 m above the pool deck.

Between the outer surface of the inside wall and the inner ssurface of the concrete enclosure wall is a 1 m wide space for pipe and seal service access.

The argon cover gas and the sodium vapor are contained by two nested sheet steel wall coverings on the inside and outside of the inner thermal wall, either of which can safely isolate the contained argon and sodium vapor from the service access space. The sheet steel wall coverings are separated from each other by a 1.0 m thick layer of argon filled ceramic fiber insulation. This argon filling is kept at a slight positive pressure to prevent Na vapor entering the wall space and degrading its ceramic fiber content.

In normal operation the FNR relies on the inner most sodium and argon containment wall covering. In the event of an inner most containment wall covering failure the FNR should be shut down at the next opportunity. A failure of the second containment wall covering indicates that the reactor must be shut down immediately, regardless of financial consequences. Note that even if there is such a failure airborne radio isotopes cannot escape due to the impermiable wall covering on the inner face of the enclosure's perimeter concrete wall.
 

ENCLOSURE STRUCTURAL INTEGRITY:
In theory a prompt critical condition might be caused by a reactor enclosure failure which crushes the core zone of the FNR's assembly of active fuel bundles. For example, a large airplane impact which causes physical collapse of the reactor enclosure.

A FNR enclosure must be designed such that a structural collapse sufficient to cause crushing of the fuel assembly core zone is not a credible risk. The reactor enclosure outside walls are protected against an external aircraft or missle impact by 2.0 m of reinforced concrete and adjacent structures such as turbo generator halls and cooling towers. The reactor dome must be structurally sufficiently robust to safely absorb the impact of a diving aircraft and safely resist a tornado. The polar gantry crane must have sufficient redudant support points to ensure that crane parts can never fall into the sodium pool.

The reactor roof structure should contain impact absorbing material, such as reservoirs of NaCl granules and polyester bands to safely distribute over the roof area the impact force of any credible projectile. If the impact causes large pieces to break off the dome the inner ceiling, NaCl reservoirs and and polyester bands must prevent the large broken pieces falling on to and crushing the reactor fuel asembly. The dome should be comparable in strength to a highway or railway overpass.

The inner ceiling immediately above the reactor should be made of light weight materials that, if they fell on the reactor fuel assembly, are not sufficiently heavy to significantly change the geometry of the assembly of fuel bundles. The impact of the material fall should be mitigated by the indicator tubes, spherical floats and the top 6 m of liquid sodium. The fixed fuel bundle plenums should provide additional shock absorption.

The dome and the external enclosure must also protect the reactor from the large liquid hydrocarbon fuel fire that might accompany the crash of a large airplane. In this respect the floor coverings over the horizontal ceiling menbers should be slightly sloped to a fire tolerant drain.

An important issue in earthquake protection is bolting the fixed fuel bundles together to form a rigid matrix. The liquid sodium above the fuel assembly can safely slosh back and forth in an earthquake provided that the surface waves do not change the fuel assembly relative geometry and hence its reactivity.
 

ENCLOSURE RELATED RISK CATEGORIES:
One of the primary functions of a FNR enclosure and dome is exclusion of air and precipitation to prevent a spontaneous Na fire. The enclosure related risks to properly sited FNRs can be divided into a several categories:
1) Extremely low probability events such as direct impact by a large meteorite or by a determined military attack. In neither case is it possible to design the FNR to continue functioning after the attack nor is it possible to fully protect the public. The best that can be done is to adopt reasonable measures to mitigate possible damage to the public.

There is no such thing as perfect public safety. That does not mean that we do not build large hydroelectric dams and nuclear power plants. What it means is that we try to mitigate risks by locating large hydroelectric dams and nuclear power plants at sites which pose minimum risks from natural causes such as earthquakes, tsunamis, etc. and we try to maintain sufficient social order that there are no determined military attacks on either large hydroelectric dams or nuclear power stations.

This situation has a public safety risk comparable to the impact of a large meteorite or a determined military attack on a large hydroelectric dam.

2) Very low probability events such as impact by an aircraft or missile with sufficient impact momentum and kinetic energy to penetrate the reactor dome, the dome fill material and the ceiling over the sodium pool. This situation will force an immediate and prolonged reactor shutdown. In this situation the main design objectives are instant reactor shutdown and rapid suppression of the likely sodium fire in a manner that minimizes or prevents the FNR emitting airborne toxic, corrosive and radioactive species.

A method of suppressing the Na fire following a major failure of the FNR dome has been developed, but that method uses NaCl which may damage the FNR's steel components.

3) Low probability events that result in an immediate reactor cool shutdown until after detailed equipment inspection. In this category are severe earthquakes, severe hurricanes and tornados, minor missile damage, minor prompt neutron criticality excursions, reactor safety trips, any fire which causes discharge of NaCl based fire suppression material, or partial failure of redundant cooling equipment.

4) Low probability events that do not result in a reactor shutdown. In this category are minor earthquakes, normal violent storms, minor Na and NaK fires that can be extinguished with argon or Na2CO3.
 

The enclosure contains four concentric barriers (the external concrete wall and three nested steel walls) that exclude ground water and rain water from the sodium.
d) The external dome over the FNR must be watertight and must be constructed to naturally shed water, snow and ice;
e) The dome's water tight membrane must be rugged and easy to maintain.

EXCLUSION OF AIR FROM SODIUM:
Air is excluded from a FNR's sodium by:
a) Proper FNR enclosure and dome design;
b) Use of argon cover gas in the primary sodium pool enclosure;
c) Use of liquid kerosene to protect the exposed primary sodium surface from oxidation during planned cold shutdowns;
d) Use of floating stainless steel balls to reduce the exposed liquid sodium surface area and to support an air exclusion fire suppression surface crust;
e) Use of a NaCl to form a non-combustable surface crust to prevent oxidation of the exposed primary sodium surface during an enclosure failure triggered emergency shutdown.
f) Use of Na2CO3 and or MgCO3 to produce CO2 bubbles to assist in fire suppression crust floatation and to establish a positive flow of CO2 outwards through any hole in the roof of the sodium pool enclosure.
 

EXCLUSION OF AIR:
There are three concentric interior reactor roofs and side walls intended for ongoing sodium vapor inclusion, argon inclusion and air exclusion. The outside structural wall and overhead dome protect the three interior gas barriers. When the liquid sodium is near ambient temperature its surface can be isolated from air by flooding the sodium surface with kerosene. The pipe paths between the argon filled spaces and the air filled spaces are isolated by bellows sealed pipe connections. Physical access is by argon-vacuum-air locks. The argon pressure is maintained at one atmosphere via the use of large argon containment bladders located within adjacent concrete protected spaces. A dual on-site cryogenic facility provides on going extraction of argon from the atmosphere.

Exclusion of oxygen:
a) Hot liquid sodium and NaK will spontaneously burn in air;
b) To prevent spontaneous sodium combustion air must be reliably excluded from the primary sodium pool space;
c) The sodium pool must be protected by a layer of argon cover gas at atmospheric pressure;
d) The argon cover gas must be contained by gas tight stainless steel wall and ceiling sheathing;
e) After loss of argon cover gas the fire suppressing agents contained in the dome must prevent oxidation of the primary sodium long enough to allow cooling the primary sodium to less than 120 degrees C.
f) Fire suppression is assisted by a layer of hollow steel balls that float on the surface of the primary liquid sodium, that support the NaCl air exclusion crust and that minimize the exposed Na surface area.

ENCLOSURE EARTHQUAKE, TORNADO AND HURRICANE TOLERANCE:
a) A steel dome roof backed up by reservoirs of sealed NaCl/MgCO3 pellets and supported by 1 m thick concrete walls that are stabilized by radial shear walls is an extremely robust way of protecting the primary sodium pool from unforseen physical events;
b) In the event of a severe earthquake or tornado the dome must remain in place preventing discharge of radio isotopes to the surrounding environment.
c) In the event of detection of a potential physical threat to a FNR an automated system should triggeer a reactor shutdown.

ENCLOSURE RADIATION CONTAINMENT:
a) The mass of the steel dome and its contained NaCl/MgCO3/Na2CO3 must be sufficient to prevent upward emission of gamma radiation;
b) In any credible accident the steel dome, NaCl/MgCO3/Floats/Na2CO3 and outside walls must safely prevent emission of airborne radio isotopes.

ENCLOSURE MISSILE RESISTANCE:
An issue that must be faced is the remote possibility of a direct overhead attack by either a diving airplane or a armor penetrating projectile. Assume that by some means the overhead dome is penetrated. The single most important immediate step is to shut down the reactor, exclude air from the primary sodium and extract heat from the primary sodium temperature as quickly as possible. The issue is that as long as the primary sodium temperature is high it will heat the gas above it causing that gas to expand. When that gas is lighter than the surrounding ambient air it will tend to rise potentially sucking in further oxygen and moisture laden air into the reactor space via any open aperture. It is essential to prevent the liquid sodium surface being exposed to a continuous supply of fresh air.

11) ENCLOSURE Protection Against Overhead Object Collapse:
Redundant support measures are used to prevent heavy overhead objects such as dome armor tiles or gantry crane components falling onto the fuel assembly.

3) ENCLOSURE FAILURE
Potential Causes:
- natural missile (air borne utility pole in a tornado)
- jhadi suicide aircraft attack
- military missile
- extreme wind (low external pressure over roof due to hurricane and tornado wind speeds)
- extreme earthquake
- extreme ice and snow accumulation
- sodium vapor pressure
- argon or air pressure regulation failure
- corrosion and UV induced deterioration

Potential Consequences:
-loss of argon cover gas
-admission of air and precipitation
- Na Fire
- airborne radioisotopes

Remedy:
- Quantified distributed dome and support wall strength
- Quantified dome resistance to point impact
- Quantified earthquake tolerance
- Quantified dome weight
- Constant radius dome patch
- Roof membrane
- Roof membrane protection
- Quantified interior ceiling and wall argon leakage
- Interior ceiling and dome differential pressure sensing
- Two independent emergency shutdown systems;
- Reserve argon
- Na cooling by HTF
- Kerosene anti-oxidation agent
 

The cover gas is contained by two nested sheet steel wall coverings either of which can safely isolate the contained argon and sodium vapor from the service space. The sheet steel walls are separated from each other by a 1.0 m thick layer of argon filled ceramic fiber insulation. This argon filling is kept at a slight positive pressure to prevent Na vapor entering the wall space and degrading its ceramic fiber content.
 

ENCLOSURE STRUCTURAL INTEGRITY:
A prompt critical condition might be caused by a reactor enclosure collapse which crushes the core zone of the assembly of active fuel bundles. For example, a falling crane or a large airplane impact which causes physical collapse of the reactor enclosure.

A FNR enclosure must be designed such that a structural collapse sufficient to cause crushing of the fuel assembly core zone is not a credible risk. The reactor enclosure outside walls are protected against an external aircraft or missle impact by 2.5 m of reinforced concrete and adjacent structures such as turbo generator halls and cooling towers. The reactor dome must be structurally sufficiently robust to safely absorb the impact of a diving aircraft. The gantry crane must have sufficient redudant support points to ensure that the crane can never fall into the primary sodium pool.

The reactor roof structure should contain impact absorbing material, such as reservoirs of NaCl granules and polyester bags of expanded polystyrene to safely distribute over the roof area the impact force of any credible projectile. If the impact causes large pieces to break off the dome the inner roof, NaCl reservoirs and and polystyrene filled polyester bags must prevent the large broken pieces falling on to and crushing the reactor fuel asembly. The dome should be comparable in strength to a highway or railway overpass.

The inner ceiling immediately above the reactor should be made of light weight materials that, if they fell on the reactor fuel assembly, are not sufficiently heavy to significantly change the geometry of the assembly of fuel bundles. The impact of the material fall should be mitigated by the indicator tubes, spherical floats and the top 6 m of liquid sodium. The fixed fuel bundle plenums should provide additional shock absorption.

The dome and the external enclosure must also protect the reactor from the large liquid hydrocarbon fuel fire that might accompany the crash of a large airplane. In this respect the floor coverings over the horizontal dome menbers should be slightly sloped to a drain.

ENCLOSURE RELATED RISK CATEGORIES:
One of the primary functions of a FNR enclosure and dome is exclusion of air and precipitation to prevent a spontaneous primary Na fire. The enclosure related risks to properly sited FNRs can be divided into a several categories:
MILD EARTHQUAKES AND SEVERE STORMS Continue normal operation

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A method of suppressing the primary Na fire following a failure of the FNR dome has been developed, but that method uses NaCl which may damage the FNR's steel components.

3) Low probability events that result in an immediate reactor cold shutdown until after detailed equipment inspection. In this category are severe earthquakes, severe hurricanes and tornados, minor missile damage, minor prompt neutron criticality excursions, reactor safety trips, any fire which causes discharge of NaCl based fire suppression material, or partial failure of redundant cooling equipment.

4) Low probability events that do not result in a reactor shutdown. In this category are minor earthquakes, normal violent storms, minor Na and NaK fires that can be extinguished with argon or Na2CO3.
 

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Na FIRE SUPPRESSION:
The immediate response to a Na fire is to cover the Na surface with several layers of buoyant steel balls. Then scatter NaCl/MgCO3 powder over sodium pool steel balls. The NaCl will form a crust supported by the floating stainless steel balls which will isolate the sodium surface from air. The CO2 will add to the crust buoyancy and will establich a positive CO2 flow out through the hole in the enclosure thus preventing inward air flow.

Then heat must be extracted from the sodium as fast as possible. When the bulk sodium temperature is down to about 120 degrees C then the density of argon (atomic weight 40) when thermally expanded by:
[(273 + 15) / (273 + 120)] X 40 = 29.31 is comparable to ambient N2 at 28 and ambient O2 at 32. We need a higher molecular weight gas for reliable fire asphixiation.
 

13 ) Sodium Fire Suppression:
Assume that there is a missile strike which makes a hole in the reactor dome that penetrates into the sodium pool space. In this event the priorities are:
a) Immediate nuclear reaction shutdown to stop formation of further nuclear heat;
b) Rapid extraction of heat from the primary sodium;
c) Sodium fire extinguishing using Na2CO3;
d) Sodium fire extinguishing using NaCl to form an air excluding crust partially supported by the spherical stainless steel floats with added Na2CO3/MgCO3 to form CO2 bubbles within the crust to help it float on top of liquid Na. This is a very serious step because the NaCl will likely damage the stainless steel pool liner, the intermediate heat exchange components and the stainless steel floats. The NaCl is held in place in storage by normally closed plugs that in an emergency are lifted by strong electromagnets. The NaCl flows downward onto a spinning head that distributes it in a manner similar to a spinning lawn sprinkler. For certainty there should be eight independent spinning NaCl discharge heads.
e) Rapid heat extraction from the sodium pool;
f)After the sodium has cooled its surface should be flooded with kerosene to prevent further oxidation.

A reference with respect to sodium carbonation reactions is: Carbonation of the EBR-II Reactor"

A reference with respect to major primary sodium fire suppression is:
Na Fires French Report

Another reference with respect to major primary sodium fire suppression is: Survey of Suppression of Sodium Fires
 

13 ) Sodium Fire Suppression:
Assume that there is a missile strike which makes a hole in the reactor dome that penetrates into the sodium pool space. In this event the priorities are:
a) Immediate nuclear reaction shutdown to stop formation of further nuclear heat;
b) Rapid extraction of heat from the primary sodium;
c) Sodium fire extinguishing using Na2CO3;
d) Sodium fire extinguishing using NaCl to form an air excluding crust partially supported by the spherical stainless steel floats with added Na2CO3/MgCO3 to form CO2 bubbles within the crust to help it float on top of liquid Na. This is a very serious step because the NaCl will likely damage the stainless steel pool liner, the intermediate heat exchange components and the stainless steel floats. The NaCl is held in place in storage by normally closed plugs that in an emergency are lifted by strong electromagnets. The NaCl flows downward onto a spinning head that distributes it in a manner similar to a spinning lawn sprinkler. For certainty there should be eight independent spinning NaCl discharge heads.
e) Rapid heat extraction from the sodium pool;
f)After the sodium has cooled its surface should be flooded with kerosene to prevent further oxidation.

A reference with respect to sodium carbonation reactions is: Carbonation of the EBR-II Reactor"

A reference with respect to major primary sodium fire suppression is:
Na Fires French Report

Another reference with respect to major primary sodium fire suppression is: Survey of Suppression of Sodium Fires
 

References:
Carbonation of the EBR-II Reactor

Na Fires French Report
 

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NaK FIRE SUPPRESSION:
The NaK loop normally operates at a pressure of ~ 0.5 MPa. There is a small tendency for NaK to leak at gasketed mechanical joints. Such leaks are potentially dangerous to service personnel. Hot NaK will self ignite in air. One way to suppress these NaK fires is to completely surround the NaK loop with an argon jacket. The jacket must be physically robust enough to reliably withstand squirting hot liquid NaK and must act as a thermal insulator.

The main method of NaK fire suppression is NaK drain down to dump tanks.

Small NaK fires can be extinguished using Na2CO3.
Reference: Handling and Treatment of NaK
 

12) NaK Fire Suppression:
These are small fires usually triggered by a NaK gasket leak or a weld failue in a heat exchange gallery. Extinguish a small fire by dump tank argon asphixiation and then use Na2CO3 extinguishers to protect stainless steel surfaces. Reference: Handling and treatment of NaK
 

NaK drains down into dump tanks.
 

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REACTOR OVERLOADING:
Na has a large thermal coefficient of expansion. If the return sodium is too cold there is a potential threat to the reactor and its enclosure. A possible danger in an an earthquake or other event which causes an accumulation of cold sodium to enter the in take port of a full temperature Na structure causing a large surge in reactor power. This power surge coik

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6) HTF FIRE;
Potential Causes:
Gasket failure;
Pipe failure
NaK/HTF or Steam generator tube failure

Potential Consequences:
Distributed oil fire

Remedy:
HTF drain down
CO2 extinguisher
Vent oil suppression
 

hHeat transfer fluid drains down into dump tanks.
 

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9) Sodium Level Maintenance:
It is essential to maintain the sodium level to ensure continued capacity to remove both fission heat and fission product decay heat and to prevent an uncontrolled reactivity increase due to vaporization of sodium within the reactor core zone.
a) In normal circumstances the sodium level is maintained by the sodium pool walls that involve three nested steel cups;
b) The cup geometery and insulating filler between the cups are chosen to prevent the sodium level falling more than 4 m on inner amd middle cup failures.
c) Hence there remain a 2 m height of intermediate heat exchange bundle immersion in sodium to permit ongoing heat removal.
d) The sodium level must fall by more than 8 m before the effect of the transient sodium level on fuel reactivity becomes a concern.

7) LOSS OF Na POOL LEVEL;
Potential causes:
- Na pool failure due to extreme earthquake:

Potential consequence:
-Inability to transport heat in normal operation
- Inability to remove fission product decay heat
- Increase in reactivity due to sodium void instability
- Fuel melt down

Remedy:
-Triple wall sodium pool with 50% between wall filler occupancy
- Two independent emergency shutdown systems;
 

Na LEVEL:
A FNR normally operates with its sodium pool surface and cover gas in the temperature range 450 degrees C to 470 degreees C. The primary sodium pool consists of three nested steel cups, any one of which can safely contain the liquid sodium and isolate it from the environment. The nested primary sodium pool walls are separated from each other by 1 m thick layers of silica sand and fire brick, which provide both thermal insulation and potential liquid sodium volume displacement.

LIQUID SODIUM LEVEL:
The primary liquid sodium is contained within three cylindrical nested open top stainless steel cups. The innermost cup is 16 m high X 20 m diameter. The middle cup is 17 m high X 22 m diameter. The outer cup is 18 m high X 24 m diameter. The 1 m wide spaces between the cups are filled with sand or fire brick. The fire brick is chosen such that if immersed in liquid sodium it will displace at least 50% of its own volume.

In the event that the inner two cups both fail liquid sodium will flow into the space occupied by all of the sand and fire brick. If the fire brick displaces a volume of sodium equal to 50% of the fire brick volume, the volume available for potential sodium occupancy up to 4 m below the normal sodium level is:
{Pi (12 m)^2 (13 m) - Pi (10 m)^2 (11 m)} (.50)
= Pi (1872 m^3 - 1100 m^3)(.50)
= 1212.65 m^3

The volume of liquid sodium available to fill this space while keeping the intermediate heat exchange tubes at least 2 m immersed in liquid sodium is:
Pi (10 m)^2 (4 m) = 1256.63 m^3

Hence as long as the outer most steel cup holds there is sufficient fire brick to prevent the sodium level in the innermost cup falling by more than 4 m. Thus:
6 m - 4 m = 2 m
of heat exchange tube remain immersed in the liquid sodium for decay heat removal.
 

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8) Pressure Safety:
a) The sodium pool, the nitrate salt lops and the HTF loops all operate at atmospheric pressure;
b) The NaK circuits normally operate in the range 0.2 MPa to 0.8 MPa which pressure is sufficient to ensure that in the event of a NaK loop leak that the NaK will always flow out of the loop, not vice versa;
c) In the event of a steam generator leak the water/steam will always flow into the nitrate salt loop or the HTF loop which loops are vented to the atmosphere;
d) Multiple small steam generators are used to minimize the amount of energy that is locally stored in high pressure steam.

There are 48 independent NaK heat transport circuits each of which contains three separate heat exchange isolation barriers. A failure in any one of these barriers results in an individual heat transport circuit shutdown. Due to the multiplicity of independent heat transport circuits, the facility can continue operating while some of the heat transport circuits are out of service.

CERTAIN REMOVAL OF HEAT:
a) In a cold shutdown condition the heat emitting nuclear fission reactions must totally stop.
b) An emergency cold shutdown condition must be attainable via two independent mechanisms.
c) Removal of heat from a FNR occurs primarily via circulation of liquid NaK and liquid nitrate salt and/or a thermal fluid.
d) There must be a sufficient number of independent heat transport circuits that no credible accident will render all of these heat transport circuits non-functional.
e) The reactor must have a negative reactivity coefficient through its entire accessible temperature range. This coefficient will limit the reactor maximum operating temperature.
f) The sodium level must always be sufficient for the heat removal system to reliably operate and to ensure stable reactor reactivity;
g) The NaK, liquid nitrate salt, thermal fluid and steam heat transport loops must be configured so that 8% of the heat transport capaciy reliably operates in all credible circumstances;
h) The water injection systems into the steam generators must be configured to reliably operate in all credible emergencies.

NaK drains down into dump tanks. Nitrate salt and heat transfer fluid drain down into dump tanks. Steam generator condensate drains down into a pumped sump. Other sump pumps expel leakage water from the reactor enclosure foundation.
 

The safety matters related to sodium cooled FNRs fall into several categories:
1) Reactivity Decline Certainty:
a) The ratios of sodium, steel and core fuel in the core zone must meet certain constraints;
b) These ratios must be maintained in spite of fuel aging and changes in movable fuel bundle insertion.

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10)Reserve Argon Supply Maintenance:
a) Reserve argon is stored in nine atmospheric pressure bladders;
b) The bladders are individually piped so that a failure of one bladder has little or no effect on the other bladders.
c) The bladders are physically protected by 1 m thick concrete walls.
d) Gas flow into the bladders is cooled to protect the bladder material.

1) CERTAINTY WITH RESPECT TO BULK NEGATIVE TEMPERATURE COEFFICIENT OF REACTIVITY;
- must take into account material variability and fuel aging
 

2) PROMPT NEUTRON CRICALITY;
Potential Causes:
-sodium void instability
-sudden fuel geometry change due to missile attack
-sudden fuel geometry change due to earthquake
-too rapid insetion of movable fuel bundles into matrix of fixed fuel bundles
-unstable fuel geometry
- fuel assembly structural failure
Control system problem
Movable fuel bundle actuator problem
Thermal expansion of the fuel assembly provides only a small range of reactivity control that can easily be overwhelmed by a fuel geomety change. Maintenance shutdown and long term compensation for fuel aging both require mechanical adjustment of the fuel geometry. Neutron absorbing control rods are unsuitable for reactivity adjustment because they waste breeding neutrons.

Potential Consequences:
- Fuel melting
- Sodium void instability
- Reactor coolant explosion
- Fuel meltdown
- Enclosure failure
- Poor fuel utilization
- Airborne radioisotopes

Remedy:
- Robust fuel assembly
- Axial fuel disassembly on approach to prompt neutron criticality;
- Two independent emergency shutdown systems;
- Limit on movable fuel bundle insertion rate
- Ball bearing fuel assembly isolation
- Distributed reactor controls
- Stable hysterisis free actuators for movable fuel bundles
- Control system limits the rate of change of fuel geometry;
- Temperature and gamma radiation scanning are used to fine adjust the movable fuel bundle actuator position settings
 

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8) LACK OF EMERGENCY POWER
Potential Cause:
Insufficient emergency power generation redundancy
 

9) LOSS OF HEAT SINK CAPACITY
Potential Cause:
Major earthquake
Major hurricane
Major tornado

Potential Consequence:
Inability to cool Na pool
Inability to reject fission product decay heat
Fuel melt down

Remedy:
Two independent emergency shutdown systems;
Twelve fold heat transport loop redundancy;
Four fold heat sink redundancy;
On-site water storage for cooling by evaporation and direct steam release;
Quantified capacity for emergency steam release for rejection of fission product decay heat from steam generators;
 

The FNR described herein is designed to ensure compliance with all of the aforementioned safety conditions.

Each of the aforementioned major safety concerns is addressed by a normal protective measure. Failure of the normal protective measure usually triggers either a zone or a total FNR shutdown. The safety system must remain continuously powered by either the external electricity grid, other on-site turbogenerators or local standby generation.
 

SECONDARY SAFETY CONCERNS:
10) There are secondary concerns relating to: chemical safety, fire safety, radiation safety, thermal power safety, credible physical threats and potential control system failures. There must be certainty regarding protection of both workers and the public from aggressive and toxic chemicals, sodium and potassium combustion, ionizing radiation and explosive thermal energy releases.

11) Quantified tolerance to intermediate heat exchange bundle, NaK-salt heat exchanger and steam generator tube failures;
11) Quantified proliferation resistance;
 

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NORMAL AUTONOMOUS OPERATION:
In the normal autonomous operation mode the entire FNR facility operates automatically. Absent an alarm there is nothing for anyone to do. The output power level is set by remote dispatch. The cooling towers act to regulate the district heating water temperature.
 

NORMAL OPERATION:
During normal opertion the FNR design discussed herein relies on a negative temperature coefficient of reactivity to maintain the Na pool surface temperature at its setpoint, where the reactivity is zero.

In normal operation there is about a 500 degree C difference between the reactor temperature setpoint (460 C) and the deep sodium coolant boilin‌g point (960 C).

The fuel assembly must have a means of gross reactivity adjustment to enable reactor setpoint adjustment.
 

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AXIAL DISASSEMBLY:
a) In normal circumstances thermal expansion of the fissile fuel must be sufficient to stop the nuclear reaction.
b) In the event of a prompt neutron criticality event high fuel temperture must cause axial disassembly of fuel inside the fuel tubes;
c) This Axial disassembly will blow the fuel rods of fixed fuel bundles towards the fuel tube plenums, which will halt the nuclear reaction. By this means prompt neutron criticality conditions are suppressed.
d) In the event of an incident or accident that causes severe fuel overheating, the fissile fuel will melt or vaporize and sink in the fuel tube displacing lower density liquid sodium. This downward flow of melted fissile fuel past the lower blanket should reduce the reactor reactivity so that the nuclear reaction stops.

SUPPRESION OF PROMPT NEUTRON CRITICALITY:
For nuclear reactors at urban sites the single biggest risk to the public is a circumstance that might cause a reactor explosion due to prompt neutron criticality. The best defense against prompt neutron criticality axial fuel disassembly.

To obtain an explosion it is necessary to cause a reactor to suddenly become neutron prompt critical. Delayed neutrons are too slow to sustain the rapid power rise needed for an explosion.

Fast neutrons are high energy neutrons (~ 20,000 km/s),
Prompt neutrons are fast neutrons that come directly from a fission reaction;
Delayed neutrons are fast neutrons that are emitted from the fission fragments a few seconds after the corresponding nuclear fission. Delayed neutrons make it possible to design and safely control both thermal neutron and fast neutron power reactors.
Note that with Pu-239 fissile fuel the ratio of delayed neutrons to prompt neutrons is smaller than with U-235.
Thermal neutrons are low energy neutrons (V = 2 km / s) that have been slowed down by scattering by low atomic weight moderator materials.

Above the prompt-critical point reactor power rise can occur quickly with thermal neutrons and much faster with fast neutrons. The power rise with prompt neutrons will quickly cause the reactor to structurally disintegrate. Structural disintegration will cause the reactor to become sub-critical which will cause the power rise to stop.

A well known case of a reactor explosion resulting from prompt thermal neutron criticality was in 1986 at:
Chernobyl

A good description of the safety measure failures that led to the accident at Chernobyl and the corresponding preventive safety measures used in CANDU reactors is contained in a report titled:
Chernobyl - A Canadian Perspective

AXIAL DISASSEMBLY TEST:
The FNR known as EBR-2 was tested under full power with sudden loss of cooling and while the control rods were deliberately inactivated to prevent automated control feedback. The EBR-2 used axial fuel dissassembly which intrinsically FNR power levels to zero within 5 minutes. This type of test was carried out almost 50 times with no apparent damage to the reactor nor to any component, with the reactor being powered up again the same day. The reason for this safe behavior was likely fuel disassembly within the fuel tubes due to internal vaporization of sodium and perhaps cesium.
 

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FUEL ASSEMBLY STRUCTURAL STABILITY:
The fuel assembly must be sufficiently robust that the fuel geometry remains physically stable during all manner of credible natural external events such as earthquakes, tornados, hurricanes, tsunami, mountain slides and natural missile impacts.
 

REACTOR FUEL GEOMETRY STABILITY:
A small step change in reactor fuel geometry causes an instantaneous change in reactor reactivity. That change in reactivity is immediately followed by a change in reactor fuel temperature sufficient to reduce the reactivity to zero.

On insertion of movable fuel bundles into the matrix of fixed fuel bundles to raise the reactor setpoint temperature extreme care must be taken to ensure that the resulting increase in fuel temperature caused by the difference between the new reactor setpoint temperature and the actual coolant temperature is not so great as to melt the fuel.

Similarly if due to reactor thermal overload the coolant temperature at the bottom of the active fuel rods becomes too low with respect to the reactor setpoint temperature the fuel will melt.

Fuel melting during fuel bundle insertion can be avoided by:
a) Inserting the movable fuel bundles very slowly so that the reactor setpoint temperature is never far above the actual coolant temperature;
b) Disconnecting the thermal load while fuel bundle insertion is taking place;
c) Keeping the reactor at its design operating temperature at all normal times except during reactor shutdowns for fuel changes or intermediate heat exchange bundle service.

The potential for fuel melting due to reactor thermal overload is eliminated by designing the heat transport system so that the maximum possible heat removal rate does not exceed the reactor fuel design limit.

There is a complicating issue that the reactor reactivity is also weakly dependent on the coolant and steel temperatures. When the coolant temperature is below the reactor setpoint temperature the coolant decreases the reactor reactivity. To compensate the reactor fuel temperature decreases sufficiently to bring the net reactor reactivity to zero. This issue will cause a decrease in the primary sodium discharge temperature.

Similarly, when the coolant temperature is above the reactor setpoint temperature the coolant increases the reactor reactivity. To compensate the reactor fuel temperature increases in order to bring the net reactor reactivity to zero. This issue will cause an increase in the primary sodium discharge temperature.

In summary, in response to a step increase in thermal load the primary sodium discharge temperature decreases and in response to a step decrease in thermal load the primary sodium discharge temperature increases.

After a step increase in reactor setpoint temperature it may take many minutes for the primary sodium pool temperature to rise. As the primary sodium pool temperature approaches the reactor temperature setpoint the fission reaction rate will decrease as indicated by reduced gamma flux.

Similarly, a step decrease in reactor setpoint temperature will cut off the chain reactions. It may take many minutes for the primary sodium pool temperature to fall and the chain reaction rate, as indicated by the gamma flux, to rise to its former level.

TOO RAPID CHANGES IN FUEL GEOMETRY:
A too rapid change in fuel geometry could be caused by too rapid insertion of movable fuel bundles into the matrix of fixed fuel bundles. Too rapid movable fuel bundle insertion can be prevented by using appropriate mechanical speed limits on the FNR actuators.

There is a transition region between a reactor being critical with delayed plus prompt neutrons and being critical with just prompt neutrons. A FNR should normally remain in that transition region. A key issue is time. If the change in reactor fuel geometry is slow enough the heat released while under control by delayed neutrons should induce sufficient negative reactivity to prevent further approach to the prompt critical condition. In a FNR controlled by the fuel temperature this feedback is almost instantaneous. The danger lies in delayed positive reactivity injections from sodium and steel that exceed the safe available negative reactivity injection available from thermal expansion of the reactor fuel.

A key issue in this respect is fuel geometric stability. With Pu-239 fuel the time required for a 0.2% _______increase in reactivity due to a change in fuel geometry must be long compared to 3 seconds.

Unlike solid fuels, liquid fissile fuels are potentially very dangerous because liquids can develop cavitation, vorticies, or surface waves that can change the reactor reactivity by more than 0.2% in a time period which is short compared to 3 seconds. It is much safer to use physically stable solid fuel as in this FNR.
 

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LARGE DROP IN REACTOR CORE ZONE INLET TEMPERATURE:
In a FNR the reactivity increases with decreasing fuel temperature. Depending upon the fuel material distribution if the primary sodium temperature entering the reactor core zone drops too quickly the resulting increase in heat flux might melt the fuel on its cenerline, vaporize the internal liquid sodium or damage the fuel tubes. It is essential to have sufficient coolant thermal mass to prevent a sudden major coolant core zone temperature drop that might lead to fuel melting or prompt neutron criticality.

Since the change in reactor reactivity with a change in temperature is negative the reactivity cannot grow due to a coolant temperature rise.

A large FNR with a 1.7 m wide liquid sodium guard band contains a lot of heat stored in its primary liquid sodium pool. Hence it can load follow using some of that stored heat without any rapid change in the reactivity of its fuel assembly. The change in reactor thermal power output can take many minutes whereas the rate of heat transfer out of the primary sodium pool can change by a similar fraction in a few seconds.
 

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PREVENTION OF PROMPT NEUTRON CRITICALITY:
Prompt neutron criticality is prevented by:
a) Proper fuel assembly design;
b) Primary sodium level maintenance;
c) Negative reactivity injection on minor prompt neutron criticality;
d) Two independent active shutdown systems, each with reactor off as a power failure default;
e) Instant reactor shutdown on occurence of a shock or pressure wave sufficient to significantly affect the FNR fuel geometry;
f) Proper control system design.
g) Ongoing monitoring to detect off-normal reactor conditions.
 

POTENTIAL CAUSES OF PROMPT NEUTRON CRITICALITY:
In a FNR there are several possible ways that prompt neutron criticality might occur.

1) Reactor power instability.

2) Too rapid changes in fuel geometry.

3) Loss of surrounding sodium.

4) Sudden large drop in reactor core zone coolant inlet temperature. This issue can be mitigated via a sufficient primary sodium thermal mass to ensure a gradual change in core zone local reactivity as a function of position.

5) Insufficient earthquake tolerance.

6) A direct attack by some form of dome penetrating bomb or missile.

7) Insufficient Murphy's Law tolerance. Generally reactors should be designed such that three independent reactivity control systems must all simultaneously fail before a major accident can occur.
 

PROMPT NEUTRON CRITICALITY DAMAGE MECHANISM:
A nuclear power plant can potentially destroy itself if its reactor is forced into a condition known as prompt neutron criticality. Usually this condition is associated with sodium void instability and is mitigated by the proprietary negative reactivity injection mechanism. In the prompt neutron critical condition there is a sudden rapid release of nuclear thermal energy potentially sufficient to melt and/or vaporize the reactor fuel and vaporize the adjacent liquid reactor coolant. Rapid coolant vapor formation will cause such a large increase in cooling fluid pressure that the reactor will literally blow itself apart. A prompt critical reactor explosion stops when the physical expansion of the fuel is sufficient to stop the nuclear reaction. The reactor explosion at Chernoblyl in 1986 was the result of rapid cooling water vaporization due to prompt neutron criticality.

A lesson here is that a determined military attack which causes prompt neutron criticality will likely seriously damage any nuclear power reactor.

One way of addressing the prompt neutron criticality issue is to design the reactor and fuel such that any credible prompt neutron critical condition will self extinguish before the resulting energy release is sufficient to be a threat to either the reactor or the surrounding public. In a FNR described herein the strategy is to inject negative reactivity in any circumstance that causes a rapid tise in fissile fuel temperature.

In a FNR this protective mechanism must operate on a time scale of the order of 10^-4 seconds, comparable to the time scale of firing a bullet from a hand gun..

One way of preventing a determined military attack from causing a prompt neutron criticality explosion is to mount tne movable fuel bundle actuator nuts in their surrounding tubes with small radial screws. If the load on these radial screws becomes much larger than the weight of a movable fuel bundles these screws will fail in shear and the movable fuel bundle will fall to its fully retracted position. In normal reactor operation these screws are protected from twisting torque by outside vertical slots in the actuator nuts that slide into matching protrusions from the inside wall of the surrounding tubes.
 

FNR PROMPT CRITICAL RISK:
When nuclei fission over 99% of the free neutrons that are emitted are prompt neutrons and less than 1% are delayed neutrons. Both the prompt and delayed neutrons have initial kinetic energies of the order of 2 MeV.

There are two classes of fission type nuclear power reactors, Fast Neutron Reactors (FNRs) and thermal neutron reactors. Most existing power reactors use water as the primary reactor coolant and neutron moderator. In these reactors the hydrogen component of the cooling and moderating water rapidly absorbs kinetic energy from high energy fission neutrons, so most of the scattered neutron flux consists of slow or "thermal" neutrons. However, if the primary coolant is a liquid metal, such as sodium which has 23X the atomic weight of hydrogen, most of the scattered neutron flux consists of higher energy or "fast" neutrons. The fast neutrons trigger many more fissions per unit time than do thermal neutrons. Hence if the reactor reactivity is positive with respect to prompt neutrons the rate of free neutron population growth and hence thermal power growth in a FNR is much greater than in a water cooled reactor. Hence a FNR must incorporate passive measures that reduce the reactor reactivity with rising fuel temperture.

Power reactors normally operate at an equilibrium point where the reactor reactivity is slightly negative with respect to prompt neutrons and is zero with the addition of delayed neutrons. At this operating point the reactor is stable and fine power control is achieved via variation of the delayed neutron flux.

However, a sudden large change in fuel geometry, coolant geometry or temperature can cause the reactor reactivity to swing positive on prompt neutrons which causes a very rapid increase in reactor fuel temperature and fuel thermal power output. It is essential to immediately suppress this positive reactivity before the reactor power rises beyond its design limit.

In a thermal neutron reactor the reactivity is controlled by mechanical adjustment of the position of control rods. In a thermal neutron reactor when the reactivity swings slightly positive the rate of neutron population growth is sufficiently slow that mechanical control rod insertion can be used for safe reactor power control, even if the reactor reactivity slightly increases with increasing fuel and coolant temperature. A practical issue with such mechanical control systems is that near reactor power equilibrium the control rod insertion control mechanism tends to slowly hunt.

In a fast neutron reactor, when the reactivity swings positive on prompt neutrons the rate of neutron population growth and hence reactor thermal power output is so fast that safe reactor power control relies on the reactor reactivity decreasing with increasing fuel temperature. Via fuel thermal expansion a fast neutron reactor should immediately converge to a new safe stable power state without relying on any mechanically driven change to the fuel assembly geometry. There is also an issue of a delayed positive reactivity injection due to thermal expansion of the sodium coolant. Hence, to achieve the required performance a FNR is subject to design constraints that are not applicable to water cooled reactors.
 

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SODIUM VOID INSTABILITY:
At the deep sodium coolant boiling point the reactivity could potentially suddenly increase causing fuel melting. A proprietary mechanism that causes temporary axial fuel disassembly is used to avoid this problem.
 

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AXIAL FUEL DISASSEMBLY:
A major safety concern relating to a power FNR is an external event such as a military attack that might rapidly inject large amounts of positive reactivity into the FNR. In these circumstances within about 1 ms a proprietary mechanism injects a large amount of negative reactivity into the FNR to suppress prompt neutron criticality while the reactor is being shut down by withdrawal of movable fuel bundles.
 

CORE FUEL MELTING PROTECTION:
A relevant paper about a comparable liquid sodium cooled reactor with metallic fuel is S Prism Reactor Margin To Accidents
 

PROTECTION AGAINST SUDDEN DROPS IN NaK RETURN TEMPERATURE:
- Temperature moderated by large thermal mass
- Thermal mass temperature moderated by induction pump flow.
 

TOLERANCE TO OVERHEAD ROOF OR CRANE COLLAPSE:
The FNR design presented herein has an additional emergency shutdown mechanism triggered by downward force on the indicator tubes caused by an event such as an overhead crane or roof collapse. However, tripping of this emergency shutdown mechanism will leave the reactor inoperative until the affected fuel bundles are replaced.
 

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TWO INDEPENDENT SHUTDOWN SYSTEMS:
The FNR design discussed herein has two independent active shutdown systems that are also used for reactor temperature setpoint control and to enable reactor maintenance and refuelling. These systems have a response time of the order of one second. These systems are described at Two Independent Shutdown Systems
 

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FISSION PRODUCT DECAY HEAT REMOVAL:
A safety concern applicable to all fission reactors is removal of of fission product decay heat after reactor shutdown. The issue is that even after the chain reaction stops fission product decay continues to produce heat at about 8% of the reactor's full thermal power capacity. This fission product decay heat output declines over time, but it is high for about one day and is significant for many weeks. Without a reliable means of fission product decay heat removal the reactor core could potentially melt down. The reactor must have a 100% reliable means of sustained fission product decay heat removal.

In this FNR reliable fission product decay heat removal is via 48 redundant pumped heat transport systems, of which at least four must be operational at all times.
 

There are 48 independent NaK heat transport circuits each of which contains three separate heat exchange isolation barriers. A failure in any one of these barriers results in an individual heat transport circuit shutdown. Due to the multiplicity of independent heat transport circuits, the facility can continue operating while some of the heat transport circuits are out of service.

SAFETY OVERVIEW:
The aforementioned features, together with sophisticated automatic fire suppression, ensure that the FNR is safe for autonomous operation at an urban site. However, safety standards relating to this matter have yet to be developed.

The issue of safety in advanced reactors is broadly discussed in the 2012 report titled:
Overview of Generation IV (Gen IV) Reactor Designs //Safety and Radiological Protection Considerations.

In Canada nuclear safety matters are regulated by the Canadian Nuclear Safety Commission (CNSC). The main regulatory document is the Canadian Nuclear Safety and Control Act. The FNR discussed herein is intended to fall under the regulatory category of Small Modular Reactor (SMR) with an electricity output of less than 300 MWe.
 

ACCIDENT DESIGN BASIS:
From a licensing point of view the FNR design must meet all the severe accident events covered in the design basis for the site. Events that can occur together must also be considered.

For example if a FNR enclosure is hit by a missile which causes an enclosure failure the movable fuel bundles must be withdrawn, the resulting sodium fire and/or NaK fire must be extinguished, the roof must be repaired and the cooling that is required for both fission product decay heat removal and net cooling must continue to operate.

A FNR must withstand earthquakes and severe tornados that potentially cause electricity transmission poles to become missiles. The list of possible hazards also includes potential steam plant accidents like main steam line breaks and associated pipe whip as well as steam turbine disintegration.

The Darlington NPP safety report is likely available to the public at the CNSC library in Ottawa and should contain a list of all the design basis accidents. The Darlington NPP has a 2 m thick south facing reinforced concrete wall intended to safely absorb jihadi attacks using passenger airplanes. Likewise the FNR NPP discussed herein is surrounded by concrete walls totalling about 2 m thick.

From the perspective of reactor financing it is essential that once the design of a particular NPP is approved there must be no further design changes for safety reasons. The issue is that further design changes forced by regulators in the name of marginal improvements in public safety have the practical effect of making the entire project unprofitable. This issue is particularly important with respect to protecting the reactor from a determined military attack. The level of protection provided to resist a determined military attack is very much a judgement call. Changing the public safety protection measures after the reactor design has been approved is extremely expensive. Funding the costs triggered by such design changes is not a risk that FNR NPP investors can reasonably accept.
 

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FNR Na POOL SPACE AIRLOCKS:
In normal FNR operation there is seldom any need for maintenance personnel to enter the FNR primary sodium pool enclosure. The FNR relies on passive physics to maintain its fuel temperature setpoint, and the nuclear reaction will passively shut down if the reactor thermal load is removed.

If it is necessary to replace a heat exchange bundle or exchange a fuel bundle the FNR temperature must be reduced to about 120 degrees C and the Na-24 component of the sodium pool, which has a half life of about 15 hours, must be allowed to decay for about one week. Then robotic equipment can be used for fuel bundle exchange or for heat exchange bundle replacement.

If for some reason robots cannot do the job then maintenance personnel need protective suits with closed circuit air systems and cooling systems, similar to space suits, to protect personnel from the 120 degree C argon atmosphere and hot surfaces in the primary sodium pool space. The practical difficulties of doing work, such as disconnecting and then reconnecting intermediate heat exchange bundle flanged pipe joints, in such working conditions should not be under estimated.
 

FNR TEMPERATURE STABILITY:
In a FNR the nuclear chain reaction progresses through successive neutron generations very quickly, so the neutron concentration and hence the reactor thermal power can potentially grow or decay equally quickly. It is important to design a FNR such that its reactivity always has a strong negative temperature coefficient so that at its operating point its reactivity always quickly decreases as its average fuel temperature increases. Then the reactor will spontaneously seek an operating point where the reactivity is zero.

This safety characteristic is near optimal when about half of the fission neutrons formed in the core zone diffuse out of the core zone and are absorbed by the adjacent blanket zones. The design of a FNR fuel assembly should closely adhere to this safety principle.

If there is a suitable negative temperature coefficient then for a particular fuel geometry and a partcular thermal load there is an average fuel temperature at which the number of free neutrons remains stable. A stable number of free neutrons corresponds to a stable thermal power output. A FNR should exhibit a declining reactivity with increasing temperature without any reliance on an external physical control system. Then varying the rate of heat removal from the reactor controls the reactor thermal power. Delayed neutrons and a large thermal mass in a FNR primary sodium pool prevent rapid wide thermal power excursions when the fuel geometry, primary coolant temperature or primary coolant flow slowly change.

FNRs should be always be operated in circumstances where coolant boiling cannot occur. Coolant boiling causes coolant voids which will increase reactor reactivity and may reduce the fuel cooling rate, causing severe uncertainty with respect to the reactor operating parameters.

A FNR should have safety features that physically limit the maximum rate of change of fuel geometry, the maximum deviation of the reactor setpoint temperature from the primary sodium temperature and the maximum thermal load, regardless of operator error.
 

FNR THERMAL POWER SURGE PROTECTION:
It is necessary to ensure that any credible change in coolant temperature or coolant flow will not cause a local reactor heat flux in excess of the fuel tube material rating.

Likewise it is necessary to ensure that a FNR will not have an uncontrolled thermal power surge due to a sudden change in its fuel or coolant geometry caused by any credible earthquake, aircraft impact, gantry crane failure or structural failure.

A FNR should use fuel designed such that any credible excursion into prompt neutron criticality causes instantaneous linear disassembly of the fuel to suppress the prompt neutron critical condition. This disassembly occurs because prompt neutron criticality will cause instantaneous boiling of cesium and sodium that is in pockets formed by the ceramic spheres that are in direct contact with the active portion of the core fuel rods. The resulting high pressure sodium and cesium vapor will blow part of the core fuel toward the fuel tube plenum reducing the reactor core zone thickness and hence the reactor reactivity.

In the event that linear disassembly of the fuel does not sufficiently suppress the prompt neutron criticality the fuel will vaporize and burst the fuel tubes to stop the nuclear reaction. In this respect it is important that when the fuel particles settle to the bottom of the sodium pool the pool bottom contour and material be such that the fuel will not again become critical. For example, the region under the fuel assembly can contain a layer of neutron absorbing gravel that will prevent a nuclear reaction close to the pool bottom.
 

NON-NUCLEAR MAINTENANCE:
On-site personnel are required to do periodic routine non-nuclear preventive maintenance on the NaK heat transport system, NaK-salt heat exchangers, salt circulation pipes and pumps, steam generators, injection pumps, turbo-generators, condensers, cooling towers and related mechanical and electrical equipment and to make repairs as necessary. However, this equipment should not involve any radioactivity. Most of it is in separate buildings isolated by three reinforced concrete walls with a total thickness of 2.5 m. There is sufficient redundancy in the FNR support equipment that some of the heat transport circuits can be shut down for maintenance or repair while the others remain in operation. Thus, the only reasons for keeping staff on the reactor site 24/7 is compliance with steam power plant regulations and maintenance of site security.
 

FNR POWER CONTROL:
For normal safe thermal power control FNRs rely on thermal expansion of reactor fuel to reduce the FNR reactivity and hence reduce the thermal power output as the fuel temperature increases. The reactor core zone fuel geometry should be slowly adjusted to change the fuel average temperature setpoint or to cause a cool or cold reactor shutdown.

Power is controlled via the NaK induction pumps.

One of the reactor design issues is prevention of sodium void instability. Formation of sodium voids would potentially increase the FNR reactivity. At all reactor operating states the decrease in reactivity due to an increase in fuel temperature must safely exceed the increase in reactivity due to structural and liquid sodium coolant thermal expansion. In this respect the large thermal coefficient of expansion of Pu plays an important role. The reactor temperature must be sufficiently low and the liquid sodium head pressure sufficiently high that coolant voids never form.

The tendency for sodium void formation is related to the local sodium temperature, the local sodium flow rate, the sodium temperature distribution in the reactor and the sodium hydraulic head. The reactor must not rely on any mechanical pumping mechanism for preventing formation of sodium voids. Typically this void free operation is achieved by operating the sodium far below its boiling point. The sodium boiling point is further raised by use of a significant liquid sodium head pressure in the reactor core zone. The reactor peak power must never be so large as to cause sodium void formation. Reactor power peaks tend to occur at times when there are changes in fuel geometry with the object of increasing the average fuel temperature.

One of the issues in FNR design is ensuring that no matter what adverse circumstances occur on loss of station power the reactor fails into a safe shutdown state.

Note that on recovery from a station power failure, if the primary liquid sodium has significantly cooled the average fuel temperature setpoint must be very slowly raised before re-establishing reactor operation. During that warming period the nitrate salt must also be melted and any water in the nitrate salt circuit must be boiled off.
 

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LOADING AND UNLOADING FUEL BUNDLES:
It is important to never let the fuel assembly accidentally go critial. In loading fuel bundles into the primary sodium pool each movable bundle should be installed in the fully withdrawn position before installing its adjacent fixed fuel bundles. Similarly the fixed fuel bundles adjacent to a movable bundle should be removed before extracting the movable fuel bundle. That strategy ensures that the fuel assembly will not accidently go critical due to pulling a movable fuel bundle through the matrix of adjacent fixed fuel bundles.
 

CORE FUEL MELTING PROTECTION:
A relevant paper about a comparable liquid sodium cooled reactor with metallic fuel is S Prism Reactor Margin To Accidents
 

CERTAIN FISSION PRODUCT DECAY HEAT REMOVAL:
For each FNR there are 48 independent passive heat removal circuits any four of which can reliably and safely remove the fission product decay heat.

Under the circumstances of a double liquid sodium containment wall failure the heat transfer capacity of each heat transfer system might fall by a factor of three. However, we only need (1 / 12) of the entire heat transfer system capacity to remove fission product decay heat. Thus in order to reliably remove fission product decay heat it is essential that (1 / 4) of the total reactor heat transfer capacity must continue to function so that under the adverse condition of a double sodium containment wall failure the remaining certain heat transfer capacity is:
(1 / 3)(1 / 4) = (1 / 12)
of system full power heat removal capacity. Hence for maximum reliability there should be at least twelve independent heat removal circuits functioning.
 

TOLERANCE OF HEAT EXCHANGE TUBE FAILURES:
A practical FNR involves many thousands of intermediate heat exchange tubes. Sooner or later one or more of these tubes will fail. Each secondary sodium system has the following features:
1) The NaK loop components which normally operate at ~ 0.5 MPa are all rated for a working presure of 2 MPa and are safety tested to 3 MPa;
2) There are NaK level sensors consisting of a long thin coils of nichrome wire suspended from an insulated feed through in an argon filled cushion tank head space. The electrical resistance of this coil to ground decreases as the NaK level increases.
3) There are NaK level sensors in the dump tanks.
4) The NaK loops normally operate at about 0.5 MPa.
5) If there is a leak in an intermediate heat exchanger the NaK level in the NaK cushion tank will decrease.
6) If there is a leak in a steam generator tube the nitrate salt level will increase and the salt will contain steam/water which will vent.
7) The nitrate salt flows through the steam generator tubes.
8) The trigger for draining the tube side of the steam generator to the nitrate salt dump tank is a formation of steam in the nitrate salt loop or a decrease in NaK level.
9) If there is water in the nitrate salt circuit it is essential to isolate the steam generator to prevent the nitrate salt circuit from being filled with water by a leak in a steam generator heat exchange tube. Since the steam generators serving a single turbine are connected in parallel it may be necessary to trip off the entire steam generator group on detection of water in a nitrate circuit. By stopping the injection water pumps we stop any possible back flow of water.

10) Note that the NaK/salt heat exchanger is at a higher elevation than all the other equipment on that same heat transfer circuit. Draining the shell side of the NaK/salt heat exchanger stops heat transfer through this circuit but potentially raises the NaK induction pump temperature up to 460 degrees C. The NaK cannot be drained to its dump tank until there is certainty that the nitrate salt loop is drained below the level of the NaK/salt heat exchanger. Otherwise there is a possibility of a major accident resulting from salt or water entering the NaK loop via a tube rupture in the NaK/salt heat exchanger.

11) If the NaK pressure falls to 0.3 MPa nitrate salt drainage to its dump tank is tripped.

12) As long as NaK is present NaK continues to flow through a NaK/salt heat exchanger tube rupture into the nitrate salt, it will produce nitrogen. If there is any water in the salt it will also produce hydrogen. The gas pressure in the salt circuit now rapidly rises and discharges more salt out the nitrate salt loop vent via a ball check.

13) There is a NaK dump tank and a nitrate salt dump tank for each heat transport circuit. Each dump tank has sufficient volume to accommodate all the NaK or salt in its circuit. If the argon pressure over the NaK dump tank is released the NaK will drain down into its dump tank.

14) If the air pressure over the nitrate salt dump tank is released the nitrate salt will drain down into its dump tank.

15) The NaK loops are vented to above the roof by vents fitted with rupture disks and gravity operated ball check valves. The vents must be sufficiently high that entrained NaK in the exhaust cannot start a roof fire.

16) The NaK dump tanks are normally filled with 0.5 MPa argon. Hence if there is a steam generator tube leak which causes steam leakage into in the nitrate salt loop the nitrate salt must be drained down into the nitrate salt dump tank.

17) If water enters the nitrate salt and the NaK level and pressure are OK the steam generator water injection is stopped and the steam generator is drained. The object is to minimize the mass of water that can leak into the salt circuit via the steam generator tube failure. As soon as there is some water in the nitrate salt circuit it will become steam which will blow salt out of the salt circuit vent.

18) An important issue is to rapidly drain water out of the steam generator to prevent that water continuing to flow into the nitrate salt via the steam generator heat exchange tube rupture. Steam or super heated water entering the nitrate salt circuit will blow salt out the nitrate loop vents. When water is detected in the nitrate salt circuit we must shut down that steam generator.

19) In order to service the NaK loop the NaK in the intermediate heat exchanger must be transferred to the NaK dump tank. While conducting maintenece work inside a particular Heat Exchange Gallery it is prudent to first drain the other nearby heat transport loops in the same gallery. Pressurized hot NaK is potentially very dangerous. Even a sudden small gasket leak could cause a life altering stream of hot NaK. Once the maintenance work is complete the affected heat transport loops can be recharged. Ideally the maintenance work should proceed fast enough that the nitrate salt does not freeze in its dump tank.

20) After repair the NaK loop must be refilled by application of argon pressure to the NaK drain down tank. The intermediate heat exchange bundle has a thin drain tube connected to its bottom. Then an overhead argon pressure permits draining the liquid NaK from the heat exchange bundle.

21) In summary any significant change in either the NaK level or the NaK loop pressure is indicative of a serious problem with that heat transfer loop. The NaK level as a function of time in both the expansion tank and the dump tank should indicate the nature of the problem.

22) On a steam generator tube rupture initially water flows from the steam generator into the nitrate salt which almost instantly raises the nitrate loop pressure blowing salt out the vents with ball checks. This transient high pressure should trip the steam generator steam pressure release valve and drain valves and turnoff the steam generator injection water pump.

23) Assume that there is a NaK/salt heat exchange tube failure and that the nitrate loop is not fully drained. A continuing NaK pressure increase will cause the rupture disk to open. Then NaK is expelled from the NaK loop via both the tube failure and via the open rupture disk.

24) A consequence of a NaK/salt heat exchanger tube failure might be NaOH accumulation in elbows at the bottom of the NaK loop. A filter system should be provided that gradually removes NaOH from the NaK loop. This filter should be installed across the induction pump. The NaOH can be periodically dissolved by raising the minimum loop temperature above 318 degrees C and then cooling it in the filter. There still may be a problem with liquid NaOH sinking to the bottom of the NaK loop. It should be expelled via the intermediate heat exchange bundle clean out tube.

25) There must be a drain that dumps the contents of the nitrate salt loop if the NaK lacks level or pressure and the nitrate salt dump tank is full. Thus lack of NaK level or pressure must turn off everything downstream.
 

EARTHQUAKE TOLERANCE:
FNR Earthquake tolerance issues are detailed on the web page titled: FNR Earthquake Protection
 

PROLIFERATION RESISTANCE:
Implementation of Proliferation Resistance
 

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MURPHY'S LAW
Murphy's Law states that if there is any way for humans to do something wrong sooner or later someone will discover it. FNRs must be engineered to be tolerant of possible human error. To the extent possible FNRs should be designed so that incompetent or irrational human activity cannot cause dangerous prompt neutron criticality.

FNRs rely on fairly complex crane manipulation of fuel bundles during fuel bundle installation and replacement. This crane manipulation is unlikely to be fully automated in the near future, so this portion of FNR work will likely be subject to potential human error.

In the event that during loading or unloading a fuel bundle is dropped and falls to the bottom of the primary liquid sodium pool the dropped fuel bundle must be immediately retrieved, not ignored or forgotten. The potential danger is a prompt critical condition arising from random overlap of the core fuel of the dropped fuel bundle with the core fuel of another dropped fuel bundle. To minimize such problems the gantry crane used for fuel loading and unloading should be fitted with a safety line to prevent such drops.

It should be assumed that sooner or later humans will make mistakes. A FNR must be designed to enable easy detection and remedy of mistakes. Any mistake that could potentially lead to reactor over heating or dangerous prompt neutron criticality must be obvious to several different individuals long before it can cause a disaster. Ideally any safety procedure that relies on human operator training or skill is open to being done wrong by someone sooner or later.
 

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THE WALK AWAY SAFETY CONCEPT:
The concept of walk away safety is that if the appropriate operating and/or maintenance employees are not present or suddenly leave when an adverse circumstance occurs the FNR must always spontaneously default to a safe condition.

The FNR facility consists of a common central heat source, 48 transport circuits and 8 to 16 independent heat to electricity conversion systems. The heat outputs are connected to supply heat to four independent district heating loops. Each district heating loop has one local cooling tower and three remote cooling towers. In order to provide maximum electricity output in the summer all of the cooling towers must be fully functional. When not in the emergency cooling mode at all times at least one generator and its associated cooling tower and heat transport circuits should remain fully functional to remove FNR fission product decay heat.

In essence outside of the reactor the facility consists of four independent power stations, each which has either two or four steam turbogenerators. In order to remove fission product decay heat, at all normaltimes at least one of the four indpendent power staions must be functional.

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FNR CONTROL SYSTEMS:
The FNR facility has multiple independent control systems:
1) Primary sodium Pool:
The primary sodium pool control system operates almost independent of the heat to electricity conversion systems. The primary sodium pool features:
a) Normal temperature control;
b) Shutdown system #1;
c) Shutdown system #2;
d) Emergency sodium pool cooling.

2) There are 8 independent heat to electricity conversion systems, each with six dedicated heat transfer circuits and one turbogenerator. There are four on-site cooling towers, each which serves two adjacent turbo generator halls.
 

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EVENTS INITIATED BY A SHUTDOWN:
In each nitrate salt heat transport loop if the temperature is too low the nitrate salt drains to its dump tank to prevent the nitrate salt freezing in its extended pipe.

Event triggering minumum power operation:
Loss of AC grid power

EMERGENCY SODIUM POOL COOLING:
Events triggering emergency sodium pool cooling include:
a) A primary sodium pool temperature high above its setpoint.
b) An imminent fire threat to the sodium pool.
 

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FISSION PRODUCT DECAY HEAT REJECTION:
The safety concept is that there must always be enough cooling water stored on the reactor site to safely remove fission product decay heat by evaporation with minimal reliance on electic power. For example, one heat to electricity conversion system can be used to provide station power, which is independent of problems on the external electricity grid.
 

LOSS OF DISTRICT HEATING WATER
If there is loss of water from the district heating system the connected condensers will not work which implies that the affected generators will not work which leads to loss of two of the eight station power systems.
 

LOSS OF CITY WATER:
The main reactor does not rely on a continuous supply of city water. However, city water pressure may be required for support services such as flushing toilets, refilling emergency water tanks, etc. so loss of city water pressure is a condition that requires ongoing manual supervision until the condition is fixed.
 

FNR SHUTDOWN STRATEGY:
At a planned and/or scheduled reactor shutdown the best strategy is to withdraw the movable fuel bundles but maintain a thermal load on the reator that balances the fission product decay heat so that the reactor maintians it soperating temperature for as long as possible. Hence electricity generation is maintained for feeding house power circuits. This strategy maintains the molten salt temperature in some of the heat transport loops and hence maintains house power electricity generation capacity. Only when the fission product decay heat is no longer sufficient to operate one turbogenerator are the reactor cooling pumps shifted to an external source of power.
 

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FORCED COLD SHUTDOWN:
On a forced cold shutdown the FNR no longer maintains temperature. The movable fuel bundles all fully withdraw. The nitrate salt circuits all drain down to their dump tanks. If the primary sodium temperature rises above its trip point fission product decay heat is removed from the reactor by the NaK and heat transfer fluid. Natural circulation of the NaK transfers heat from the primary sodium pool to NaK and then heat transfer fluid and then water in the steam generators which heat is vented as steam.
 

LOSS OF THE EXTERNAL AC GRID:
On loss of the external AC grid the NPP disconnects from the grid and the turbogenerators revert to local frequency control. That local frequency can be phase locked to either the grid or a local time base. Everything continues to operate as normal but only one generator can meet the station parasitic load. All the pumps continue operating as before.

Loss of Grid AC power means that the remote cooling tower and remote building water cooling pumps will no longer operate. It is necessary to power the local cooling towers from the station power bus so that these local cooling towers continue to function when there is no grid AC power.

Typically each cooling tower has two of everything so that half of the cooling tower equipment is powered by one generator parasitic power circuit and the other half is powered by the other generator parasitic power circuit.

Thus on loss of AC grid power the FNR continues normal operation at reduced power.
 

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LOSS OF STATION POWER:
In normal opertion the station power circuits continue operation after loss of AC grid power.

If there is loss of station power the related cooling tower water pump, NaK pumps and nitrate salt pumps will immediately stop and the nitrate salt will drain to its dump tank. Hence that system can no longer remove fission product decay heat.

Each station power parasitic load requires heavy duty standby power to restart by melting the nitrate salt in each dump tank and circulating the salt prior to local house power generation. Generally this start power must come from either the AC grid, a large local diesel generator or one of the other station power systems. Thus, if possible we do not want an AC grid failure to precipitate a station power failure. Loss of station power causes nitrate salt drain down in all the affected heat transfer circuits. On loss of power to the house power busses the FNR facility must default to a forced cold shutdown.
 

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LOSS OF PRIMARY SODIUM POOL POWER:
In normal ongoing operation the primary sodium pool monitoring system consumes very little power and is easily battery backed for a long period of time. Hence the primary sodium pool monitoring system does not lose power until long after all eight station power systems have failed. On loss of sodium pool control power the movable fuel bundles remain in their last set position. If there is a credible physical threat to the reactor battery power should be used to withdraw the movable fuel bundles.

The sodium pool has a filter pump which can be powered from the station power circuit. This filter pump can be off for a long period of time with little negative effect.

However, if the batteries for the sodium pool electronics become depleted the reactor must fail to a cool shutdown. These batteries should be charged from the station power bus.

If the sodium pool temperature becomes too high it is likely indicative of net sodium heating by fission product decay, which indicates a requirement for more cooling.

In a forced cold shutdown the nitrate salt is already drained to its dump tanks. NaK and HTF are used to remove fission product decay heat from the Na. There must be reliable sources of argon pressure sufficient to transfer NaK ans HTF from their dump tanks to the top of the NaK-HTF heat exchangers.

Note that if the FNR has been operating for a significant length of time producing just station power the potential thermal power of the fission products will be low. However, care needs to be taken that emergency cooling water is not wasted.

Reconnection of a station power circuit to the AC grid requires resynchronization. Most such reconnections are manually supervised.

Recovery from a forced cold shutdown requires manual intervention.
 

POWER SYSTEM MAINTENANCE:
If only one heat transport circuit is involved:
Drain down the relevant NaK loop;
Drain down the nitrate salt;
Drain down the HTF.

If only one generator is involved:
Take generator to minimum power;
Disconnect generator from AC grid;
Turn off makeup water to its steam generators;
Drain down the six associated nitrate salt loops;
Drain down the six associated NaK loops;
Drain down the six associated HTF loops.

If one cooling tower is involved:
Turn off the associated generators, as necessary for safe work.
 

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EARTHQUAKE TOLERANCE:
FNR Earthquake tolerance issues are detailed on the web page titled: FNR Earthquake Protection
 

STRONGER EARTHQUAKES AND VIOLENT STORMS Continue normal reactor operation but be prepared for loss of electricty grid

An important issue in earthquake protection is bolting the fixed fuel bundles together to form a rigid matrix. The liquid sodium above the fuel assembly can safely slosh back and forth in an earthquake provided that the surface waves do not change the fuel assembly geometry and hence its reactivity.
 

EXTREME EARTHQUAKES AND EXTREME STORMS
May cause reactor safety shutdown.

2) Very low probability events such as impact by an aircraft or missile with sufficient impact kinetic energy to penetrate the reactor dome, the dome fill material and the ceiling over the primary sodium pool. This situation will force an immediate and prolonged reactor shutdown. In this situation the main design objectives are instant reactor shutdown and rapid extinguishing of the likely primary sodium fire in a manner that minimizes or prevents the FNR emitting airborne corrosive and radioactive species.

A method of suppressing the primary Na fire following a failure of the FNR dome has been developed, but that method uses NaCl which may damage the FNR's steel components.

3) Low probability events that result in an immediate reactor cold shutdown until after detailed equipment inspection. In this category are severe earthquakes, severe hurricanes and tornados, minor missile damage, minor prompt neutron criticality excursions, reactor safety trips, any fire which causes discharge of NaCl based fire suppression material, or partial failure of redundant cooling equipment.

SEVERE EARTHQUAKES, LARGE METEORITES AND DETERMINED MILITARY ATTACKS:
A reality that people must face is that FNRs provide the only fuel sustainable means of fully displacing fossil fuels. If extinction by global warming is to be avoided mankind must accept widespread deployment of FNRs. Ill considered safety regulations that have the effect of delaying or preventing widespread deployment of FNRs will prevent sustainable displacement of fossil fuels, which will eventually lead to extinction of humans by global warming. People must choose between the extremely low risk of a serious FNR related accident, possibly caused by an extreme earthquake, a large meteorite or an irrational military action, and the certainty of thermal extinction by global warming due to failure to deploy a sufficient fleet of FNRs.

There is no such thing as perfect public safety. Consider extremely low probability events such as a severe earthquake, a direct impact by a large meteorite or a determined military attack. The public safety risk of living near a power FNR is comparable to living downstream from a large hydroelectric dam, which could also be damaged by such events.

It is not possible to design the FNR to continue functioning after such an event nor is it possible to fully protect the public. The best that can be done is to adopt reasonable measures to minimize potential consequences.

An issue that arose during the 2022 Russian invasion of Ukraine is what happens if a FNR is subject to a determined military attack, such as by a large missile or a laser guided armour penetrating bomb dropped from a high altitude. As was shown in WWII through the use of large armour penetrating bombs with tungsten carbide noses (Tall Boys) to attack German U-boat pens which had 5 m thick concrete roofs and to sink the heavily armoured battle ship Tirpitz, it is simply not practical to build FNR enclosures that can reliably resist determined attacks by precision guided armour penetrating munitions. If a determined military attack is a credible risk to a FNR, either the reactor should be cold shut down with the movable fuel bundles fully withdrawn or some other means of preventing a determined military attack, such as surface to air missiles (Patriot Battery) should be implemented.

That does not mean that we do not build large hydroelectric dams and nuclear power plants. What it means is that we try to mitigate risks by locating large hydroelectric dams and nuclear power plants at sites which pose minimum risks from natural causes such as earthquakes, tsunamis, etc. and we try to maintain sufficient social order that there are no determined military attacks. We are not always successful. In June 2023 Russia destroyed the Nova Kakhova hydroelectric dam in Ukraine.

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This web page last updated December 1, 2023