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

This web page focuses on the FNR enclosure. The elevation of the enclosure must be chosen to be sufficient that the contained primary sodium pool will never be flooded.

The primary sodium pool is locateed at the center of the nuclear building at the center of the FNR Nuclear Power Plant site. The sodium pool has a robust rigid octagonal external concrete enclosure and a less robust but thermally insulated and gas tight octagonal internal enclosure. Together these two enclosures reliably contain heat, sodium vapor and argon, absorb gamma ray emissions and exclude air and water. The external concrete enclosure wall is stabilized by 12 X 1 m thick external shear walls.

Around the perimeter of this nuclear structure are 8 heat exchange galleries that are further protected by a 1 m thick outside wall.

Spaced 10 m away from this 1 m thick concrete outside wall are 8 turbine halls and four cooling towers which together provide additional protection for the nuclear building against low angle attack by either ground based military weapons or low flying aircraft.

On top of the octagonal concrete enclosure is a very robust steel octagonal dome described on the web page titled: FNR Dome which contains sandbags for tornado resistance and missle impact absorption

There are 53 argon gas pressure zones:
a) Gas pressure over the primary sodium pool;
b) Argon pressure inside the fiberfrax filled wall cavity
d) Outside air pressure = pressure in the heat exchange galleries;
e) Argon bladder pressure = access space pressure
f) Argon pressure over each of 48 secondary liquid sodium dump tanks.

The gas pressure in the reactor space slowly follows the access space pressure due to the bladder(s) in the access space that are connected to the pool space via a gas cooler.

The gas pressure in the access space slowly follows the outside air pressure due to an orifice betwen the acces space and the outside.

A tornado can cause a relatively rapid change in outside air pressure. This change is pressure must appear across the structural wall, not the inner gas tight wall. Hence air exchange between the access space and the outside is limited to about the same gas flow that flows into or out of the bladders which are connected to the pool space.

The gas flow rate between the reactor space and the bladders is limited by the gas flow rate through the argon cooler. In order to minimize differential gas pressure forces on the gas tight reactor wall the pressure in the access space must accurately follow the pressure in the reactor space.

Hence the rate of exchange of gases between the access space and the outside must also be small.

The argon pressure inside the fiberfrax cavity is kept at a small constant above the pressure in the reactor space and the access space to prevent either air or sodium vapor entering the fiberfrax cavity.

A FNR enclosure safety feature, prompted by Russian shelling of power reactor sites during the 2022 Russian invasion of Ukraine, are two layers of sand bags supported by the dome's horizontal tie rods, directly over the FNR primary sodium pool. The weight of these sand bags is sufficient to hold the dome in place during a tornado. The sand bags will further prevent projectiles or large roof pieces, from free falling up to 30 m, penetrating 5 m of liquid sodium and then impacting the reactor fuel with enough momentum to cause physical damage. It is not necessary to provide comparable protection from sand or small falling roof pieces. Their impact momentum should be sufficiently absorbed by the ~ 5 m of liquid sodium above the reactor fuel. Note that silica sand will not rapidly chemically react with liquid sodium.

The primary sodium pool is a 20 m diameter X 15 m deep pool of liquid sodium with a normal top surface temperature of 460 degrees C. This pool is located on the central axis of a 31 m outside face to outside face octagonal concrete enclosure. This enclosure is protected on top by a robust structural steel dome fitted with a water tight surface membrane.

The FNR enclosure's 1 m thick reinforced concrete walls surrounding the primary sodium pool are stabilized on the outside both above and below grade by the 12 X 1 m thick X 9.0 m deep reinforced concrete shear walls of the heat exchange galleries, by the 1 m thick concrete external wall of the heat exchange galleries and by a 17 m wall depth below grade.

The the octagonal concrete wall and dome roof are designed to withstand repeated direct exposure to maximum possible hurricane and tornado force winds which can cause a transient differential pressure drop across the wall and dome surfaces of about 10,000 Pa.

The octagonal concrete wall is reinforced against a tornado induced net internal positive pressure by a perimeter wound steel cable. On 1 m of enclosure height the tornado induced differential pressure across the wall exerts a force of:
10,000 Pa X 30 m X 1 m = 300,000 Newtons.

Hence 1 m of wall height must have wound steel cable reinforcement sufficient to resist:
300,000 Newtons / 2 = 150,000 Newtons

Steel reinforcement is good for:
10,000 psi X 101,000 Pa / 14.7 psi
= (101 X10^7 Pa)/ 14.7
= 6.87 X 10^7 Newtons / m^2

Hence the required steel cross section in 1 m of wall height is:
[(150,000 Newtons) / (6.87 X 10^7 Newtons / m^2)] / m
= [21.834 X 10^-4 m^2] / m
= 21.834 cm^2 / m

A (1 / 2) inch diameter steel cable has a cross sectional area of:
Pi (1 / 4)^2 inch^2 X (2.54 cm / inch)^2 = 1.2668 cm^2

Thus if the reinforcement is (1 / 2) inch diameter steel cable the required number of cable windings per m of wall height is:
(21.834 cm^2 / m) / (1.2668 cm^2 / cable turn)
= 17.2 cable turns / m

In order to resist tornados the dome has a weight of about 2 tonnes / m^2. This weight is achieved by addition of sand bags. The sand is fire resistant and will distribute projectile impact forces over a wide area.

The inside bottom of the dome is formed from horizontal tie rods. In addition to balancing radial forces these tie rods support the lower layer of sand bags. These sand bags also prevent large dislodged roof pieces penetrating the FNR primary pool inner ceiling and falling into the primary sodium pool.

The dome's nearly flat floor is sloped to drain off burning liquid fuel, such as might be present after a jihadi aircraft attack.

The FNR foundation is gravity drained to a discharge point which is above the maximum posssible elevation of the surrounding water table.

The primary sodium pool enclosure is in the middle of the FNR Site Plan shown below.


The octagonal concrete wall supports the FNR dome and its loads, supports the secondary sodium pipes, absorbs above pool level gamma emissions from Na-24 and forms the inner wall of the heat exchange galleries. The inside surface of this concrete wall is coated with a rubber like sealing material which has the effect of preventing unplanned air exchange between the access space and the outside.

Immediately inside this concrete wall is a 1 m wide access space which provides an air flow path for closed circuit reactor leakage heat removal. Below the pool deck this space is > 2.5 m wide, of which about 1.5 m is occupied by an argon bladder. This space could contain relative lateral FNR primary sodium pool wall stabilization to prevent primary sodium pool damage during a very severe earthquake.

Above the pool deck there are both inner sheet stainless steel wall and the outer gas tight sheet metal wall separated by 1 m of compressible ceramic fiber thermal insulation. The outer sheet metal wall must be continuously edge soldered and bubble tested to be gas tight. There is a gas tight flexible connection between the pool deck and the inner stainless steel wall of the primary sodium pool.

The wall must flex to allow thermal expansion and contraction of the wall surface, pool and pool deck. Above the pool deck there is further side wall flexing due to thermal expansion and contraction of the secondary sodium pipes connecting to the intermediate heat exchangers located in the primary sodium pool.

The wall space containing the fiberfrax ceramic insulation contains argon at slightly above atmospheric pressure. The inner sheet stainless steel wall is used to contain the neutron activated sodium vapor within the primary sodium pool space. The outer sheet metal wall is used to contain the argon and exclude air. The inner sheet stainless steel wall normally operates at about 460 deg C and the outer sheet metal wall normally operates near ambient temperature.

The inner wall (shown in faint purple) is a 1 m thick layer of rigid fiber-frax insulation covered on both the inside and outside by gas tight sheet stainless steel. This inner wall contains cool side structural members (studs) that are supported from the top and bottom so that the wall can flex to allow thermal expansion and contraction of the top supports, the bottom supports and pipes passsing through the wall. The outer containment is actually sheet stainless steel fastened to the inside surface of the studs. Hence the studs act as fins to dissipate leakage heat into the cooled access space. Inside the outer stainless steel sheet is a layer of rigid fiberfrax which is held inplace by self tapping screws which go through the sheet stainless steel. On the inner side of the layer of fiberfrax is another layer of sheet stainless steel which is held in place by self tapping screws into the fiberfrax. The inside stainless steel layer must be gas tight to contain both hot argon and sodium vapor. The hot sodium vapor will potentially attack the fiberfrax insulation if it leaks through the inner sheet of stainless steel.

A small gas pump fed by cool argon keeps the argon pressure inside the fiberfrax containment cavity slightly above atmospheric pressure to prevent sodium vapor leaking into the fiberfrax. If this argon leaks into the 1 m wide cooled access space this leak is easily located by soap bubble testing. The intake to this gas pump has a cooling system which condenses sodium vapor to prevent sodium vapor entering the fiberfrax containing cavity.

The argon pressure inside the fiberfrax will tend to drive the inner and outer stainless steel wall coverings apart. These stainless steel sheets must each be independently screwed into the fiberfrax with self tapping screws. Higher density and thicker fiberfrax is chosen for this semi-structural purpose.

Any small leaks of argon into the 1 m wide access space are located by soap bubble testing. These leaks are fixed by allowing the pressure inside the fiberfrax to temporarily fall to atmospheric and then by applying sealing goop to the access space side of the outer stainless steel wall wherever needed. The use of this sealing material must be limited because it will tend to increase the thermal resistivity for heat conducting into the 1 m wide access space.

The purpose of the fins on the access space inner wall is to transfer leakage heat from the reactor space to the gases in the 1 m wide access space. This gas mixture is circulated through a dome mounted closed loop refrigeration system which constantly dumps the leakage heat to outside air. A major benefit of this closed access space is that it protects the inner reactor space wall from differential pressure damage in a tornado. In normal reactor operation, when no service access is required, the access space can be oxygen depleted, which will minimize the rate of long term oxygen absorption by the primary liquid sodium.

The outside surface of the 1 m wide access space is sealed with another sheet stainless steel layer screwed to studs. Gas leaks in this layer are plugged by sealing goop applied from inside the access space.

In normal reactor operation there should be no argon in the 1 m wide access space. The lower oxygen partial pressure in the 1 m wide service space than inside the fiberfrax wall prevents oxygen in the 1 m wide access space penetrating the primary pool space. The presence of any argon in the access space indicates an interior wall leak. Locating the vicinity of that leak requires scanning the inner wall of the 1 m wide access space with a mass spectrometer tuned to argon detection. Once the general vicinity of the leak has been detected the exact location can be found with a bubble test. A small controlled amount of outside air may be admitted into the 1 m wide space to gradually expel leakage argon and to provide a breathable atmosphere for persons working inside the access space.

Personnel access to the 1 m wide access space is via a double door airlock similar to the airlocks used on many commercial buildings.

Both the inner and outer stainless steel coverings of the fiberfrax containment space must be able to sustain the small differential pressure. The fibrefrax inside the stainless steel containment consists of three staggered layers so that heat leakage at fiberfrax board joints will be minimal. The staggered fiberfrax boards must be fastened together with recessed staggered metal hardware. The exact interior layout of the screws forming this wall needs to be resolved.

Note that any argon which leaks from the reactor space into the access space may bring with it small amounts of Kr-85 or Na-24. This issue needs more study. If too much radioactivity is present the access space may need to be vented before any personnel enter.

The pool deck is slightly sloped toward the pool so that under normal circumstances sodium vapor condensation on the inside walls drains back into the sodium pool. At the edge of the sodium pool there is a safety rail to prevent a suited worker accidentally slipping into the sodium pool.

Radiating out from the edge of the primary sodium pool are 96 secondary sodium pipes that connect the intermediate heat exchange bundles to the equipment in the adjacent heat exchange galleries. The 48 intermediate heat exchange bundles are supported from the bottom by a table but are free to slide radially to allow for pipe thermal expansion/contraction.

The main components of the FNR enclosure height are:
Footings = 2 m
Ventilation space under pool = 1 m
Supporting Fire brick = 2 m
Primary sodium depth = 15 m
Sodium surface to pool deck = 1 m
[Pool deck is 2 m above grade]

Pool deck to inner ceiling = 14.5 m
Ceiling insulation = 1 m
Flat open ceiling space for ventilation, instrumentation and roof structural access = 2 m
Dome = 8.0 m

The sodium pool is mostly below grade. The top 2 m of the pool walls are above grade to prevent surface flood damage and to enable easy horizontal transfer of fuel bundles and intermediate heat exchange bundles via horizontal airlocks between the space over the sodium pool and flat deck truck mounted horizontal shielded fuel bundle shipping containers.

The pool deck is a 26 m diameter almost flat plate with the 20 m diameter primary sodium pool at its center. This plate fits partially under the fiberfrax filled side wall.

The enclosed space over the liquid sodium pool is filled with the inert gas argon which will not chemically react with either the liquid sodium or steel. The argon pressure is maintained at one atmosphere by argon filled bladders located under the pool deck and under the heat exchange galleries. This simple argon pressure control system does not require AC power. Fail safe argon pressure control is essential for sealed reactor enclosure structural integrity.

If the reactor is shut down for an extended period the sodium surface should be flooded with kerosene to prevent sodium oxidation. However, that kerosene itself is a potential fire hazard and is difficult to remove. The number one objective is to keep oxygen out of the enclosed primary sodium pool space. Any oxygen or water vapor that leaks into this space will gradually form Na2O or NaOH which must be filtered out of the primary sodium.

The concrete portion of the FNR enclosure normally remains at ambient temperature. The main function of the concrete is provide structural strength and to attenuate gamma radiation emitted by sodium pool Na-24 decay while the reactor is operating. Other functions of the concrete include:
1) Exclusion of ground water;
2) Exclusion of rain water;
3) Exclusion of flood water;
4) Protection from physical attack;
5) Reserve fire containment;
6) Supporting the dome roof structure, the ceramic fiber insulated ceiling and wall, the polar gantry crane, the secondary sodium pipes and the fuel bundle electronic monitoring equipment;
7) Guiding cooling air flow;
8) Gamma radiation absorption

9) Resist tornados.  

The functions of the inner sheet metal wall sheathing include:
1) Sodium vapor containment;
2) Argon containment;
3) Fiber-frax insulation protection.

The functions of the outer sheet metal wall include:
1) Air exclusion;
2) Argon containment;
3) Fiber-frax insulation protection.

Note that at points where the secondary sodium pipes pass through the sheet stainless steel there are bellows sleeves over the pipe and the sleeves are filled with a special sealing cement that:
1) Matches the TCE of the pipe;
2) Is a good thermal insulator;
3) Remains gas tight over the temperature range 20 deg C to 530 deg C;
4) Is sufficiently soft as to not crack due to being hot on the inside while being cool on the outside.

The functions of the access spaces include:
1) Space sufficient for circulation of the cooling air flow;
2) Space for inspection and service access;
3) Clearance for inner wall and primary sodium pool deck movement due to thermal expansion.

The primary function of the external domed roof is to provide physical protection against violent storms (hurricanes and tornados) and aerial attack. It must also exclude rain water, house forced air ventilation and cooling equipment, and provide structural support for the inner wall, ceiling, gantry crane and electronic monitoring system.

The inner ceiling contains the 1 m of fiber frax insulation used to contain heat. The outer sheet metal covering the fiber-frax contains the argon and provides secondary protection against a roof rain water leak. The inner sheet metal covering contains the fiber frax insulation and prevents penetration of the fiber frax by sodium vapor.

The innermost ceiling must be high enough (14.5 m above the pool deck) to allow fuel bundle and intermediate heat exchanger repositioning and replacement using the internal polar gantry crane. The primary sodium pool enclosure must be gas tight and must dependably exclude both air and rain water under the most adverse circumstances.

Between the steel and concrete roof structure and the top of the inner ceiling is a 2 m high open space which allows easy access to the ceiling mounted reactor monitoring system and to the access space cooling equipment.

The reactor enclosure domed roof must be strong enough to safely absorb a credible aircraft, bomb or missile impact. The roof must have a steel content comparable to a major highway overpass. The sandbag layers are below the roof and are high enough above the reactor monitoring equipment to allow the sandbags and roof structure to safely absorb most of the kinetic energy contained in a diving aircraft, bomb or missle. The sandbags operate by both dissipating projectile kinetic energy and by distributing impact momentum over a wide area.

If an imminent airborne threat to the FNR is detected the moveable fuel bundles should be withdrawn forcing a reactor cool shut down. To the extent that time permits sufficient heat should be extracted from the primary liquid sodium pool to prevent spontaneous combustion of sodium with air if there is a subsequent major roof failure.

If at any time a significant amount of air leaks into the argon cover gas the immediate requirement is to withdraw the movable fuel bundles to take the reactor to a cool shutdown and to lower the primary liquid sodium temperature below 200 C to prevent spontaneous sodium combustion in air. That rapid cooling is achieved by draining the nitrate salt circuits and then allowing water into the nitrate salt circuits. The water will dissolve the salt and will boil into steam which is immediately vented. The secondary sodium temperature will drop to about 110 degrees C which will soon pull the primary sodium temperature down to about 120 degrees C.

The robust concrete and steel structure of the heat exchange galleries located around the FNR enclosure perimeter and the related shear walls make the FNR enclosure very resistant to a low angle physical airborne attack against its sides. Within a few seconds of an alarm the secondary liquid sodium in selected heat exchange galleries can drain down into below grade argon covered dump tanks to rapidly extinguish almost any secondary sodium fire.

The sodium pool enclosure roof is an octagonal structural steel dome. The roof design is constrained by the use of prefabricated structural steel members that are limited by road and rail transportation constraints to < 15.8 m in length. This objective is achieved by a domed roof.

Protecting the FNR primary sodium pool against a direct overhead air attack requires a roof structure which safely stops falling projectiles. The object of the sandbag layers is to absorb and dissipate part of the projectile kinetic energy if the projectile penetrates the roof. Any projectile explosion should be forced to occur outside the roof dome so that the explosive shock wave is distributed over a wide area.

In an emergency an argon gas cover can be maintained over the liquid sodium pool while a temporary new roof patch is applied. In such circumstances the liquid sodium pool should be cooled below 140 degrees C and then covered with a kerosene to prevent sodium combustion with air. Circumstances that might lead to such a roof failure include a direct overhead impact by an armour penetrating bomb, missle or meteorite. In these circumstances it is critical that the pumps injecting water into the nitrate salt loops have reliable backup power supplies.

There should be a sufficiently large supply of stored argon on-site to prevent sodium combustion while the liquid sodium temperature is being reduced following a sudden major roof failure.

There should be stock of thin sheet metal roof patch material stored in the roof space together with appropriate tools and supplies sufficient for temporarily blocking of any hole in the ceiling caused by a penetrating missle. In order to execute this fix the gamma radiation in the roof space from Na-24 in the primary sodium pool must be quantified. There may need to be a shielded access route and work cart also used for accessing the reactor monitoring system and roof space mounted ventilation equipment.

A 300 MWe FNR has 48 fully independent heat removal circuits connected to four independent district heating-cooling loops. This level of independence provides certainty against loss of fission product decay cooling.

The reactor enclosure inner ceiling will have an inside height above the pool deck of 14.5 m. The gantry crane I beam is supported by dollys running on two circular tracks which are in turn supported bythe roof structrure. The inner circular track is Pi(11.4 m) long. The outer circular track is Pi(22.8 m) long. Each circular track is composed of multiple pieces.

The transverse I beam rail is 24.8 m long. This transverse I beam rail may need to be fabricated by field joining of two shorter I beam lengths. Alternatively the transverse I beam rail may have to be delivered by helicopter due to its 24.8 m length.

There are further complications related to mounting the monitoring electronics package. The monitoring system has four overlapping quadrents so as to see past the polar gantry crane transverse rail. Each electronics package will consist of two parts, one which illuminates the fuel bundle indicator tubes and one which receives and processes return data from these indicator tubes. The monitoring system electronics will require continuous cooling. Loss of this cooling must immediately trigger a reactor cool shutdown.

The inner ceiling over the primary sodium pool should be supported by hangers attached to the the outer ceiling and outer structural roof via thermal breaks. The inner ceiling is made from sheet stainless steel and normally operates at about 460 degrees C. On top of the inner ceiling is a 1 m thick layer of high temperature rated fiber ceramic insulation so that the access space between the outer metal ceiling and the structural roof is cool. This space is normally kept cool by the mechanical cooling system.

Unrestricted safe access to the inside roof structure is only available after the reactor has been shut down for a week so that the gamma emission from Na-24 in the liquid sodium is low. Hence provision should be made for safe servicing of the ceiling mounted electronics package.

This safe access may involve a shielded route via the stairwells adjacent to the heat exchange galleries and a shielded cat walk and/or work cart. Each monitoring electroncs package can be mounted on a radial track which moves it from a shielded area to near the center of the ceiling over the primary sodium pool. The worst case level of Na-24 gamma radiation in the ceiling space needs to be identified.

The polar gantry crane must work reliably after being in the 460 degree C sodium vapor environment for an extended period of time. Temperature and sodium vapor sensitive components must be removed when the gantry crane is not in use.

The roof structure supports three rings of hangers with thermal breaks (ceramic egg insulators) that in turn support the interior walls, interior ceiling ceiling and the two circular gantry crane tracks. These track supports are further cross stabilized to the outside wall.

The gantry crane transverse rail is rotated by motors located at the dollies that run on the circular tracks. These motors are removed via roof access ports when the gantry crane is not in use.

THe gantry crane transverse trolley movement is also controlled by a similar motor.

The gantry crane remote manipulator attachment is only used when the primary liquid sodium pool is relatively cool (120 degrees C). Normally when the reactor is operating this remote manipulator is kept in cool storage. The remote manipulator is designed to fit through the equipment transfer air locks. The detail of the connection methodology between the gantry crane remote manipulator and the gantry crane trolley remains to be resolved. This remote manipulator must be able to precisely place fuel bundles in their sockets and bolt fixed fuel bundle corners together 6 m below the liquid sodium surface.

Outside the primary sodium pool outer cup wall is a 1.0 m wide space for air cooling, earthquake shake clearance and for maintenance access to the below pool deck space.

There is an inner steel primary sodium steel cup, a middle steel primary sodium steel cup and an outer primary sodium steel cup. As long as at least one of these steel cups maintains its physical integrity the liquid sodium will be sufficiently contained to maintain its minimum required level for fission product decay heat removal following FNR shutdown. Between the steel cups is fire brick and a small space to allow for differential thermal expansion of the steel cups. The pool deck is integral with the inner steel cup. The bottom of the pool deck which extends under the walls of the primary sodium pool has a 1 m thick layer of fiberfrax insulation.

Outside the outer steel cup are radial supports that reinforce the steel cups during severe earthquakes. These reinforcements also assist in outer cup assembly.

During normal reactor operation there is no requirement for personnel to enter the argon filled pool enclosure. Such entry should only occur after the reactor has been in cold shut down for at least a week to allow Na-24 gamma emission and the pool temperature to subside. Even so such entry requires personal protectrive equipment against the ~ 120 degree C temperature and requires closed circuit breathing equipment.

The fuel bundles are centrally positioned in a 20 m diameter X 15 m deep liquid sodium pool. A 1.7 m wide band of liquid sodium at the perimeter of the liquid sodium pool is dedicated to intermediate heat exchangers and secondary sodium pipes. The remaining primary sodium guard band serves as a fuel bundle service access corridor.

The central core region of the reactor together with the top and bottom blanket regions involve _____vertical active fuel bundles. Each fuel bundle is 6.5 m high and has an additional 1.5 m bottom leg projection. Fixed fuel bundles are supported by 1.5 m high bottom legs (corner girder bottom extensions on the fixed fuel bundles) that plug into sockets fixed to the open steel lattice located near the bottom of the primary sodium pool. Each movable fuel bundle has a 1.5 m long bottom probe and a removeable 7.5 m _____high top indicator tube.

The movable fuel bundles slide into the fixed fuel bundle matrix from the bottom. The insertion distance is set by a liquid sodium hydraulic actuator with 1.1 m of travel. For each movable fuel bundle an indicator tube projects above the primary liquid sodium surface to indicate the movable fuel bundle's actual vertical position. This vertical position is constantly monitored using an overhead monitoring system similar to a laser measuring tape.

The reactor core region is surrounded on its outer perimeter by a 1.33 m thick blanket formed from 4 rings of passive fuel bundles.

There are two further outer rings of used fuel bundles in which natural decay of fission products occurs over a six year period before the fuel bundles are removed from the primary liquid sodium pool. Thus the total effective blanket width is 2.0 m.

One of the most important aspects of fission reactor design is provision for fission product decay heat removal under adverse circumstances. If some event occurs which causes a reactor shutdown the fission products will for a short time continue to produce decay heat at up to 8% of the reactor's full power rating. It is essential that there be a 100% reliable means of ensuring ongoing removal of the fission product decay heat under adverse conditions such as shortly after a severe earthquake when station power may be lost.

To provide certainty regarding fission product decay heat removal a FNR has 48 independent heat transport loops. In the event of loss of station power fission product decay heat removal should be achieved by natural fluid circulation with just half of the heat transport circuits in service. This objective is achieved by lowering the salt circuit temperature to 100 degrees C by injecting water which causes strong natural circulation of the secondary sodium.

During normal reactor operation for safety certainty at any instant in time at least 2 of the 8 heat exchange galleries connected to two different cooling towers should be operational.

For a liquid sodium cooled FNR all heat removal is via primary liquid sodium, so it is essential that:
1) Under no circumstances will the primary liquid sodium level ever fall to the point that adequate heat transfer via the intermediate heat exchangers is no longer possible.
2) The liquid sodium pool walls are designed such that if the inner and middle nested cup walls fail and the primary liquid sodium leaks into the space between the walls, the leakage into the space between the walls will not lower the primary liquid sodium level more than 4 m, so 2 m of intermediate heat exchanger tube length is still immersed in the primary sodium.
3) Even if the intermediate loop sodium induction pumps fail when the salt circuit is at 100 degrees C there is enough secondary liquid sodium natural circulation to ensure safe removal of the fission product decay heat.
4) The secondary liquid sodium dumps its heat into the salt circuits. The salt circuits remain at atmospheric pressure so that their injection water pumps do not face a pressure load.
5) There must be enough clean water in on-site storage such that in an emergency the fission product decay heat can be removed by evaporating that water. The cooling towers normally operate dry with pumped steam condenser cooling water circulating through the heat exchange coils located near the bottom of the cooling tower.

The fuel bundles are repositioned and/or replaced from time to time using the overhead polar gantry crane and remote manipulation. Note that the ceiling height is sufficient to allow extraction and replacement of individual fuel bundles and individual intermediate heat exchangers. During the extraction process used fuel bundles are lifted 6.5 m to clear the other bundles and then are then moved horizontally to the reactor perimeter zone of the primary sodium pool where the irradiated fuel bundles are stored until they lose most of their fission product decay heat before being removed from the primary sodium pool. During the removal process the fuel bundles must be permitted to drip dry over the primary sodium pool before being moved int an airlock.

The FNR fuel bundle can be thought of as being centrally located in a 15 m deep liquid sodium pool with a liquid sodium top surface which is 1 m below the pool deck.

In an earthquake with a 1.5 g horizontal component sustained liquid sodium surface waves might slosh right up to the ceiling. The inside walls must withstand the related forces.

In an earthquake the fuel bundles tend to stay in position due to their own inertia while the primary sodium pool moves with the surrouing ground.

The concrete walls surrounding the primary sodium pool are stabilized on the outside by the about 17 m of surrounding ground depth and by the reinforced concrete shear walls of the heat exchange galleries.

The main chemical threat from a power FNR is the 4700 m^3 of liquid sodium contained in the primary sodium pool. If this liquid sodium contacts water there will be a rapid chemical reaction which liberates hydrogen that will spontaneously ignite in an air atmosphere. Hence one of the main issues in FNR application is choice of a reactor site where the sodium will NEVER be exposed to flood water. The reactor site is chosen such that the reactor foundation will always gravity drain.

Even if the foundation drain becomes blocked and the reactor enclosure floods 18 m deep the outer stainless steel wall around the primary sodium pool should prevent any contact between water and the primary sodium.

The other potential threat is a sodium fire. Quite apart from the release of Na2O and NaOH the big threat is melting of the fuel tubes leading to potential release of air borne plutonium and fission products. It is essential that the reactor be designed and sited such that a large sustained sodium fire cannot occur. In order to extinguish a primary sodium fire the oxygen concentration over the sodium must be minimized and heat must be extracted from the primary sodium. That heat extraction requires reliable operation of at least a fraction of the secondary sodium heat transport systems. Under no circumstances can water be allowed to contact either the primary or secondary sodium.

The soil and bedrock outside the concrete enclosure should be sufficiently dry, dense and stable to safely contain the liquid sodium in the extremely unlikely event that a major earthquake ruptures the inner, middle and outer stainless steel walls of the liquid sodium pool and cracks the enclosing concrete wall.

It is equally important that there be an effective non-water based fire suppression system. The local fire department must be trained that water should NEVER be used to fight a FNR fire. Inappropriate use of water carried by a fire truck could change a minor fire into a major disaster. The FNR has excess argon in silo storage for emergency use.

To mitigate the fire threat the primary liquid sodium is covered by argon cover gas, a sodium vapor resistant inner metal ceiling, a gas tight outer metal ceiling and a gas tight inner seal on the concrete wall. In the event of air penetration into the argon cover gas the reactor should be immediately shut down and heat removed from the primary liquid sodium pool to lower the primary liquid sodium temperature below 200 degrees C so as to prevent spontaneous combustion of sodium in air. As the argon temperature over the primary sodium pool decreases stored argon from bladders in the argon storage silos is automatically added to the argon cover gas to maintain the 1 atmosphere pressure in the argon cover gas.

Once the liquid sodium temperature is below 140 degrees C the surface of the liquid sodium can be flooded with a thin layer of kerosene to prevent the liquid sodium oxidizing during prolonged work such as roof repair.

Similarly if there is an enclosure roof failure the immediate objective is to extract heat from the sodium to reduce its temperature to the point where kerosene can be safely used to prevent sodium oxidation. Until the heat is removed from the sodium argon must be used to exclude oxygen from the sodium surface. That heat extraction might easily take half an hour, depending on the available heat sinking capacity. The fastest way to emergency cool the system is to directly vent steam from the nitrate salt circuit. It is important to have enough water in on-site tank storage to remove the fission product decay heat by latent heat of vaporization. Then the limiting factor is the maximum safe heat transfer capacity of the intermediate heat exchanger tube bundles and the sodium/salt heat exchanger tube bundles. If there is a FNR roof failure it is essential to prevent the exhaust steam from the salt circuit condensing overhead and falling onto the exposed liquid sodium surface. These steam vents should go into the on-site cooling towers. This issue highlights the importance of FNR enclosure roof and ceiling integrity and immediate availability of material for temporary exclusion of rain or other water falling from overhead.

This web page last updated April 19, 2022

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