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

It is recommended that students, before attempting to read the material on this web page, first study the web page titled: FNR CONCEPT so that they have an elementary understanding of what a Fast Neutron Reactor (FNR) is and how it works.

The term Fast Neutron Reactor (FNR) is generically quite broad. For the purpose of this web site the definition of an FNR is narrowed to be a fast neutron reactor with metallic fuel rods, sealed metal fuel tubes, liquid sodium primary and secondary coolants, a fertile fuel blanket and a liquid sodium guard band.

A Fast Neutron Reactor (FNR) nuclear power plant has eight major components.
a) There is an enclosed low pressure pool type reactor. The pool contains nuclear fuel bundles which automatically keep the primary liquid sodium pool surface temperature at about 500 degrees C. These fuel bundles also breed fertile fuel into fissile fuel;
b) There are 56 intermediate heat exchangers immersed in the primary sodium pool. These heat exchangers transfer heat from the radioactive low pressure primary sodium to 56 independent non-radioactive higher pressure secondary sodium loops;
c) There are 8 heat exchange galleries which use heat from secondary liquid sodium loops to safely produce non-radioactive high pressure steam in 16 steam pressure zones to feed 16 turbogenerators;
d) There are 16 steam condensers and 4 cooling towers;
e) There is a control system which varies the electric power output from each turbogenerator by controlling the corresponding secondary sodium flow rates;
f) There are redundant nuclear safety systems which monitor the reactor and subsystems' performance and which safely shut down the reactor and/or subsystems if any condition is abnormal.
g) There is a fire safety system that can suppress secondary sodium fires by dumping secondary sodium from selected heat transfer loops into argon covered dump tankes.
h) There is sufficient storage volume for primary and secondary sodium to enable long term equipment maintenance.
i) There is enough cooling water stored on-site to remove fission product decay heat by evaporation in the total absence of both station power and city water service.
j) Thhere is provision for automatic suppression of secondary sodium fires.

A practical fast neutron reactor is an assembly of metallic nuclear fuel rods inside sealed metal fuel tubes. Inside the metal fuel tubes, along with the metallic fuel rods, is sufficient liquid sodium to provide good thermal contact between the core fuel rods and the inside wall of the metal fuel tubes. In the fuel tubes, above the metallic fuel, is empty plenum space for collection and storage of inert gas fission products.

The nuclear properties and physical geometry of the fuel assembly cause the core fuel rods to maintain an average temperature of about 520 degrees C. Then when cooler primary liquid sodium circulates past the outside of the fuel tubes the primary liquid sodium coolant acquires heat which can be used to generate electricity. The rate at which heat is extracted from the FNR is proportional to the temperature difference between the core fuel and the circulating primary liquid sodium.

The metal fuel tubes maintain a stable fuel geometry, reliably contain the fuel and fission products and conduct heat from the fuel to the circulating primary liquid sodium. The maximum thermal power output and fuel cycle time of a FNR is limited by the properties of its fuel tubes.

The Fast Neutron Reactor (FNR) referred to herein uses a cylindrical pool of primary liquid sodium 20.0 m in diameter X 15.0 m deep. Immersed in middle of this primary sodium pool is an assembly of about 401,280 vertical 1.27 cm OD X 6 m tall steel fuel tubes that maintain the liquid sodium pool surface temperature at about 500 degrees C and that breed new fissile fuel atoms from fertile fuel feedstock. The central 243,712 fuel tubes contain the reactor core fuel. Each such core fuel tube can continuously emit about 4 kWt of heat.

Around the primary sodium pool perimeter are 56 immersed intermediate heat exchangers which input hot primary liquid sodium from near the top of the primary sodium pool and discharge cooler primary sodium 5.3 m deeper in the primary sodium pool.

When there is no heat being extracted the liquid sodium pool surface temperature will rise to about 520 degrees C. Under design full load conditions the liquid sodium pool surface temperature falls to about 490 degrees C.

The FNR is assembled from truck portable modules. The fuel bundles, heat exchange bundles, pumps, steam generators, turbogenerators, condensers, etc. used to make a FNR power plant are all modular and individually road truck portable.

The primary sodium tank is assembled on site from pre-formed (3 / 4) inch thick stainless steel plate using deep pentration inert gas welding

FNR design concepts are significantly different from water cooled reactor design concepts. FNR design concepts are reviewed below:

1) There is a single triple wall primary sodium pool operating at atmospheric pressure. The primary sodium pool enclosure is designed to safely withstand a severe earthquake that produces horizontal accelerations of up to 1.25 g.

2) For safety there are no penetrations through the primary sodium pool bottom or vertical side walls below 1 m above the normal primary liquid sodium surface level;

3) The primary sodium naturally density stratifies so that the hottest sodium floats on the top surface of the primary sodium pool;

4) Natural circulation causes the primary sodium to flow through both the fuel bundle cooling channels and along the outside of the intermediate heat exchanger tubes;

5) The walls and floor of the primary sodium pool and the intermediate heat exchangers are protected from long term neutron damage by a 3 m thick primary sodium guard band. This sodium guard band, in addition to absorbing neutrons, provides thermal mass that limits the overall rate of change of the primary sodium pool temperature;

6) The reactor fuel is an assembly of alternating fixed and mobile fuel bundles. The mobile fuel bundles are inserted into or withdrawn from the matrix of fixed fuel bundles by liquid sodium hydraulic piston actuators to adjust the FNR core zone width and hence the core zone operating temperature;

7) The controlled mobile fuel bundle insertion into the fixed fuel bundle matrix provides incremental control of the fuel bundles' core fuel temperature set point. The insertion positions of the mobile fuel bundles should be periodically adjusted over the fuel bundle operating life to keep the mobile fuel bundle sodium discharge temperature at full load uniform across the FNR at about 490 degres C;

8) As long as its fuel concentration and fuel geometry are stable and the thermal load exceeds fission product decay heat emission a FNR passively maintains its fuel temperature setpoint.

9) For safety the FNR heat transport system is designed so that in a cold shutdown state natural circulation of both the primary and secondary sodium is sufficient to safely remove fission product decay heat, even if the top 4 m of liquid sodium is lost due to up to two pool steel wall failures;

10) For practical primary sodium pool structural maintenance there must be available storage containers into which the primary sodium can be transferred;

11) The primary sodium mass, the intermediate heat exchangers, the induction pumps and the steam generators must all be rated to safely accommodate worst case power transients;

12) A FNR power reduction to a warm shutdown is achieved by reducing the secondary sodium flow rate which slows heat extraction from the intermediate heat exchangers and hence from the primary sodium pool. With no thermal load the primary sodium temperature will gradually rise to about 520 degrees C at which point thermal expansion of the fuel and primary sodium causes the nuclear chain reaction to stop.

13) Cool shutdown of a FNR is achieved by withdrawal of mobile fuel bundles from the matrix of fixed fuel bundles.

14) Cold shutdown of a FNR is achieved by performing a cool shutdown and then venting the steam generators directly to the atmosphere. That venting lowers the steam generator water temperature from 320 degrees C to 100 degrees C which in turn will gradually force the primary sodium pool temperature down to about 110 degrees C. That venting of steam also reduces the pressure head seen by the pumps which inject condensate water into the steam generators.

15) Heat transport system maintenance and secondary sodium fire suppression are enabled by freeze plugs that allow selected secondary sodium circuits to drain into dump tankes.

16) The secondary sodium circuit normally operates at a pressure part way between the steam pressure and atmospheric pressure. In the event of a steam generator tube failure a small amout of water leaks into the secondary sodium circuit. This water chemically reacts with the seconday sodium toproduce hydrogen. The pressure in the secondary sodium circiut rises preventing further water ingress and blowing a rupture disk that allows the secondary sodium to drain into its dump tank. Simultaneously the relevant steam generator water side is drained to prevent water back flow through the tube rupture into the secondary sodium circuit;

17) A fall in secondary sodium circuit pressure, such as might result from a secondary sodium circuit leak to the atmosphere or a leak into the primary sodium, stops the fan which maintains the freeze plug. A short time later the freeze plug melts and the remaining secondary sodium drains into its dump tank.

The FNR core zone normally operates at fission criticality. The requirement for fission criticality in combination with practical core fuel concentration and geometry sets the nominal core zone thickness with new fuel at about 0.35 m. As the fuel ages the core zone thickness should be gradually increased to about 0.70 m. When the fuel is new the active fuel tube area is smaller than when the fuel is old, which potentially imposes a heat flux constraint.

The reactor core zone maximum outside diameter is a function of the liquid sodium pool inside diameter. Thesee diameters are constrained by practical structural issues related to the reactor enclosure roof. With a 10 m diameter reactor core zone the corresponding sodium pool inside diameter is 20.0 m. It is necessary to have a roof covered 5 m wide perimeter strip around the liquid sodium pool for the 2 m of insulating fire brick, 1 m of inner enclosure wall thickness, 1 m maintenance access and air cooling, earthquake tolerance and 1 m for gamma ray absorbing outer enclosure concrete. Hence the nominal reactor enclosure outside dimensions are 30 m X 30 m.

The reactor core zone thickness, the core zone diameter and the fuel tube geometry establish the active heat transfer area of the fuel tubes.

There is also a heat transfer limit relating to the rate at which liquid sodium will naturally circulate through the cooling channels between the fuel tubes with a 150 degree C temperature rise.

For practical reasons some FNR sizes are more economic than others. If FNRs are made too small the breeding efficiency will be poor due to insufficient neutron capture and economies of scale are lost. If FNRs are made too large economies related to energy transmission, reserve power backup and enclosure size are lost. Hence a core zone diameter of about 10 m is close to optimum.

For energy supply dependability it is usually better for an electricity and/or district heat utility to have a network of smaller power reactors that can mutually support each other during refueling and maintenance shutdowns rather than to rely on just one large power reactor.


For simplicity, in the above diagram the air locks and the open steel lattice supporting the fuel bundles and intermediate heat exchangers are not shown.

The FNR described herein has a primary sodium pool consisting of a cylindrical steel tank with outside dimensions 18 m high X 24 m diameter. The tank walls are three layers of steel separated from each other by two 1 m thick layers of fire brick, so the tank inside dimensions are 16 m high X 20 m in diameter. Inside this tank is a liquid sodium pool 15 m deep. The liquid sodium top surface is 1 m below the pool deck and about 1 m above grade.

Centrally positioned in the primary liquid sodium pool is a ~ 14 m diameter array of about 401,280 vertical closed end chrome-steel fuel tubes, each 6 m high X 0.0127 m outside diameter.

Surrounding the fuel tube array is a 3 m wide guard band of liquid sodium.

The initial weight fractions of the core fuel rods are:
Pu = 20%
U = 70%
Zr = 10%

The initial weight fractions of the blanket rods are:
U = 90% Zr = 10%

Details with respect to metallic FNR fuel are available at: Metallic Fuels: The EBR-II legacy and recent advances

Each fuel tube contains a 4.3 m high stack of metallic core and blanket fuel rods and sufficient liquid sodium to provide good thermal contact between the metallic fuel rods and the steel fuel tube inner wall. In each active fuel tube the fuel rod stack consists of 1.8 m of blanket rod on the bottom, 0.7 m of core rod in the middle and 1.8 m of blanket rod on top. Above the fuel rod stack is a 1.6 m high empty space known as the fuel tube plenum, in which fission product inert gases accumulate and are stored.

Thus the ~ 0.35 m thick X 10 m diameter reactor core region (shown in red on the profile diagram) is surrounded by a 1.45 m to 1.8 m thick blanket region (shown in orange on the profile diagram). The blanket regions are realized with blanket fuel rods.

The 3 m wide sodium guard band absorbs atomic particles and gamma radiation directly emitted by the fuel and the fuel tubes.

Immersed in the liquid sodium around the perimeter of the primary liquid sodium pool are 56 intermediate heat exchangers that transfer heat from primary liquid sodium to secondary liquid sodium.

Connected to the mobile fuel bundles in the middle of the primary sodium pool is a grid of vertical steel indicator tubes which project 0.3 m to 1.5 m above the primary liquid sodium surface. These indicator tubes convey to overhead monitoring instrumentation data relating to each mobile fuel bundle's: insertion depth in the matrix of fixed fuel bundles, primary sodium discharge temperature and gamma ray production. The indicator tubes are buoyand and are attached to the crane lifting points of individual mobile fuel bundles.

The exposed top surface of the primary liquid sodium pool is covered by steel floats. The purpose of these floats is to minimze the exposed primary liquid sodium surface area. In the event that air leaks into the reactor cover gas the reduced exposed primary liquid sodium surface area minimizes the rate of spontaneous sodium oxidation and the related fire risk.

Connected to the intermediate heat exchangers are 112 secondary sodium pipes that transport heat from the primary sodium to the steam generators.

The FNR is designed to safely withstand earthquake induced horizontal acceleration of up to 1.25 g. Such a sustained earthquake acceleration might potentially cause the primary liquid sodium to slosh almost to the ceiling level of the primary sodium pool enclosure.

The metal fuel tubes are in an array of alternating octagonal and square fuel tube bundles. Within each fuel bundle the fuel tube square array center to centre distance of:
5 / 8 inch = 0.015875 m
is chosen to provide dependable natural circulation of liquid sodium, efficient heat transfer from the fuel tubes to the primary liquid sodium and a critical fissile fuel concentration in the core zone that can be realized using single step electrolytic fuel reprocessing.

The fixed octagonal fuel tube bundles each contain 416 fuel tubes and the square fuel tube bundles each contain 280 fuel tubes. Each of 340 mobile square fuel bundles can slide vertically 1.2 m with respect to the adjacent fixed fuel tube bundles.

The reactor core zone thickness is set by inserting mobile fuel bundles into the matrix of fixed fuel bundles. The more the overlap of the mobile fuel bundle core fuel rods with the fixed fuel bundle core rods the larger the reactor core zone thickness.

Maximum withdrawal of the mobile fuel bundles from the matrix of fixed fuel bundles causes a reactor cool shutdown.

A dedicated liquid sodium hydraulic piston actuator is used to insert each mobile fuel bundle into the matrix of fixed fuel bundles from the bottom. The mobile fuel bundle's indicator tube projects above the primary liquid sodium surface to indicate the vertical position of the mobile fuel bundle. This vertical position of each mobile fuel bundle is constantly monitored using an overhead laser scanner. If a mobile fuel bundle actuator loses power that fuel bundle withdraws due to gravity causing a local cool shutdown.

The relative overlap of the mobile and fixed fuel bundles is adjusted so that the core fuel rods maintain an average temperature of about 520 degrees C. The temperature of the sodium discharge from each mobile fuel bundle is measured using an overhead infrared scanner.

Each mobile fuel bundle has associated with it an actuator that vertically positions the mobile fuel bundle based on the indicator tube vertical position with respect to its position setpoint. The actuator piston moves in response to liquid sodium hydraulic pressure. Thus a single actuator jam or mobile fuel bundle jam does not physically affect the other mobile fuel bundles except via a changed neutron flux in the horizontal plane. If a hydraulic circuit problem is detected the liquid sodium hydraulic pressure falls to zero and gravity causes the affected mobile fuel bundle to withdraw 1.2 m from the fixed fuel bundle matrix.

The mobile fuel bundle positioning actuators rely on a common liquid sodium hydraulic pressure pump that will run occasionally due to small leaks past the hydraulic actuator pistons. This pump is submerged to maintain suction head but its motor is above the primary sodium pool and is cooled with flowing argon. If this pump fails the mobile fuel bundle actuators will lose pressure causing the mobile fuel bundles to withdraw 1.2 m due to gravity and thus cause a reactor cool shut down.

The reactor contains two outer perimeter fuel bundle storage rings. Used fuel bundles are stored in these two outer perimeter rings for six years to allow natural decay of fission products before the used fuel bundles are removed from the primary liquid sodium. The reactor also contains four inner perimeter rings of passive fuel bundles that are loaded with just blanket rod fuel. Thus the perimeter blanket around the core zone is effectively six fuel bundle rings (2 m) thick.

At full load the hot liquid sodium primary coolant flows by natural circulation from the top of the liquid sodium pool at 490 degrees C down along the intermediate heat exchanger tubes and then back to the bottom of the liquid sodium pool at about 340 degrees C.

The change in both primary and secondary liquid sodium temperature while flowing through the intermediate heat exchangers is about 150 degrees C. The axial temperature gradient is about:
150 degrees C / 6 m = 25 degrees C / m which is modest. The transported heat is normally used for electricity generationand/or district heating.

The maximum safe thermal output of this FNR is limited by the properties of its fuel tubes.


In the above diagram the outer dark ring is the fire brick filled primary sodium pool wall. This wall has an inside diameter of 20 m and an outside diameter of 24 m. Inside this wall is an 18 m diameter ring which indicates the location of the intermediate heat exchangers. There are four gaps in this ring indicating the positions of the air locks. Inside the ring of intermediate heat exchangers are the active and passive fuel bundles. Only one quadrant of fuel bundles is shown in detail. The mobile square fuel bundles forming the reactor core are shown in red. The fixed square fuel bundles forming the reactor blanket are shown in purple. The fixed octagonal fuel bundles in both the core and the blanket are shown in orange.

The fuel tubes are packaged in bundles. Each active fuel tube contains both blanket fuel rods and core fuel rods. There is a two dimensional matrix of fixed fuel bundles in which the reactivity at any point in the matrix can be adjusted by adjusting the insertion depth of mobile fuel bundles into the matrix of fixed fuel bundles.

The central core region together with the top and bottom blanket regions involve 697 vertical fixed and mobile fuel bundles. There are 357 fixed active fuel bundles that form a 2 dimensional matrix into which 340 mobile active fuel bundles are inserted from the bottom. Each fixed fuel bundle is 0.3651 m long X 0.3651 m wide X 6.0 m high and is supported by 1.5 m high legs (corner girder extensions underneath the fixed fuel bundles). Each mobile fuel bundle is 0.3016 m long X 0.3016 m wide X 6 m tall and has attached to its top a 7.5 m tall buoyant indicator tube. The mobile fuel bundles are supported by hydraulic actuators contained in the 1.5 m high steel lattice that in turn is supported by the primary sodium pool inner liner floor.

The FNR fuel tube assembly is completely surrounded by a ~ 3.0 m thick guard band of high purity liquid sodium. This sodium guard band absorbs all neutrons that escape from the assembly of fuel tubes. These absorbed neutrons convert natural sodium (Na-23) into Na-24. The Na-24 naturally decays by electron emission with a half life of 15 hours to become stable Mg-24. In the decay process a 1.389 MeV gamma photon is emitted. After a reactor shutdown it takes about a week for this radiation to drop by a factor of about 2000. If any of the fuel tubes are leaking or if the primary sodium purity or filtering is inadequate there could be other radio isotope emissions. For certainty with respect to gamma radiation safety, reactor physical security, certain sodium containment and certain water exclusion a FNR is surrounded by a ~ 1 m thick concrete wall resting on an ~ 2 m thick concrete foundation.

Due to the sodium guard band no neutrons reach the primary sodium pool containment walls or floor, the intermediate heat exchangers, the secondary sodium, or the overhead structures. This prevention of neutron activation and neutron damage outside the fuel bundle assembly enables a very long facility working life and prevents formation of radioactive decommissioning waste.

There are the 56 immersed intermediate heat exchangers.

There is an overhead gantry crane in the reactor space that is used to install, reposition and remove fuel tube bundles and to position primary sodium pool and intermediate heat exchangers and related piping.

There are 1.3 m wide X 2.8 m high X 9.7 m long air-vacuum-argon locks that that used to bring fuel bundles and intermediate heat exchanger equipment into the reactor space or remove fuel bundles and intermediate heat exchanger equipment from the reactor space without loss of argon and without mixing of air and argon.

There is also an airlock for personnel entry and exit. This air lock uses chemicals to absorb oxygen which would otherwise mix with the argon cover gas.

The argon cover gas above the primary sodium is kept at atmospheric pressure. Its normal operating temperature is about 500 degrees C. When the reactor is cold shut down for service this temperature decreases to about 120 degrees C. Each FNR has four redundant argon bladder tanks in external silos to accommodate thermal expansion-contraction of the argon cover gas.

To the extent possible all work in the reactor enclosure is done using robotic equipment. Any entry by service persons requires both a prolonged (one week long) reactor shutdown to allow Na-24 decay and a thermally insulated and cooled suit fitted with a closed circuit breathing apparatus. Such entry might be necessary to repair a gantry crane defect or to replace an intermediate heat exchanger.

In the event of a fuel tube leak or fuel melting the contained nuclear fuel is denser than liquid sodium, so any leaked material will tend to sink to the bottom of the primary sodium pool where, provided that it is prevented from forming a critical mass, it poses no risk and can be recovered by the sodium pool filter system. Any potentially corrosive fission products are chemically trapped by the liquid sodium.

Outside the pool side walls and bottom is a 1 m wide ventilated air space stabilized with structural steel which in turn is enclosed by a ~ 1 m thick square concrete wall. The concrete extends 19 m below grade and 17 m above grade. The surrounding earth fill is of sufficient density and dryness to safely contain the liquid sodium even in the event of combined sodium pool and concrete wall structural failures. The earth fill also protects the FNR from the ground impact and fire that might result from the crash of a large airplane.

Outside the reactor enclosure is a ring of up to 56 steam generators and induction pumps mounted in 8 isolated heat exchange galleries on an upper level. Below that level are 56 drain down tanks. The heat exchange galleries have additional 1 m thick concrete side walls and 1 m thick shear walls.

The heat exchange galleries and related shielding walls and shear walls provide the FNR above grade protection against a low level airplane strike. The spare space underneath the heat exchange galleries is available for drum storage of primary sodium in the event that work is required on the inner pool liner.

For safety reasons the secondary liquid sodium pressure is normally kept midway between atmospheric pressure and the steam generator steam pressure. This heat transfer arrangement is safe in the presence of either an intermediate heat exchanger tube failure or a steam generator heat exchanger tube failure. If the secondary sodium pressure goes too high a rupture disk opens causing the secondary sodium to flow into its drain down tank.

The concrete walls outside the heat exchange galleries are stabilized on the outside by the surrounding 17 m depth of dry ground. Even if there is a violent earthquake causing a total sodium pool and concrete wall rupture this dry ground will safely contain the liquid sodium. Its elevation with respect to the local water table should prevent rain or flood water reaching the sodium, even after a total wall collapse.

There are 12.75 inch OD secondary sodium pressure pipes from the intermediate heat exchanger discharge manifold top caps connecting to the corresponding steam generator inlet manifold top caps. There are 12.75 inch OD secondary sodium pipes from the steam generator discharge bottom cap to the intermediate heat exchanger bottom caps via an induction type circulation pump. The aforementioned pipes have accessible disconnection flanges close to the intermediate heat exchangers.

These flanged connections are both bolted and perimeter seal welded. The flanges are fabricated so that the seal welds can be removed with an edge grinder.

The steam generators are located at an elevation higher than the reactor so that the secondary sodium will natuarally circulate even if the secondary sodium induction pumps lose power.

Normal secondary sodium circulation is by induction pumping. The steam discharged from the steam generators is ducted to turbogenerators and steam condensers located across a laneway from the heat exchange gallery space.

The heat exchange gallery space is fitted with rupture panels which in the event of uncontrolled sodium-water contact will vent the resulting hydrogen to the atmosphere. In the event of a sodium fire the pressure in the steam generator is released, the freeze plugs are openned and the secondary sodium flows by gravity into argon covered drain down tanks to extinguish the fire. Secondary fire protection is provided by the extinguishing agent NaHCO3.

Natural circulation of the secondary sodium for fission product decay heat rejection when the intermediate sodium induction pumps are not energized is achieved by venting the steam generators to the atmosphere which lowers their contained water temperature to 100 degrees C. Then the large temperature differece between the primary sodium and the steam generator drives natural secondary sodium circulation.

The secondary sodium loop components must be sized so that if the steam generator water temperature is 100 degrees C the natural circulation rate of the secondary sodium will be sufficient to safely remove fission product decay heat after the nuclear chain reaction is shut down, even if the top 4 m of primary liquid sodium is missing from the primary sodium tank.

Each secondary sodium drain down tank has sufficient volume to accommodate all the secondary sodium in its secondary sodium heat transport circuit. Sodium is transferred from this drain down tank to the secondary sodium heat transport circuit by applying argon pressure over this drain down tank while venting argon from the corresponding secondary sodium circuit high point to the argon atmosphere over the primary sodium pool. The drain down tanks require electric immersion heaters for liquid sodium melting/temperature maintenance. Note that the drain down tanks must be rated as pressure vessels. Each secondary sodium circuit must be fitted with rupture disk high pressure relief vented to the corresponding draindown tank.

In the heat exchange galleries all equipment and pipes containing potentially high pressure sodium must be further surrounded by an argon filled thermally insulating containment jacket for minimizing loss of heat, for fire prevention/mitigation and for personnel safety.

The steam pressure in each steam generator is normally controlled by a motorized steam discharge valve which maintains a constant pressure (11.2 MPa) in the steam generator. That pressure, via the pressure-temperature relationship for saturated steam, determines the water temperature in the steam generator (320 C). The difference between the liquid sodium primary discharge temperature and the water temperature in the steam generator, less two heat exchange wall temperature drops, determines the change in temperature across the secondary sodium loop. Thus controlling the secondary liquid sodium flow rate controls the mass flow of constant pressure steam delivered to the corresponding turbo-generator.

The steam discharged from the steam generators is ducted to turbogenerators and steam condensers located across a laneway from the heat exchange galleries. There are up to 16 steam turbogenerators and condensers. The condensers sink heat to a district heating system and to four independent natural draft dry cooling towers. The cooling towers are each sized for safe rejection of the reactor's maximum fission product decay heat.

The 16 steam to electricity conversion systems are completely isolated from one another. Each intermediate heat exchanger feeds a dedicated steam generator and has a dedicated drain down/expansion tank. Hence in the event of an equipment problem only a fraction of the reactor output capacity need be shut down while the balance of the reactor capacity remains fully operational.

A FNR is controlled by several micro-computers. The main function of these micro-computers is to safely maintain the desired fuel temperature and to trigger reactor shutdowns if any significant out-of-normal condition is detected. The monitoring software must include hardware watchdogs that can detect any significant problems within the reactor monitoring system or the mobile fuel bundle insertion control systems. If any such problems are detected there should be an immediate cool shutdown of some or all of the reactor fuel bundles.

An FNR designed for utility electric power production typically has 697 active fuel bundles. Sooner or later through accident, negligence or malevolent behavior there will be a defective active fuel bundle and/or a defective active fuel bundle control portion positioning apparatus. In these circumstances the major concern is potential fuel melting. In response to local overheating the adjacent mobile fuel bundles must be immediately withdrawn to ensure chain reaction shutdown in the defective fuel bundle.

If fuel melting occurs fuel alloy droplets might collect on the primary liquid sodium pond floor covering under the fuel tube assembly. It is essential that these droplets do not accumulate together to form a critical mass. The floor cover must be made of neutron absorbing material that is geometrically shaped to prevent fuel droplets gathering to form a critical mass. There should also be a practical means of selectively removing and replacing sections of this floor cover.

The FNR fuel tube assembly consists of 697 active fuel tube bundles surrounded by up to 672 passive and cooling fuel tube bundles. Each octagonal fuel tube bundle has 1.5 m long lower legs and 0.30 m top extensions which together with a 0.1 m grating allowance and a 0.1 m core zone swelling allowance give it a total height of 8.0 m.

Each fuel bundle is composed of vertical 0.5 inch OD steel tubes X .035 inch wall, 6.0 m high that form a square lattice spaced (5/8) inch center to center. The steel fuel tubes are sealed closed at both ends and contain the reactor core and blanket rods as well as internal liquid sodium to enhance thermal contact between the fuel rods and the steel tubes and to chemically absorb corrosive fission product gases such as F, Cl, Br and I. Each fuel tube has a plenum space for storage of inert fisson product gases.

Each square mobile fuel tube bundle contains 280 X 0.5 inch OD HT-9 steel tubes. Each octagonal fixed fuel tube bundle contains 416 X 0.5 inch OD HT-9 steel tubes.

The fuel tube to fuel tube spacing is maintained by spiral windings around the fuel tubes and by diagonal plates within the fuel bundles. The tube lattice bottom spacing is fixed by the fuel bundle bottom gratings which support and position the fuel tubes and which permit vertical liquid sodium coolant flow. The fuel tube top plugs have radial bumps which assist in inter-fuel tube spacing control. Some fuel tubes are missing from the regular lattice to allow space for shrouds and fuel tube bundle diagonal steel plates which further stabilize the fuel tube positions.

Each active fuel tube contains 6 X 0.30 m long blanket fuel rods, 2 X .35 m long core fuel rods and then another 6 X 0.30 m long blanket fuel rods. Each passive fuel tube contains 14 X ~ .30 m long blanket fuel rods.

During prolonged reactor operation the core fuel rods swell from 0.35 m long to about 0.40 m long. Each fuel tube contains a measured amount of liquid sodium. The top 1.6 m of each active fuel tube are nominally empty to provide sufficient plenum volume to relieve pressure stress resulting from formation of inert gas fission products and to store sufficient spare sodium to compensate for fuel tube material swelling. The plenum portion of the fuel tube length also serves as a chimney to enhance the natural circulation of the primary liquid sodium.

Each octagonal fixed fuel tube bundle is supported and stabilized by its 1.5 m high legs that plug into sockets on the top of the 1.5 m high steel lattice which rests on the bottom of the primary sodium pool. The fixed fuel bundles at their tops are bolted to their four nearest neighbours to provide the fixed fuel bundle matrix lateral stability. Additional lateral stability can potentially be realized using a formed steel belt around the outside of the assembly of fuel bundles.

The vertical insertion/withdrawal of each mobile square fuel bundle is controlled by liquid sodium pressure applied to a piston type hydraulic actuator located within the steel lattice. The mobile bundle's vertical position is indicated by a vertical indicator tube attached to the top of the mobile bundle. This indicator tube projects above the surface of the liquid sodium and is horizontally stabilized by its own buoyancy.

The core fuel rods are initially by weight: 10% zirconium; 20% plutonium, U-235 and fissionable transuranium actinides; and 70% U-238.

The blanket rods are initially by weight: 10% zirconium and 90% U-238.

The purpose of the zirconium in both the core and blanket rods is to prevent plutonium from forming a low melting point eutectic with the steel fuel tube material.

Hydraulic pressure lines routed through the bottom of the open steel lattice provide the controlled liquid sodium pressure that raises or lowers each mobile fuel bundle.

The mobile fuel bundle actuators are periodically automatically cycled to ensure that their corresponding mobile fuel bundles slide freely.

Heat is removed from the fuel tube assembly by primary liquid sodium which flows upwards via natural convection through the fuel tube support gratings and then up through the gaps between the HT-9 steel fuel tubes. The fuel bundles are fitted with bottom filters to trap any particles with dimensions over (1 / 32) inch. There is space downstream of the filters to allow for liquid sodium cross flow so as to ensure liquid sodium can flow past all the active fuel tubes even if a particular filter section is obstructed.

This web page last updated October 12, 2020

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