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

Elsewhere on this website Fast Neutron Reactors (FNRs) have been identified as a sustainable solution to mankind's energy problems. This web page focuses on FNR design parameters that are necessary to achieve inherent safety. The issue is that FNRs must be designed so that they can be safely installed, operated and maintained in high density urban locations.

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 and liquid sodium primary and secondary coolants. The contemplated FNR has a core region 0.35 m high X 10.4 m diameter that is surrounded above, below and on the perimeter by a blanket region 1.2 m thick.

The central core region together with the top and bottom blanket regions involves 532 active vertical fuel bundles. Each fuel bundle is 0.4 m long X 0.4 m wide X 6.0 m high. In addition there are 3 m corner girder extensions on the fuel bundle bottom and there is a 4.5 m indicator tube extension on the top. Each active fuel bundle consists of a vertically sliding central control portion and a fixed position surround portion. The control portion is inserted into the surround portion from the bottom. The indicator tube projection above the liquid sodium surface shows the vertical position of the control portion.

The core region is surrounded on its perimeter by a 1.2 m thick blanket formed from 272 more passive vertical fuel bundles.

The active fuel bundles are centrally positioned in a 25.4 m long X 18.4 m wide X 13.5 m deep liquid sodium pool. The region 3.0 m long X 18.4 m wide at each end of the liquid sodium pool is dedicated to intermediate heat exchange bundles. There is a central service aisle through the intermediate heat exchange bundles. The 3.5 m long liquid sodium channel in front of the intermendiate heat exchange bundles is kept open to allow easy exchange of both fuel bundles and heat exchange bundles.

Much of the inherent safety of an FNR is due to minimal moving parts in the FNR primary sodium system. Unless there is an external failure the reactor justs sits in the pool of liquid sodium and maintains the pool temperature. The primary sodium moves by natural circulation. When there is no thermal load that circulation stops and the fission reactions stop. When there is a thermal load natural circulation recommences and fission reactions restart. In normal reactor operation the control portions of the active fuel bundles remain in fixed positions. The only moving parts are the primary liquid sodium filter pump and the hydraulic liquid sodium pump.

The active fuel bundle control portion positioning actuators rely on a common hydraulic liquid sodium pump that runs occasionally due to small leaks past the actuator pistons. It is anticipated that this pump will be submerged to maintain suction pressure but its motor will be out of the primary sodium pool and will be cooled with flowing argon. If this pump fails the control portion actuators will lose pressure causing control portion withdrawal due to gravity and the reactor will shut down.

A situation to be avoided is pumping too much liquid sodium into a control portion positioning actuator which could lead to fuel bundle overheating and fuel melting. If an overhead indicator tube position sensor detects that an indicator tube is too high or if the gamma / neutron flux from a fuel bundle is too high the reactor safety system causes redundant valves to turn off the liquid sodium flow to the relevant control portion positioning actuator and to drain the liquid sodium from that actuator.

As an additional safety measure if a control portion position fails to respond to control signals or if the emitted gamma flux is too high or if the indicator tube temperature is too high then the reactor control computer automatically reduces the position setpoints of the surrounding nearest neighbor active fuel bundle control portions to force the non-responsive fuel bundle into a sub-critical state.

A fuel bundle control portion positioning actuator inserts a fuel bundle control portion into its surround portion. The vertical position of that control portion is indicated by the height of its indicator tube. The corresponding fuel bundle power is indicated by the strength of the gamma / neutron radiation propagating up that indicator tube. The indicator tube temperature should respond to a change in gamma /neutron flux with a time lag. If either the fuel bundle gamma / neutron signal or the indicator tube temperature is too large the control portion insertion must be immediately reduced. A high indicator tube temperature without a high gamma signal may signal an obstructed liquid sodium flow. Failure of the indicator tube temperature to track the gamma / neutron signal indicates an equipment problem.

If a fuel bundle's discharge temperature, gamma / neutron flux or control portion insertion depart from normal values the control portion should be automatically withdrawn and that fuel bundle locked out of service until the cause of the problem is determined and rectified.

The actuator valves for each fuel bundle control portion have three states: add high pressure hydraulic sodium, do nothing, drain hydraulic sodium. Most of the time the actuator valves should be in the do nothing state. If the actuator valves spend too much time in the add sodium state it may indicate a control portion positioning actuator sodium leak or a drain valve seal failure. If the drain valve spends too much time open it may indicate a seal problem with the actuator fill valve.

Normal thermal power control in a FNR is achieved by thermal expansion of the liquid sodium and the active fuel bundles. The amount of insertion of a fuel bundle control portion into its surround portion determines the fuel bundle discharge temperature setpoint. A FNR fuel bundle operates at maximum temperature when the fuel bundle control portion is fully inserted into its surround portion. Fuel bundle control portions are withdrawn to achieve a reactor cold shutdown. On loss of control power all the control portion actuator drain valves open and gravity causes the fuel bundle control portions to withdraw from the bottom of the fuel bundle surround portions to ensure certain reactor shutdown.

It is crucially important that fuel bundle control portions not be inserted too far, or the safe operating temperature of the fuel and the fuel tube could be exceeded leading to fuel tube distortion or melting. Hence the fuel bundle control portion positioning actuator system must be extremely reliable and the rate of insertion of the control portions must be limited.

For safety purposes each active fuel bundle must remain subcritical when its control portion is fully inserted but the four nearest neighbor control portions are all withdrawn. This feature ensures an immediate safe reactor cold shutdown in the presence of a single control portion jam and/or a single control portion positioning system failure. Note that under this criteria the thermal power output from the outer ring of active fuel bundles will be relatively small. For each member of this perimeter active fuel bundle ring one or two of the nearest neighbor active fuel bundles are already missing.

Reactor safety is improved by requiring that the reactor will fully shut down even if two adjacent fuel bundle control portions jam in the full power position. The core portion of each fuel bundle can be regarded as cube 15 inches to a side. In normal full power reactor operation neutrons are lost from 2 faces of this cube for centrally located fuel bundles and neutrons are lost from 3 or 4 faces of the cube for active fuel bundles that are members of the perimeter ring. To ensure reactor shutdown with two adjacent jammed control portions a fuel bundle must shut down if neutrons are lost from either 6 or 5 cube faces. Hence the reactor should be fuelled such that when the contol portions are inserted the the fuel bundle shutdown threshold is at neutron loss through 4 cube faces. In reality the fuel mix will change through the operating life of a fuel bundle, so to achieve a higher safety margin it is necessary to move the shutdown threshold from 4 faces of neutron loss to 3 faces of neutron loss. Under these circumstances the perimeter ring of the active fuel tubes may generate little or no thermal power.

If a control portion jams such that it will not withdraw due to gravity an overhead gantry crane can either lift or push down the corresponding indicator tube to release the jammed control portion.

To prevent fuel melting in adverse conditions the fuel bundle positioning mechanisms must be highly reliable. Indicator tubes that project above the surface of the primary liquid sodium provide a reliable indication of the control portion vertical position, the liquid sodium temperature and the corresponding fuel bundle gamma / neutron flux.

The individual fuel bundles are engineered such that if they are outside the reactor environment they remain sub-critical. When the fuel bundle assembly is formed with the control portions withdrawn the reactor will be sub-critical. When all of the control portions are fully inserted the reactor should operate at or above maximum rated power. As a fuel bundle reaches its design operating temperature thermal expansion of the primary liquid sodium and the fuel bundle materials should cause the fuel bundle reactivity to drop below criticality.

The indicator tube temperature distribution is monitored with an infrared camera. In normal operation the indicator tubes should be uniformly at 440 degrees C. A small change in indicator tube temperature will cause a large change in the infrared emitted thermal power per unit area.

In normal operation the control portion vertical position setpoints are adjusted so that at full rated reactor power the gamma / neutron outputs by every active fuel bundle inside the perimeter ring of active fuel bundles are identical. This state corresponds to uniform thermal power loading of the active fuel bundles that are inside the outer ring of active fuel bundles.

A control portion's vertical position is indicated by the height of its indicator tube. In normal operation the vertical position setpoints of the indicator tubes should be nearly identical.

As the reactor thermal load varies the gamma flux varies for all the active fuel bundles.

The fuel bundles are engineered to allow sufficient lateral liquid sodium flow to provide sufficient cooling to prevent fuel and fuel tube damage if an isolated cooling channel becomes blocked.

A liquid sodium flow obstruction will cause a small local temperature increase and hence a local decrease in gamma / neutron flux as compared to neighboring fuel bundles. If a relative decrease in gamma/neutron flux is detected that fuel bundle's control portion should be withdrawn until the cause of the abnormally low gamma / neutron flux is identified and remedied and/or the faulty fuel bundle is replaced.

The height of each indicator tube is sensed by a laser scanner. The fuel bundle gamma / neutron emission is sensed by an overhead thermally isolated and cooled radiation monitoring apparatus, likely using He-3 as a sensing element. The indicator tube temperatures are monitored with an infrared camera.

In plan view the active fuel bundles can be divided into two groups a "red" group and a "black" group as on a chequer board. Each "red" active fuel bundle has four surrounding "black" active fuel bundles. Each "black" active fuel bundle has four surrounding "red" fuel bundles. Neutrons in the core region flow in three dimensions. Hence turning off either all the "red" bundles or all the "black" bundles turns off the entire reactor. Hence there are two independent reactor shutdown systems, one of which turns off all the "red" active fuel bundles and the other which turns off all the "black" active fuel bundles. For reactor service both shutdown systems are normally activated.

The reactor primary and secondary coolants are both pure liquid sodium. A Na-K mixture is specifically excluded. A Na-K mixture offers superficial advantages in terms of melting point. However, in a power reactor the presence of potassium in the primary sodium leads to formation of K-40 which has a half life of 1.26 X 10^9 years. If there are many power reactors operating for many years K-40 will become an increasing environmental problem. The best solution to this problem is to not produce K-40 in the first place. Hence the primary coolant should be pure sodium.

The secondary coolant circuits will operate until something fails, likely an intermediate heat exchange bundle or a steam generator tube. In an intermediate heat exchange bundle failure secondary coolant will mix with the primary coolant. Hence to keep potassium out of the primary coolant the secondary coolant must also be pure sodium.

The reactor thermal output power is sensed via the secondary sodium temperature differential and secondary sodium flow rate. It is important to keep the reactor thermal output power within its design range to prevent damage to the active fuel tubes.

The FNR core fuel rods are primarily metalic U-238 with Pu-239 and 10% zirconium alloyed with it. The purpose of the zirconium is to prevent formation of a fuel tube low melting point plutonium-iron eutectic. The purpose of the Pu-239 is to drive the nuclear reaction. The purpose of the U-238 is to capture neutrons to breed more Pu-239 The core rods are enclosed in HT-9 steel fuel tubes. The core rod diameter is intentionally only about 86% of the initial HT-9 steel tube ID to allow for core rod swelling due to internal formation of gaseous fission products. Inside the HT-9 steel tubes, along with the fuel rods is liquid sodium, which provides good thermal contact between the fuel rods and the fuel tubes and which chemically absorbs the potentially corrosive nuclear reaction products fluorine, chlorine, bromine and iodine. The HT-9 steel tube plenum volume above the fuel and blanket rods contains sufficient extra sodium to compensate for eventual fuel tube swelling and provides sufficient empty volume to allow for volumetric expansion of the fuel rods and liquid sodium and to allow for sealed containment of inert gaseous fission products at allowable pressures.

As the primary liquid sodium warms up its density decreases, taking the reactor core zone reactivity below its critical point. Then the only significant heat produced is fission product decay heat. Provided that there is adequate decay heat removal at a low thermal load the reactor temperature will stabilize at its maximum rated temperature. As heat is removed from the primary liquid sodium the primary sodium pool temperature decreases increasing the primary sodium density. This density increase restores reactor criticality and raises reactor power. The reactor can be shut down and cooled off by withdrawal of its fuel bundle control portions.

One of the most important aspects of fission reactor design is provision for decay heat removal under extremely adverse circumstances. If some event occurs which causes a reactor shutdown the fission products will continue to produce decay heat at about 5% of the reactor's full power rating. It is essential to have sufficient stored cooling water to ensure ongoing removal of the fission product decay heat under accident conditions such as a severe earthquake.

In the case of 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 the fuel rods are not fully immersed in liquid sodium or where thermal contact with the intermediate heat exchangers is lost.
2) The liquid sodium pool walls must be designed such that if the inner wall fails and the primary liquid sodium leaks into the space between the inner and outer pool walls, the leakage into the space between the two walls will not lower the primary liquid sodium level below the tops of the fuel rods or below the lower ends of the intermediate heat exchanger tubes.
3) Even if the intermediate loop sodium pumps fail there must be enough secondary liquid sodium natural circulation to ensure safe removal of the fission product decay heat.

An FNR designed for utility power production typically has 532 active fuel bundles. Sooner or later through accident, negligence or malevolent behavior there will be a defective fuel bundle and/or a defective control bundle positioning apparatus. In these circumstances a major concern is fuel melting. In response to local overheating the adjacent fuel bundles must be immediately be withdrawn to ensure shutdown of the defective fuel bundle.

If a fuel melting occurs fuel alloy droplets might collect on the primary liquid sodium pond floor under the fuel tube assembly. It is essential that these droplets do not accumulate together to form a critical mass. If a critical mass is on the verge of forming fuel melting should cause the molten fuel to penetrate the floor cover in a pattern that is inconsistent with existence of a critical mass. Thus the bottom of the primary liquid sodium pond should have a removeable liner that has a high neutron absorption cross section and has controlled size boron containing bumps and cavities such that if the fuel melts a critical mass cannot form. There should also be a practical means of selectively removing and replacing sections of the primary liquid sodium pool floor liner.

It is contemplated that hot liquid sodium will naturally circulate vertically up through the reactor between the active fuel tubes, horizontally along the top of the primary liquid sodium pool, down between the vertical intermediate heat exchange tubes and then horizontally along the bottom of the primary liquid sodium pool back to below the reactor.

Each active fuel bundle control portion has associated with it a control portion actuator that vertically positions the control portion based on the indicator tube vertical position with respect to its setpoint. The actuator piston moves in response to liquid sodium hydraulic pressure. If a problem is detected the liquid sodium hydraulic pressure falls to zero and the fuel bundle control portion withdraws causing a fuel bundle shutdown.

The steam generators are located at an elevation higher than the reactor and on the return side of the intermediate sodium loop so that the intermediate sodium will natuarally circulate even if the intermediate sodium pump has no power. The equipment should be sized so that the natural circulation rate is sufficient to safely remove reactor decay heat after the reactor is shut down.

The individual active fuel bundle surround portions and the passive fuel bundles are supported and held in position by square vertical tubes that mate with a steel frame located on the bottom of the primary sodium pool. The indicator tubes are attaached to the top of the active fuel bundle control portions and are horizontally stabilized by the buoyancy of the indicator tubes and the steel floats. The fuel bundles mechanically clip to their nearest neighbours to provide assembly lateral stability.

The fuel bundles are repositioned and/or replaced from time to time using an overhead gantry crane and remote manipulation. Note that the ceiling height over the liquid sodium pond must be high enough to allow extraction and replacement of individual fuel bundles. Spent fuel bundles are lifted 2 m to clear the vertical square support tubes and are then moved 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.

The FNR is designed to safely withstand earthquake induced horizontal acceleration of up to 1.0 g. At a sustained 1.0 g horizontal acceleration the surface of the liquid sodium could adopt an angle that is 45 degrees to the horizontal. Under these circumstances the liquid sodium height on one end of the pool could theoretically reach up to 25 m _______above the liquid sodium pool floor. The gantry crane is located higher than this maximum liquid sodium height. The saw cut lava rock blocks forming the pool thermal insulation must be firmly attached to the surrounding concrete walls and to upper level steel walls to prevent a structural failure in severe earthquake conditions. Hence the FNR can be thought of as being located in a 12.5 m deep liquid sodium pool with a liquid sodium top surface which is 1 m below the pool deck.

The main chemical threat from a power FNR is the 5608 m^3 of liquid sodium contained in the primary sodium pool. If this liquid sodium contacts water there will be an explosive chemical reaction which liberates hydrogen that will spontaneously ignite in an air atmosphere. Hence one of the main issues in FNR design is choice of a reactor site where the sodium will never be exposed to flood water.

The other chemical threat is a spontaneous reaction between hot liquid sodium and air. To mitigate this threat the liquid sodium is covered by floating steel covers, an argon cover atmosphere, a gas tight suspended inner metal ceiling, and a gas tight suspended outer metal ceiling. In the event of air penetration into the argon cover gas the reactor should be immediately shut down and heat dumped from the primary liquid sodium pool to lower the primary liquid sodium temperatre below the threshold for spantaneous combustion of sodium in air.

The reactor must be sited at sufficient elevation above the maximum possible local flood level that the liquid sodium will never be exposed to flood water. However, sufficient cooling water must be reliably available to provide evaporative cooling of fission product decay heat. The soil and bedrock around the liquid sodium pool must be sufficiently dry, dense and stable to safely contain the liquid sodium in the unlikely event that a major earthquake ruptures both the inner and outer stainless steel walls of the liquid sodium pool and the enclosing concrete wall. Outside that containment must be course gravel that ensures gravity drainage of water away from the sodium pool.

An ideal site for a FNR is a dense igneous rock plateau that drops off quickly to a lake or ocean with a high water mark over 20 m below the plateau grade. The lake or ocean provides the required water for evaporative cooling. The lake or ocean bottom depth should be sufficient to prevent silt and/or sand being sucked into the cooling tower water makeup system. The sodium pool elevation above the water level must be sufficient that no natural geophysical event will ever lead to water threatening the stored sodium. Channels or tunnels can be cut into the bedrock surrounding the FNR to ensure positive gravity drainage. The plateau should provide sufficient area for a FNR, a reserve sodium pool, dry cooling towers, emergency wet cooling tower, turbogenerators, condensers and transformer/switchyard facilities.

Note that the elevation of a FNR with respect to a nearby major water body is much higher than the elevation of a reactor that uses direct lake or sea water cooling. Thus Fukushima Daiichi like problems that might be caused by a tsunami or a comparable flood mechanism are avoided.

The space above the liquid sodium pool is filled with the inert gas argon which will not chemically react with either the liquid sodium or steel.

Floating on top of the liquid sodium are shallow draught steel floats which reduce the exposed surface area of liquid sodium by about 99%. These floats have holes through them for the indicator tubes.

If for any reason air leaks into the argon cover gas the immediate requirement is to lower the liquid sodium temperature below 200 C to prevent spontaneous sodium combustion.

The primary function of the roof structure is to contain the argon cover gas and heat and to keep both rainwater and air away from the liquid sodium. The roof structure must be high enough above the liquid sodium pool to allow individual fuel bundle and intermediate heat exchange bundle replacement. The roof structure must be gas tight and must reliably exclude both air and rain water under the most adverse circumstances, including violent storms (tornados), long term corrosion and deliberate aerial attack. There should be an inner metal ceiling, thermal insulation, an outer metal ceiling, ventilation space and an arched or A frame outer roof.

If an imminent threat to the FNR from an air born object is detected the FNR should be immediately shut down and sufficient heat should be extracted from the primary liquid sodium pool to ensure that there will be no spontaneous combustion of sodium with air if there is a roof failure.

The inner ceiling is normally supported by the internal argon pressure, but should that support mechanism fail then the inner ceiling 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 450 degrees C. On top of the inner ceiling is a layer of high temperature rated insulation so that the space between the outer metal ceiling and the structural roof is relatively cool. This space is normally cooled with forced air to enable roof level service.

An arched concrete roof Can be fabricated from precast arch shaped pieces. It must have sufficient strength to withstand an overhead tornado and to support the inner two metal ceilings. The upper surface of the concrete roof could be sod covered so that the exact reactor position is difficult to visually pinpoint from an overhead aircraft.

Engineering a roof that can withstand a direct overhead air attack is one of the most difficult aspects of liquid sodium cooled FNR implementation. It may ultimately be necessary to locate FNRs deep underground with arched rock roofs to provide security against intentional overhead attack.

A reasonable compromise is to locate the liquid sodium pool for the new FNR below grade such that in an emergency an argon gas cover can be maintained over the liquid sodium pool while a temporary roof is applied. In such circumstances the liquid sodium pool must be cooled below 200 degrees C to avoid spontaneous sodium combustion with air. Circumstances that might lead to such a roof failure include a direct bomb, missle or meteorite strike.

There should be a sufficiently large supply of liquified argon on-site to temporarily prevent sodium combustion in the event of a sudden major roof failure.

Sooner or later it will be necessary to do maintenance work on the sodium pool inner wall. To do such work it will be necessary to temporarily remove the primary liquid sodium from the primary sodium pool. Hence there must be a nearby reserve pool with sufficient capacity to hold the entire volume of the primary liquid sodium while the aforementioned maintenance work is being carried out. The reserve pool must also accept hot fuel bundles removed from the primary liquid sodium pool. One reserve pool might be shared by several adjacent FNRs. The reserve pool can be located underneath a dry cooling tower. When not required for storing sodium the reserve pool can be used for storing extra emergency cooling water.

Heat is removed from the FNR via intermediate heat exchanger bundles. The heat exchange tubes have low pressure radioactive primary liquid sodium on the outside and have high pressure non-radioactive secondary liquid sodium on the inside. The high pressure secondary sodium exchanges heat to water in a steam generator. Thus, even if there is a heat exchanger or steam generator tube rupture there is no contact between radioactive sodium and the turbogenerator working fluid (clean water). There is further heat exchange isolation between the turbogenerator working fluid (clean water) and the external cooling water.

The contemplated heat sink is a multiplicity of dry cooling towers. The cooling towers must be piped to ensure reactor decay heat removal by natural circulation even if the dry cooling towers are partially out of service. These dry cooling towers must be designed such that they can safely remove fission product decay heat by natural circulation. If the dry cooling is out of service in an emergency wet cooling can be used for fission product decay heat removal.

This web page last updated September 1, 2017

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