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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 sufficiently safe that they can be installed 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 blankets involves 532 active vertical fuel bundles. Each fuel bundle is 0.4 m long X 0.4 m wide X 6.0 m high. Each active fuel bundle consists of a vertically sliding central control bundle that is surrounded by a fixed position surround bundle. The control bundle is inserted into the surround bundle from the bottom.

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 3.5 m long X 18.4 m wide at each end of the liquid sodium pool is dedicated to intermediate 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 some sort of 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.

The control bundle positioning actuators rely on a common liquid sodium pump that must run 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 sodium pool and will be externally cooled with argon. If this pump fails the actuators will lose pressure and the reactor will shut down.

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

As an additional safety measure if a control bundle position fails to respond to control signals or if the fuel bundle discharge temperature becomes too high then the reactor control computer must automatically reduce the setpoint temperature of the eight surrounding bundles to force the non-responsive fuel bundle into a sub-critical condition.

A fuel bundle positioning actuator inserts a control bundle. The vertical position of that control bundle is indicated by the height of the indicator tube. The corresponding fuel bundle power is indicated by the strength of the gamma / neutron radiation flowing up the indicator tube. The indicated fuel bundle discharge temperature should respond with a time lag. If either the fuel bundle gamma / neutron signal or the fuel bundle discharge temperature is too large the control bundle insertion must be immediately reduced. A high fuel bundle discharge temperature without a high gamma signal likely indicates obstructed liquid sodium flow. A high gamma signal without a high discharge temperature may indicate other problems. Failure of the control bundle to maintain a stable position setpoint may indicate an actuator system problem.

If a fuel bundle's discharge temperature, gamma / neutron flux or control bundle insertion depart from normal values the control bundle 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 control bundle have three states: add high pressure sodium, do nothing, drain 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 bundle positioning actuator sodium leak or drain valve seal failure. If the drain valve spends too much time open it may indicate a seal problem with the fill valve.

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 control bundle determines the discharge temperature setpoint of that fuel bundle. A FNR fuel bundle operates at maximum temperature when its control bundle is fully inserted. Control bundles are withdrawn to achieve a reactor cold shutdown. On loss of control power all the actuator drain valves open and gravity will cause the control bundles to withdraw from the bottom of the FNR fuel tube assembly to ensure certain reactor shutdown.

It is crucially important that control bundles not be inserted too far, or the safe operating temperature of the fuel and the fuel tube could be exceeded or worse the fuel tube could melt. Hence the control bundle positioning actuator system must be extremely reliable and the rate of insertion of the control bundles must be limited.

For safety purposes each fuel bundle should remain subcritical when its control bundle is fully inserted but the eight nearest neighbor control bundles are all withdrawn. This feature ensures an immediate safe reactor cold shutdown in the presence of a single control bundle jam and/or a single control bundle positioning system failure.

If a shutdown problem remains the alternative is to use an overhead gantry crane to push down on the indicator tube to release a jammed control bundle.

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 bundle vertical position, the fuel bundle's liquid sodium discharge 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 bundles withdrawn the reactor should still be sub-critical. When all of the control bundles are fully inserted the reactor should operate at maximum rated power. As a fuel bundle reaches its design operating temperature thermal expansion of the liquid sodium coolant and the fuel bundle materials should cause the fuel bundle reactivity to drop below criticality.

To prevent some fuel bundles operating at higher temperatures than other fuel bundles each fuel bundle has its own liquid sodium discharge temperature control system.

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

A liquid sodium flow obstruction increases the fuel bundle discharge temperature which reduces the fuel bundle operating power. The decrease in power causes a decrease in gamma / neutron flux.

A reduction in relative operating power of one fuel bundle as compared to its nearest neighbours is revealed by a high resolution gamma / neutron flux camera scan. A relative drop in gamma /neutron output indicates a fuel bundle with a cooling flow obstruction. This methodology hinges on accurate measurement of each fuel bundle's discharge temperature.

The height of each indicator tube is sensed by a laser scanner. The discharge temperature of each fuel bundle changes the vapor pressure inside the indicator tube which deflects an end diaphragm. This diaphragm deflection is also 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 reactor primary and secondary coolants are both pure liquid sodium. A Na-K mixture is specifically excluded. An 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 the intermediate heat exchange bundle. 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 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 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 steel fuel tubes. The core rod diameter is intentionally only about 86% of the initial steel tube ID to allow for core rod swelling due to internal formation of gaseous fission products. Inside the steel tubes, along with the fuel rods is liquid sodium, which provides good thermal contact between the fuel rods and the steel tubes and which chemically absorbs the nuclear reaction products fluorine, chlorine, bromine and iodine. The steel tube plenum volume above the fuel and blanket rods contains extra sodium to compensate for fuel tube swelling and provides sufficient empty volume to allow for volumetric expansion of the fuel rods and liquid sodium and to allow for containment of inert gaseous fission products at allowable pressures.

As the liquid sodium warms up its density decreases, taking the reactor core zone reactivities below their critical points. Then the only significant heat produced is fission daughter decay heat. Provided that there is adequate decay heat removal the reactor temperature stabilizes at its maximum rated temperature. As heat is removed from the liquid sodium the sodium pool temperature decreases increasing the sodium density. This density increase restores reactor criticality and raises reactor power. The reactor can be shut down and cooled off by withdrawal of all of the control bundles.

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 6% of the reactor's full power rating. Hence it is essential to ensure ongoing removal of the fission product decay heat.

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 liquid sodium leaks into the space between the two walls, the leakage into the space between the two walls will not lower the liquid sodium level below the tops of the fuel tubes 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 safely remove the fission product decay heat.

An FNR designed for utility power production 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 should be immediately be withdrawn.

If a fuel melting occurs fuel and fuel tube 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 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 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 flow vertically up through the reactor, horizontally along the top of the primary liquid sodium pool, down along the vertical intermediate heat exchange tubes and then horizontally along the bottom of the primary liquid sodium pool back to below the reactor.

Each control bundle has associated with it a vertical motion actuator that positions the control bundle based on the fuel bundle discharge temperature. The actuator operates using liquid sodium hydraulic pressure. If a problem is detected the control bundles withdraw causing a reactor 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 fuel bundles are supported and held in position by 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 fuel bundles and are horizontally stabilized by the buoyancy of the indicator tubes and by the steel floats.

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 moved to the perimeter zone of the pool where they 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 top surface which is 1 m below grade.

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 site where the liquid sodium will never be exposed to 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 roof, and a gas tight suspended outer metal roof. Provision is made for rapid dumping of heat 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 local water table that the liquid sodium will never be exposed to flood water. However, sufficient cooling water must be reliably available to provide 1000 MWt of evaporative cooling. 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.

An ideal site for an FNR site 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 large quantities of 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 sodium pool. 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 group of FNRs, their cooling towers, turbogenerators and transformer/switchyard facilities.

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

If for any reason oxygen 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 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, long term corrosion and deliberate aerial attack. There should be an inner metal roof, thermal insulation, an outer metal roof, ventilation space and an arched concrete roof.

If an air born object is detected that might be a threat to the FNR 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 with air.

The inner roof is normally supported by the internal argon pressure, but should that support mechanism fail then the inner roof should be supported by hangers attached to the the outer roof via thermal breaks. The inner roof is made from sheet stainless steel and is normally at about 460 degrees C. On top of the inner roof is a layer of high temperature rated insulation so that the space between the outer roof roof and the ached concrete roof is relatively cool. This space is normally cooled with forced air to enable roof level service.

The concrete roof is fabricated from precast arch shaped pieces. It must have sufficient strength to withstand an overhead tornado and to support the two metal roofs. The upper surface of the concrete roof should be sod covered so that the 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 more 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. 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 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 can be shared by several adjacent FNRs.

Heat is removed from the FNR via intermediate heat exchanger bundles. These heat exchange bundles 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 natural draught evaporative cooling towers. The water supply to these cooling towers must be extremely reliable. The cooling towers must be piped to ensure reactor decay heat removal by natural circulation even if the cooling towers are partially out of service or if the primary water supply is not available.

If the water supply is limited dry cooling towers must be used. These towers must be designed such that they can safely remove fission product decay heat by natural circulation.

This web page last updated May 26, 2017

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