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The term Fast Neutron Reactor (FNR) is generically quite broad. For the purpose of this web page the definition of an FNR is narrowed to be a fast neutron reactor with metallic fuel rods, sealed metal fuel tubes and liquid sodium primary and intermediate coolants.
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 insetion depth of mobile fuel bundles into this matrix of fixed fuel bundles.
In external appearance a FNR appears to be a cylindrical steel tank with outside dimensions 18 m high X 24 m diameter. The tank walls consist of three layers of steel separated from each other by 2 X 1 m thick layers of fire brick, so the net tank inside dimensions are 16 m high X 20 m in diameter. Inside this tank is a liquid sodium pool 15 m deep.
FAST NEUTRON REACTOR SIDE ELEVATION:
For clarity in the above diagram the overhead gantry crane and the air locks are not shown.
Also not shown but resting on the inside bottom of the tank is the fuel assembly ball bearing based earthquake protection system, the open steel lattice which supports all of the fuel bundles and the steel columns which support the heat exchange bundles.
Immersed in the liquid sodium are the fuel tubes (shown in red, pink and orange) and intermediate heat exchange tubes (shown in green) that supply and remove heat from the primary sodium pool.
The assembly of fuel bundles has a pancake shaped core region (shown in red) 0.45 m to 0.90 m thick X 10 m diameter that is surrounded above, below and around its perimeter by a blanket region ~ 1.3 m to 1.8 m thick and a fuel bundle cooling region 0.66 m thick.
The blanket region and cooling region are further surrounded by a ~ 3 m thick guard band of liquid sodium which absorbs any neutrons that escape from the blanket and cooling regions, and provides space for damping fuel assembly motion during a severe earthquake.
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 indicator tube. The mobile fuel bundles are supported by hydraulic actuators embedded in the 1.5 m high steel lattice that rests on the primary sodium pool inner floor.
The mobile fuel bundles insert into the matrix of fixed fuel bundle from the bottom. The insertion distance of each mobile fuel bundle is set by a dedicated liquid sodium hydraulic piston actuator. The mobile fuel bundle's indicator tube projects above the primary liquid sodium surface to indicate the actual vertical position of the mobile fuel bundle. This vertical position is constantly monitored using an overhead device similar to a laser measuring tape.
The reactor core region is surrounded above, below and on its outer perimeter by a 1.3 m to 1.8 m thick blanket region. The top and bottom blanket regions are realized with blanket fuel rods above and below the core fuel rods in the active fuel tubes. The perimeter blanket regions are formed from four rings of passive fuel bundles.
There are two outer perimeter rings of used fuel bundles where natural decay of fission products occurs over a six year period before the fuel bundles are removed from the primary liquid sodium.
The active fuel bundles are centrally positioned in a 20 m diameter X 15 m deep liquid sodium pool. The region 2.5 m wide at the edge the liquid sodium pool is dedicated to intermediate heat exchange bundles and a service access corridor. The service access corridor also forms part of the sodium guard band.
A FNR is designed so that about half of the fission neutrons produced diffuse into the surrounding blanket zones where they are absorbed to convert U-238 into Pu-239 and Pu-240. Of the remaining fission neutrons about (2 / 3) are required to sustain the fission chain reaction at a reactivity of about unity and (1 / 3) are used to breed U-238 into Pu-239 and Pu-240 within the reactor core zone. Each mobile fuel bundle is insertion position adjustable to allow precise setting of that fuel bundle's reactivity at a particular temperature.
CHAIN REACTION CONTROL:
Consider the core of an ideal FNR that has uniform fuel geometry throughout the core. If the reactivity at ambient temperature is slightly greater than unity the core temperature will rise until the local reactivity is zero. Then if the liquid sodium is stationary there will be no fission heat injection and the core temperature will remain constant.
If warmer sodium flows through this core zone the core zone temperature will rise to the liquid sodium temperature but the reactivity will be less than unity so there will be no fission heat injection.
However, if cooler sodium flows into the local core zone the local reactivity will increase above unity which will cause local fission heat injection. This heat will cause sodium thermal expansion until the local reactivity is again unity.
Thus if cooler sodium flows upwards through a FNR core the temperature of the flowing sodium will rise to keep the local reactivity in the core zone at or below unity.
Now consider a real FNR. To spacially distribute the local heat injection it is desirable to design the reactor core such that as sodium rises through the core zones the local reactivity set by the fuel geometry gradually rises before it suddenly decreases. Then as an atom of sodium rises through the lower and middle core zones the local sodium temperature will gradually rise to inject negative reactivity to balance the positive reactivity injection due to the changing local fuel geometry. Thus as the sodium rises through this region the net local reactivity remains close to zero because the temperature increases. As the flowing sodium rises past the top of the middle core zone, due to the changing local fuel geometry the local reactivity will start to decrease. Hence above the top of the middle core zone there is no more nuclear heat injection except due to fission caused by fringe neutrons. At the top of the upper core zone the fissile fuel concentration drops to zero so there is no more fission heat injection.
There is a small amount of fission heat injection near the bottom of the lower core zone due to fringe neutrons.
In the blanket regions there is negligible fission heat injection because the fissile fuel concentration in both blanket regions is close to zero.
Thus in summary the main region of nuclear heat injection is the upper portion of the lower core zone and the middle core zone. In this region the rising liquid sodium gains heat but the local reactivity remains close to zero.
In FNR operation the reactor local reactivity is controlled by local fuel geometry and the local liquid sodium temperature. As liquid sodium rises through the lower core zone and into the middle core zone the local reactivity set by the local fuel geometry increases. The increase in local reactivity causes local fission heat injection which increases the local sodium temperature. That increase in local sodium temperature causes sodium and fuel thermal expansion which injects negative reactivity to balance the increasing local reactivity due to the fuel geometry. Thus in this main region of fission heat injection the local reactivity remains close to zero. Elsewhere the local reactivity is below zero.
At the transition between the lower core zone and the middle core zone care must be taken that the local reactivity does not rise so quickly as to enable either a concentration of local nuclear heat injection beyond the capacity of the fuel tubes to transfer or worse, a prompt critical condition.
COLD SHUTDOWN CONTROL:
There are two completely redundant nuclear reaction shutdown systems. These systems operate by gravity withdrawal of every second mobile fuel bundle in a chequer board pattern. Hence in total there are two independent nuclear reaction cold shutdown systems as well as the warm shutdown system which is normally used for maintaining the reactor setpoint temperature.
LIQUID SODIUM CONTAINMENT:
There is an inner steel primary sodium enclosure, a middle steel primary sodium enclosure and an outer steel primary sodium enclosure. As long as at least one of these enclosures maintains its physical integrity the liquid sodium will be sufficiently contained to maintain the minimum required primary liquid sodium level for safe fission product decay heat removal following a FNR shutdown.
FISSION PRODUCT DECAY HEAT REMOVAL:
Fission product decay can produce heat at a peak rate equivalent to about 8% of a nuclear reactor's full power rating, even after the nuclear chain reaction is shut down. Hence there must be certainty about passive removal of this heat. A FNR has at least four completely independent heat transport systems, any one of which is sufficient for safe fission product decay heat removal. In the event of loss of station power fission product decay heat removal is achieved by natural sodium circulation. In order to meet these requirements at least 33% of the intermediate heat exchange tube length must always be immersed in liquid sodium. This requirement effectively imposes geometrical constraints on the primary sodium pool wall design.
SAFETY ADVANTAGES OF FNRs AS COMPARED TO WATER COOLED REACTORS:
Liquid sodium cooled FNRs inherently provide several major safety advantages as compared to water cooled reactors. At a pressure of one atmosphere water boils at 100 degrees C, a temperature far below the water cooled reactor maximum operating temperature of 320 degrees C. Sodium at a pressure of one atmosphere boils at 883 degrees C, a temperature far above the FNR maximum operating temperature of 502 degrees C. This difference in boiling point relative to reactor operating temperature has important implications.
1) If water is used as a reactor coolant any coolant leak will flash into radioactive steam. In an accident situation this radioactive steam cannot be safely vented to the atmosphere at an urban reactor site. Liquid sodium cooled FNRs avoid this problem by use of low pressure liquid sodium as the primary coolant. The primary liquid sodium is isolated from the turbo-generator working fluid steam by two separate heat exchangers. Thus there is no radiotoxicity hazard in venting turbo-generator steam to the atmosphere.
2) Water cooled reactors have a potentially dangerous condition known as transient void formation in which when the coolant pressure drops steam bubbles form in the reactor fuel tubes. These steam bubbles reduce the heat transfer capacity and reduce the reactor reactivity. To compensate for the reduced reactor reactivity the water cooled reactor power control system may automatically withdraw control rods to maintain the desired power setpoint. Then if the cooling feedwater temperature suddenly drops or the cooling water pressure is restored the steam voids can rapidly collapse leading to a reactor explosion due to prompt neutron criticality. The causes of void formation include coolant circulation failure or coolant pressure drop due to a pipe rupture, a pump failure or a loss of pump power. By contrast the contemplated FNRs rely on natural circulation of low pressure primary liquid sodium which has a boiling point about 380 degrees C above the maximum liquid sodium operating temperature. Hence in a liquid sodium cooled FNR there is no issue of transient void formation.
3) Water cooled reactors rely on ongoing mechanical movement of control rods to control the reactor power. This control system has many possible failure modes. This control system must constantly operate because water cooled reactors are inherently unstable. Absent operation of the mechanical control system the reactor power would spontaneously rise or fall. By contrast in a FNR thermal expansion and contraction of the primary liquid sodium provides stable passive primary liquid sodium temperature control.
The FNR fuel geometry is chosen such that void formation in the primary liquid sodium will stop the nuclear chain reaction, regardless of the amount of insertion of the active fuel bundle control portion into its surround portion. Unlike in a water cooled reactor there is no automatic rapid change in fuel geometry to compensate for void formation. Void formation causes a chain reaction shutdown.
4) During severe accident conditions water cooled reactors vent radioactive gases to the atmosphere. By contrast in FNRs the only radioactive gases present remain sealed inside steel fuel tubes.
5) Most existing water cooled power reactors rely on mechanical pumps for primary coolant circulation. Loss of coolant or loss of coolant pumping are constant and potentially dangerous threats. By contrast the FNR design contemplated herein uses highly reliable natural sodium circulation for primary cooling. At worst loss of pump power causes diminishment in earthquake tolerance.
6) Most existing water cooled power reactors rely on use of concrete casks for on-site dry storage of spent fuel bundles. By contrast after removal from the reactor spent fuel bundles from FNRs are placed in lead shipping containers and are transported by road truck to a central fuel reprocessing site.
7) Water cooled reactors generate large amounts of spent fuel containing highly radiotoxic transuranium actinides. By contrast FNRs eliminate transuranium actinides by fissioning them.
8) Existing water cooled reactors generate large amounts of sevice and decommissioniong waste. By contrast each FNR contemplated herein has a liquid sodium guard band which almost eliminates production of service and decommissioning wastes. In the event of a fuel tube rupture the primary sodium filter system will collect the released fuel and fission product material for reprocessing.
9) Water cooled reactors rely on complex electro-mechanical control systems to keep the reactor fuel and fuel tube temperatures within design limits. By contrast in a FNR the fuel and fuel tube temperatures are constrained by the sodium temperature and the FNR fuel tube heat flux is limited by the maximum intermediate liquid sodium flow rate.
As in all nuclear reactors, during FNR operation geometric stability of the reacting fuel rods with respect to one another is essential. In all nuclear fission reactors a rapid increase in core reactivity can potentially lead to an explosive condition known as prompt neutron criticality. In the FNR design discussed herein gravitional forces and sodium voids tend to reduce reactor criticality. The fuel tube melting point is much greater than the temperature at which primary sodium voids can form. Formation of primary sodium voids causes a chain reaction shutdown. The reactor is designed so that too rapid insertion of the FNR mobile fuel bundles is both mechanically and electronically prevented because a too rapid insertion might cause a transient increase in reactivity sufficient for prompt neutron criticality. However, rapid withdrawal of the mobile fuel bundles is permitted for reactor cold shutdown.
To ensure control stability the mobile fuel bundle insertion rate and withdrawal rate are both normally mechanical orifice limited. Mobile fuel bundle insertion control is not used for normal ongoing reactor power control. Reactor cold shutdowns use separate full port hydraulic valves to enable rapid mobile fuel bundle withdrawal from the matrix of fixed fuel bundles.
The top and bottom of each octagonal fuel bundle is physically attached to its four nearest neighbour octagonal fuel bundles to realize fixed fuel bundle matrix stability.
The assembly of fuel bundles and its supporting open steel lattice rests on a layer of one inch diameter ball bearings which provide the fuel assembly protection against a 3 g horizontal ground acceleration that may accompany a violent earthquake. During an earthquake the fuel assembly stays nearly still while the ground moves underneath it. Provided that the net ground movement does not exceed the 3 m width of the liquid sodium guard band there should be no earthquake caused damage to the fuel assembly. The hydraulic pressure lines connected to the fuel asembly must have sufficient length and flexibility to accommodate the potential relative movement of the fuel assembly with respect to the primary sodium pool during an earthquake. The inside floor of the primary sodium pool must be slightly depressed in the center to keep the assembly of fuel bundles centered in the primary sodium pool.
FEW MOVING PARTS:
Much of the inherent safety of an FNR is due to absence of mechanical moving parts. The fuel bundles simply sit motionless in a pool of liquid sodium. Thermal expansion and contraction adjusts the reactor thermal power to maintain the desired liquid sodium temperature. Under thermal load the liquid sodium naturally circulates. When there is no thermal load the natural circulation stops which causes the fission chain reaction to stop. When the thermal load is restored natural circulation recommences and the fission chain reaction restarts. In normal reactor operation the mobile fuel bundles remain in fixed positions. The only mechanical moving parts in the reactor enclosure are the primary liquid sodium filter pump impeller, the hydraulic liquid sodium pressure maintenance pump and occasionally the hydraulic actuator sodium flow control valves. These valves operate by gravity and by argon pressure. The argon pressure control transducers are located outside the reactor enclosure. Loss of argon pressure causes a reactor cold shutdown.
The intermediate sodium receives a circulation assist from electric induction pumps that are located on the intermediate sodium return pipes outside the reactor enclosure. If there is a loss of intermediate sodium level or intermediate sodium pressure in a particular heat transport system that entire heat transport sytem is automatically shut down.
MOBILE FUEL BUNDLE POSITIONING SYSTEM:
Each active mobile fuel bundle is posioned by a dedicated liquid sodium hydraulic piston actuator. Thus a single actuator jam or mobile fuel bundle jam does not physically affect the adjacent mobile fuel bundles except via a changed neutron flux in the horizontal plane.
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 gradually lose pressure causing the active fuel bundle control portions to withdraw due to gravity and thus cause a reactor shut down.
MOBILE FUEL BUNDLE INSERTION RATE:
The mobile fuel bundle positioning system is robust and extremely reliable and the rate of insertion of the mobile bundles is both mechanically and electronicly limited.
A situation that must be avoided is too rapid insertion of a mobile fuel bundle which could potentially lead to fuel bundle overheating, fuel melting, sodium void formation and possibly even prompt neutron criticality. Hence the rate of insertion of the mobile fuel bundles is limited by both hardware and software.
To avoid potential problems related to too rapid mobile fuel bundle insertion orifices on the hydraulic actuator control valves limit the active fuel bundle control portion insertion rate. If the reactor monitoring system detects that a mobile fuel bundle insertion is either too deep or too fast or if the gamma / neutron flux from a mobile fuel bundle is too high or if the fuel bundle discharge temperature is too high the reactor safety system causes redundant full port drain valves to open causing rapid withdrawal of the relevant active fuel bundle control portion. Simultaneously the hydraulic liquid sodium flow to the relevant control positioning actuators is turned off.
As an additional safety measure if the position of a mobile fuel bundle fails to respond to position control signals or if the emitted gamma flux is too high or if the active fuel bundle discharge temperature is too high then the reactor control computer automatically causes withdrawal of the surrounding nearest neighbor mobile fuel bundles to force the relevant portionof the reactor into a sub-critical state.
CONTROL SYSTEM DETAILS:
If there is a step increase in reactor reactivity the fuel rod temperature increases almost instantly. However, the temperature of the surrounding liquid sodium may take more than a second to respond. Hence there is a small transportation delay between an increase in reactivity and the corresponding liquid sodium thermal expansion. To avoid reactor power oscillations it is important to keep this transportation delay to under 3 seconds to allow stable reactor power control with delayed neutrons which themselves have an average 3 second delay.
The mobile fuel bundle insertion rate is limited to prevent the reactor going critical on prompt neutrons before liquid sodium thermal expansion has had time to respond to the increased reactor power level. The mobile fuel bundle must not be inserted so far that thermal expansion of the liquid sodium cannot stop the fission chain reaction at an acceptable liquid sodium temperature.
The indicator tube height sensor must have good resolution with negligible hysterisis. The corresponding mobile fuel bundle power is immediately indicated by the strength of the gamma / neutron radiation propagating up the hollow portion of that bundle's indicator tube. The fuel bundle liquid sodium discharge temperature should respond to a change in gamma /neutron flux with some time lag. If either a fuel bundle gamma / neutron signal or a fuel bundle discharge temperature is too large the mobile fuel bundle insertion must be immediately reduced. Sustained failure of the fuel bundle discharge temperature to track either the gamma / neutron signal or the indicator tube height indicates an equipment problem. That mobile fuel bundle should be withdrawn and locked out of service until the cause of the problem is determined and rectified.
The actuator valves normally used for position control of each mobile fuel bundle have three states: add high pressure hydraulic sodium via a small orifice, close, and drain hydraulic sodium via a small orifice. Most of the time these actuator valves should be in the closed state. If an actuator valve spends too much time in the add sodium state it may indicate an actuator piston ring leak or a drain port seal failure. If the drain valve keeps reopenning it may indicate a fill port seal leak.
In addition to the orifice restricted actuator control valves for each active fuel bundle there is a full port shutdown valve which can rapidly drain liquid sodium from the hydraulic actuator causing a rapid reactor cold shutdown. On loss of argon control pressure gravity operation of this full port valve causes an immediate fuel bundle shutdown.
FUEL BUNDLE SAFETY ENGINEERING:
The FNR control system assumes that the core rods in a particular fuel bundle all behave the same way. Hence during fuel rod production it is important that 100% of the core rods in a fuel bundle come from the same alloy batch or be individually tested to check that their alloy mix meets design specifications.
The FNR control system assumes that all the liquid sodium cooling channels in a fuel bundle are open. Hence it is essential that the liquid sodium be sufficiently filtered to eliminate particulate matter that over time might obstruct the liquid sodium cooling channels. Each fuel bundle has sufficient cross flow to tolerate a few isolated blocked liquid sodium cooling channels but the fuel bundle cannot tolerate a large cluster of blocked liquid sodium cooling channels. Of particular concern are carbon, boron, beryllium, oxygen and fluorine based particulate compounds that might potentially float or be nearly neutrally buoyant in liquid sodiuum. Denser particulate species will naturally settle to the bottom of the primary liquid sodium pool where they will likely be caught by the primary sodium pool filter.
For additional certainty each active fuel bundle has its own dedicated Vee liquid sodium entrance filter. Behind each filter is a cross channel which ensures proper fuel bundle operation even if half of its Vee filter is blocked.
In normal FNR operation the minimum temperature of the primary sodium is kept at about 340 C to prevent formation of solid NaOH which might potentially deposit on heat exchange surfaces to reduce heat transfer or might obstruct the liquid sodium coolant flow.
The active fuel bundles are designed so that they rely on their nearest neighbors for operation. If the position of a mobile fuel bundle fails to respond to control signals that mobile fuel bundle can be shut down by shutting down its 8 nearest neighbour mobile fuel bundles. This reactor feature also allows implementation of two completely redundant reactor emergency shutdown systems.
THERMAL POWER CONTROL:
Normal thermal power control in a FNR is achieved by material thermal expansion and contraction of sodium and steel. The amount of insertion of a mobile fuel bundle into the matrix of fixed fuel bundles determines the fuel bundle discharge temperature setpoint. As the fuel bundle liquid sodium inlet temperature drops below the discharge temperature setpoint the fuel bundle thermal power increases. However, care must be taken because under a very heavy thermal load the fuel and fuel tube temperatures could potentially exceed their material ratings. On reactor turn-on the fuel bundle discharge temperature setpoint must be raised gradually over about a half hour time period so that the fuel bundle discharge temperature setpoint does not exceed the fuel bundle inlet temperature by more than 160 degrees C.
Once the reactor reaches normal operating conditions the reactor thermal power is controlled by varying the intermediate sodium flow rate. Under these circumstances the role of thermal expansion of the primary liquid sodium is to keep the fuel bundle discharge temperature in the range 490 degrees C to 511 degrees C.
The reactor thermal output power is sensed via the intermediate sodium temperature differential and intermediate sodium flow rate. It is important to keep the reactor thermal output power below its design maximum to prevent fuel rod center line melting and/or damage to the fuel tubes.
Mobile fuel bundles are withdrawn to achieve a reactor cold shutdown. On loss of control system argon pressure all the mobile fuel bundle position control full port actuator drain valves open and gravity causes the mobile fuel bundle to withdraw from the matrix of fixed fuel bundles. Under these circumstances the primary liquid sodium pool will gradually cool and argon in bladder tanks will flow into the reactor space to maintain one atmosphere of argon pressure in that space. Otherwise the integrity of the reactor enclosure may be threatened by the decrease in cover gas argon pressure as the primary sodium temperature decreases. Note that argon flowing into the bladder tanks must be cooled to protect the bladder material.
If the gamma flux for a particular fuel bundle exceeds specification, indicating potential fuel bundle over heating, that fuel bundle should be immediately shut down.
If a fuel bundle discharge temperature gets too high, indicating fuel bundle overheating, that fuel bundle should be immediately shut down.
If an indicator tube top rises too high, indicating a potential active fuel bundle control portion positioning system problem that fuel bundle should be immediately shut down.
REDUNDANT SHUTDOWN SYSTEMS:
For safety purposes each mobile fuel bundle must remain subcritical when it is fully inserted but the eight nearest neighbor mobile fuel bundles are all withdrawn. This feature ensures an immediate safe reactor cold shutdown in the presence of a single mobile fuel bundle position jam or actuator failure. Note that under this criteria the thermal power output from the outer most ring of active fuel bundles may be relatively small. For each mobile fuel bundle in the outer ring of mobile fuel bundles at least three of the eight nearest neighbor mobile fuel bundles are already missing.
Reactor safety is further improved by requiring that the reactor will fully shut down if two mobile fuel bundles jam. If two jammed mobile fuel bundles are detected all the remaining mobile fuel bundles should be immediately withdrawn to force a reactor cold shutdown.
If a mobile fuel bundle jams such that it will not withdraw due to gravity the overhead gantry crane can push down the corresponding indicator tube to release the jammed control portion. If there is a jam in a fuel bundle's hydraulic actuator system that actuator should be shut down and the problem fixed at the next opportunity.
The individual fuel bundles are engineered such that when they are outside the reactor environment they remain sub-critical. When the fuel bundle assembly is formed with the mobile fuel bundles withdrawn the reactor must always be sub-critical. When all of the mobile fuel bundles are fully inserted the reactor should operate at or above its 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 fuel bundle discharge temperature distribution is monitored with an overhead scanner. In normal full power operation the discharge temperatures should be uniformly at 500 degrees C. As the reactors thermal load decreases the fuel bundle discharge temperature should rise to about 510 C at which temperature the reactor should become subcritical.
In normal operation the mobile fuel bundle vertical position setpoint is adjusted so that at full rated reactor power the gamma / neutron outputs from every interior active fuel bundle are identical. This state corresponds to uniform thermal power loading of the mobile fuel bundles.
The vertical position of a mobile fuel bundle is indicated by the height of the top of its indicator tube. In normal reactor operation the vertical position setpoints of the various 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 the cooling required to prevent fuel and fuel tube damage if an isolated cooling channel becomes blocked.
A liquid sodium flow obstruction will cause a local primary sodium flow decrease and hence a corresponding temperature increaase and hence a corresponding local decrease in gamma / neutron flux as compared to neighboring fuel bundles. If a relative decrease in gamma/neutron flux is detected that mobile fuel bundle 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.
Unlike the EBR-2 the fuel tubes are in a square array instead of a hexagonal array. The square array is more tolerant of fuel tube swelling and allows use of fuel bundle cross rods to stabilize the positions of the individual fuel tubes.
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. The discharge temperature of each mobile fuel bundle is indicated by the liquid sodium temperature inside the indicator tube.
TWO INDEPENDENT SAFETY SHUTDOWN SYSTEMS:
In plan view the mobile fuel bundles can be divided into two groups, a "red" group and a "black" group as on a checker board.
Each "red" mobile fuel bundle has four surrounding "black" mobile fuel bundles. Each "black" mobile fuel bundle has four surrounding "red" mobile fuel bundles. Neutrons in the core region flow in three dimensions. Hence shut down of either all the "red" mobile fuel bundles or all the "black" mobile fuel bundles shuts down the entire reactor. Hence there are two independent reactor shutdown systems, one of which shuts down all the "red" mobile fuel bundles and the other which shuts down all the "black" mobile fuel bundles. For reactor service both groups of mobile fuel bundles are shut down.
Reliable operation of these two independent shutdown systems may impose constraints on the FNR core rod fuel alloy mix and hence on the FNR core rod fuel reprocessing cycle time.
The withdrawal of each fuel bundle is accomplished by removing argon control pressure from a normally open full port actuator drain valve. When this argon pressure is removed the valve opens by gravity acting on a weight which rapidly drains liquid sodium from the mobile fuel bundle actuator.
REACTOR PRIMARY AND INTERMEDIATE COOLANTS:
The reactor primary and intermediate 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 breeder reactor the presence of potassium in the primary sodium leads to increased neutron absorption. The best solution to this problem is to not use potassium in the first place. Hence the primary coolant should be pure sodium.
The intermediate 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 intermediate coolant will mix with the primary coolant. Hence to keep potassium out of the primary coolant the intermediate coolant must also be pure sodium.
REACTOR THERMAL POWER:
The reactor thermal output power is sensed via the intermediate sodium temperature differential and the intermediate sodium flow rate. It is important to keep the reactor thermal output power below its design maximum 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 85% 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 fission 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 to allow for sealed containment of inert gaseous fission products at allowable pressures. Another important role of the long fuel tubes is to act as chimneys that enhance primary sodium natural circulation.
PRIMARY SODIUM THERMAL POWER CONTROL:
As the primary liquid sodium temperature increases 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 of 511 degrees C. As heat in excess of fission product decay 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 mobile fuel bundles.
FISSION PRODUCT DECAY HEAT REMOVAL:
One of the most important aspects of fission reactor design is provision for fission product 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% - 8% of the reactor's full power rating. It is essential to have a 100% reliable means of ensuring ongoing removal of the fission product decay heat under accident conditions such as after 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 liquid sodium thermal contact with the intermediate heat exchange bundles 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 walls will not lower the primary liquid sodium level more than 4 m, so that (1 / 3) of the heat removal capacity of the heat exchange bundles is retained. This condition restricts the volume of the fire brick between the primary sodium containment walls.
3) Even if the intermediate loop sodium pumps fail there must be enough intermediate liquid sodium natural circulation to ensure safe removal of the fission product decay heat.
4) The intermediate liquid sodium dumps its heat into steam generators. Hence on a reactor shutdown the pressure in the steam generators must be released so that the system that injects cool water into the steam generators in emergency conditions does not face a back pressure load.
5) There must be enough clean water in storage, above the elevation of the steam generators, such that the steam generators can be gravity fed and the fission product decay heat can be removed by evaporating that water and condensing the resulting steam in adjacent natural draft cooling towers.
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 fuel melting. In response to local overheating the adjacent mobile fuel bundles must be immediately withdrawn to ensure shutdown of the defective fuel bundle.
If fuel melting occurs fuel alloy droplets might collect on the primary liquid sodium pond floating floor under the fuel tube assembly. It is essential that these droplets do not accumulate together to form a critical mass. The floating floor must have a covering 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 floating floor cover.
PRIMARY LIQUID SODIUM FLOW:
The 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, down to the bottom of the primary sodium pool and then horizontally along the bottom of the primary liquid sodium pool back to below the reactor.
REACTOR SHUT DOWN CONTROL:
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. If a problem is detected the liquid sodium hydraulic pressure falls to zero and gravity causes the mobile fuel bundle to withdraw 1.2 m from the fixed fuel bundle matrix.
REACTOR MONITORING ELECTRONICS:
A FNR is controlled by several micro-computers. The main function of these computers is to safely maintain the desired fuel bundle discharge temperature setpoint 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 and the reactor fuel bundle discharge temperature control systems. If any such problems are detected the reactor control must either shift to a fully redundant electronic monitoring and control system or trigger a reactor shutdown.
SECONDARY LIQUID SODIUM FLOW:
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 induction type circulation pump loses power. The equipment should be sized so that the natural circulation rate is sufficient to safely remove fission product decay heat after the reactor is shut down.
FUEL BUNDLE ASSEMBLY SUPPORT:
The fixed fuel bundles are supported by 1.5 m high legs that plug into sockets on the top of the 1.5 m high bottom steel lattice located on the bottom of the primary sodium pool. The indicator tubes are attached to the top of the mobile active fuel bundles and are horizontally stabilized by the buoyancy of the indicator tubes and their surrounding steel floats. The fixed fuel bundles at their tops are bolted to their four 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 air lock size must be sufficient to allow extraction and replacement of individual fuel bundles and individual intermediate heat exchange bundles. During the fuel bundle removal process used fuel bundles are lifted 0.5 m to clear the fuel bundle support sockets and then are then moved horizontally to the perimeter 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. Earthquakes will cause sloshing of the liquid sodium.
The gantry crane is located above maximum anticipated liquid sodium height. The bricks forming the pool thermal insulation must have surrounding structural steel elements that firmly stabilize the outer steel wall to the surrounding concrete walls and to upper level steel walls to prevent a structural failure in severe earthquake conditions. Hence 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.
The concrete walls surrounding the primary sodium pool are stabilized on the outside by earth/rock embankments with a one unit rise for each two horizontal units. These embankments should be drained at the bottom, should have grass growing on top with sets of concrete stairs. Rain should either run off the surface or should drain out via a sloped drains near the bottom. Even if there is a violent earthquake causing a total sodium pool rupture this embankment must contain the liquid sodium and must prevent rain or flood water reaching the sodium.
The main chemical threat from a power FNR is the 4600 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 main 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 release of air borne plutonium and fission products. It is essential that the reactor be designed and sited such that a sodium fire cannot occur. In order to extinguish a sodium fire the oxygen concentration over the sodium must be minimized and heat must be extracted from the sodium. Under no circumstances can water be allowed anywhere near the sodium. Perhaps a high molecular weight inert gas such as radon, xenon, krypton could be used to extinguish a sodium fire. The key issue is to not have any makeup fresh air flow that could blow the high molecular weight inert gas away.
Small sodium fires can be extinguished with sodium carbonate.
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 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 sodium fire into a major disaster.
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 temperature below 200 degrees C, the threshold for spantaneous combustion of sodium in air. As this heat is dumped stored argon molecules from bladders in storage silos must be added to the cover gas to maintain the 1 atmosphere pressure in the argon cover gas.
Once the liquid sodium temperature is down to about 120 degrees C the surface of the liquid sodium can be flooded with a thin layer of low density oil such as kerosene to prevent the liquid sodium oxidizing during 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 cooling capacity. The fastest way to emergency cool the system is to directly vent steam from the steam generator discharges. It is important to have enough water in tank storage in or near the steam generator building to remove the fission product decay heat via latent heat of vaporization. Then the limiting factor is the maximum safe heat transfer capacity of the intermediate heat exchanger tube bundles and the steam generator tube bundles. If there is a FNR roof failure it is essential to prevent this steam condensing and falling onto the exposed liquid sodium surface. This issue highlights the importance of FNR enclosure ceiling integrity. As a minimum ther should be at least two structurally independent ceiling support systems.
Molten aluminum is far more dangerous than liquid sodium yet many millions of tons of aluminum are refined from bauxite every year with few accidents of note. The Hall-Heroult electrothermic reduction cells operate at around 830 C. If aluminum catches fire the last thing one dares put on it is water. Also, you dare not let water come in contact with molten aluminum for it will explode in your face. An molten aluminum fire has to be smothered with sand or similar.
The liquid sodium fearing people need to take a look at hazardous realities that are far more dangerous and are being handled safely on large scale in many countries around the globe. The fear of liquid sodium is like the fear of low doses of radiation. It is being amped up by people who should know better. To be fair, molten aluminum forms an oxide film that prevents it from catching fire readily in air while molten sodium will quickly ignite in air. Aluminum dust (fuel for solid propellant rockets) explosions are among the most devastating known but the plants get rebuilt anyway.
A significant public concern is that FNRs be engineered and operated in a manner that does not allow bad actors to obtain Pu-239 in a form suitable for making atomic bombs. The solution to this problem is to exchange FNR fuel bundles on a first-in first-out basis so that every fuel bundle, in addition to containing Pu-239 also contains a sufficient fraction of Pu-240 to prevent the contained Pu being used for bomb manufacture. Pu-240 cannot be chemically separated from Pu-239 and is extremely difficult to physically separate from Pu-239. In a bomb assembly Pu-240 causes pre-ignition, which prevents a large scale explosion. Ensuring first-in first-out exchange of FNR fuel bundles requires keeping a non-volatile record of the neutron flux exposure history of every FNR fuel bundle.
INTERMEDIATE HEAT EXCHANGE BUNDLES:
Heat is removed from the FNR via intermediate heat exchange bundles. The heat exchange tubes have low pressure radioactive primary liquid sodium on the outside and have high pressure non-radioactive intermediate liquid sodium on the inside. The high pressure intermediate 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 external cooling water.
This web page last updated November 9, 2019
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