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

This web page is concerned with practical details related to FNR operation. It is presumed that before reviewing this web page students have already studied the web pages titled:

Consider the core zone of an ideal FNR that has uniform fuel geometry throughout the core zone. If the neutron generation rate in the core zone is slightly greater than the neutron loss rate in the core zone the core zone temperature will rise until the neutron loss rate equals the neutron generation rate. Then if the liquid sodium is stationary there will be no fission heat injection and the core temperature will remain constant.

However, if cooler sodium flows into the core zone the neutron generation rate in the core zone will rise and the neutron loss rate will fall which will cause a neutron surplus and hence local fission heat injection. This heat will cause sodium thermal expansion until the neutron generation rate and neutron loss rates are again equal.

Thus if cooler sodium flows upwards through a FNR core the temperature of the flowing sodium will rise to keep the rate of generation of fission neutrons equal to the rate of loss of fission neutrons.

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 temperature set by the fuel geometry gradually rises. Then as an atom of sodium rises through the core zone the local sodium temperature also gradually rises. As the flowing liquid sodium rises past the top of the core zone, due to the changing local fissile fuel atom concentration the local temperature setpoint will drop. Hence above the top of the core zone there is no more nuclear heat injection into the sodium except due to fission caused by fringe neutrons.

Similarly, there is a small amount of fission heat injection in the lower core zone due to fringe neutrons.

In the blanket regions there is negligible fission heat injection because the fissile fuel concentration in the blanket regions is very small.

Thus in summary the main region of nuclear heat injection is the core zone.

The core zone of a FNR has a pancake like geometry which gives it a large [(surface area) / (volume)] ratio. In a FNR core zone a major cause of neutron loss is neutron diffusion out of the core zone. The neutron diffusion loss rate increases with increasing core zone temperature while the neutron production rate via fission reactions decreases with increasing core zone temperature. In a FNR operating at its setpoint temperature the free neutron production rate equals the free neutron loss rate and the heat output is proportional to the number of free neutrons. If the core zone temperature drops due to heat removal by flowing liquid sodium the fission reaction rate increases and the neutron loss rate by diffusion decreases which together increase core zone heat production to restore the core zone temperature to its set point. If the core zone temperature is above its set point the fission reaction rate decreases and the neutron loss rate by diffusion increases. Hence a FNR reduces its free neutron concentration and core zone heat production to maintain its core zone temperature setpoint.

If the reactor's external heat load is less than the reactor thermal power output the excess heat is absorbed by the liquid sodium causing the liquid sodium temperature beside the fuel assembly to increase, reducing the primary sodium natural circulation which turns off the chain reaction and hence causes the reactor's thermal power output to drop to zero.

If an external heat load removes heat from the liquid sodium the liquid sodium temperature beside the fuel assembly decreases which restarts primary liquid sodium natural circulation and hence the nuclear chain reaction restarts. The reactor core zone fuel runs at a constant temperature of about 520 degrees C. The rate of heat extraction from the core zone fuel varies with the elevation of the liquid sodium thermal stratification layer.

Thus when the mobile fuel bundle insertion into the fixed fuel bundle matrix is correct the FNR thermal power output automatically varies to meet the external thermal load.

When the FNR is operating at full rated power liquid sodium coolant enters the bottom of FNR fuel tube bundles at about 340 C, flows upwards through the flow channels between the active fuel tubes, and emerges from the top of the active fuel tube bundles at about 490 C. The fuel operating temperature setpoint (520 degrees C) of each active fuel bundle is controlled by the amount of mobile fuel bundle insertion into the matrix of fixed fuel bundles. Withdrawing a mobile fuel bundle from the matrix of fixed fuel bundles reduces the fuel temperature setpoint.

The lower density hot liquid sodium rises to the top surface of the liquid sodium pool, flows across the top of the liquid sodium pool and near the pool edge enters the intermediate heat exchangers. Primary sodium with a temperature of about 340 degrees C emerges from the bottom of the intermediate heat exchangers and falls to the bottom of the primary sodium pool. This cooler higher density primary liquid sodium flows along the bottom of the primary liquid sodium pool and then rises again through the fuel bundle flow channels.

The fuel tubes emit heat at a variable rate which at full load raises the primary liquid sodium discharge temperature from the reactor core zone coolaant channels to about 490 deg C. The mechanism which controls the fuel tube heat emission rate is thermal expansion of the fuel, steel and liquid sodium. Due to its lower density and hence buoyancy hot liquid sodium naturally rises vertically through the coolant channels between the fuel tubes. With no thermal load, when the upper 6.6 m of primary liquid sodium reachs 520 deg C the primary sodium natural circulation stops and heat production stops.

The reactor thermal power is controlled by controlling the rate at which heat is extracted from the intermediate heat exchangers. Controllable heat extraction is realized by use of a lower temperature steam generator heat sink and by controlling the flow rate of secondary liquid sodium flowing between the intermediate heat exchanger and the stean generator. A makeup water recirculation pump serving the lower portion of each steam generator is used to reduce thermal stress on the steam generator heat exchange tubes that are immersed in water and hence provide liquid to liquid heat transfer.

The reactor thermal power output is limited by the rate of extraction of heat from the primary liquid sodium pool. This heat extraction rate is a function of the difference between the secondary liquid sodium supply temperature and the thermal load temperature as well as the secondary liquid sodium flow rate. The maximum rate of heat extraction must be kept within reactor fuel tube design limits. Otherwise the reactor fuel tubes could overheat.

The FNR heat extraction system is designed to limit the maximum heat extraction rate while the reactor is operating to the maximum rated thermal output power of the fuel tubes. Otherwise the core fuel could potentially overheat and crack the fuel tubes. Similarly, during a reactor cold start the safe rate of liquid sodium pool warmup is again limited by the thermal output power rating of the active fuel tubes.

When the primary sodium surface temperature reaches 500 degrees C that surface glows faint red. To put this temperature in perspective the melting points of some common metals are:
sodium = 98 C, tin = 232 C, cadmium = 321 C, lead = 327 C, zinc = 419.5 C, iron = 1538 C and chromium = 1907 C.

Advantages of higher temperature FNR operation, as compared to water moderated reactors, include increased efficiency in electricity generation and reduced embrittlement of fuel tubes. However, at still higher temperatures there may be significant problems related to plutonium in the core fuel rod alloy melting at 640 deg C and chemically interacting with iron in the fuel tube alloy. A thermal analysis which takes into account the temperature drop across the core fuel rod radius indicates that 520 degrees C is a prudent fuel set point temperature.

When there is no external thermal load the heat extraction rate is zero. When the top 6.6 m of primary sodium reach 520 degrees C the nuclear chain reaction stops. Hence with no thermal load the primary liquid sodium pool surface temperature stabilizes at 520 degrees C. However, there may still be fission product decay heat. If the primary liquid sodium pool temperature exceeds 530 degrees C a cool shutdown is triggered.

Each mobile fuel bundle can have its insertion position adjusted to allow precise setting of that fuel bundle's operating temperature.

If there is a step change in FNR core zone thickness the fission rate and hence the fission gamma photon flux and the prompt neutron flux respond almost instantly. However, the liquid sodium temperature, which limits the fission power, takes longer to respond.

To prevent uncontrolled explosive power growth FNRs should always remain subcritical with respect to prompt fission neutrons, which constitute about 99.8% of the total neutron flux. The remaining 0.2% of the total neutron flux consists of delayed neutrons from fission fragments emitted about 3 seconds after the corresponding nuclear fission. Provided that most of the delayed neutrons participate in reactor power control, the rate of fission power growth is safely limited by the rate of delayed neutron production. This time delay in reactor power growth normally allows sufficient time for the liquid sodium temperature to rise and suppress the core reactivity to safely control the fission power in a FNR.

There is a requirement that during a reactor cold startup the insertion rate of the mobile fuel bundles must be sufficiently low to prevent fuel rod melting and to prevent the FNR becoming critical on prompt neutrons. This insertion rate limit is ensured via flow orifices on the hydraulic actuator positioning valves as well as by programmed actuator position slew rate limits. Hence the insertion of the mobile fuel bundles into the matrix of fixed fuel bundles must be very slow and very carefully controlled. By contrast on loss of control power the withdrawal rate of the mobile fuel bundles is very fast. To enable fast mobile fuel bundle withdrawal a parallel connected full port hydraulic actuator drain valve is used to achieve rapid fuel bundle cool shutdown. The hydraulic control valves must be rated for continuous use with liquid sodium at the highest possible liquid sodium operating temperature. To achieve the required temperature isolation these valves should be argon pressure actuated. On loss of argon pressure gravity should cause the full port drain valve to open. The argon pressure to each actuator drain valve is controlled by an electronic transducer located in a cool environment outside the reactor space.

In FNR operation the reactor fuel temperature is controlled by the fuel geometry. The heat production rate is controlled by the liquid sodium temperature with respect to the fuel temperature. A rise in local sodium temperature causes sodium thermal expansion which causes liquid sodium natural circulation.

At the transition between the lower blanket and the core zone bottom care must be taken must be taken to ensure that the local nuclear heat injection is not so large as to cause thermal stress failure of the fuel tubes.

Parameters of importance in practical FNR operation are the local mobile fuel bundle penetration of the fixed fuel bundle matrix, local liquid sodium discharge temperature and the local gamma radiation flux. These parameters may be affected by mobile fuel bundle insertion change, long term change in the core fuel fissile atom concentration or by shorter term change in liquid sodium coolant flow through the core zone due to flow obstruction. Decrease in fissile atom concentration is compensated for by adjustment of the local core zone thickness so that the liquid sodium discharge temperature remains uniform at its target value over the reactor core zone area.

To prevent liquid sodium flow obstruction it is necessary to keep the liquid sodium coolant clean, well filtered and free of impurities that can form solids in the liquid sodium operating temperature range. The liquid sodium should not contain any potassium which could potentially form KOH. At liquid sodium temperatures below 360 degrees C KOH will precipitate. If not filtered out it could potentially cause flow and heat transfer problems.

To detect a local sodium flow obstruction it is necessary to have gamma and thermal radiation monitoring equipment that can detect formation of local hot spots in the reactor. Should such a hot spot form the affected reactor region should be temporarily disabled by reducing the core thickness at the hot spot until the next reactor shutdown for maintenance or fuel exchange.

In addition to ongoing liquid sodium filtering an important periodic maintenance procedure is to temporarily remove the reactor thermal load and allow the reactor to raise the temperature of all of its steel and liquid sodium components up to 520 degrees C. This maintenance procedure will dissolve most impurities that deposit on cooler surfaces and will anneal reactor related steel components.

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 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 circulation of secondary sodium. In order to meet this requirements the primary sodium surface level must not fall below the intermediate heat exchanger primary sodium inlet port. This requirement effectively imposes geometrical constraints on the primary sodium pool wall design.

As in all nuclear reactors, during FNR operation geometric stability of the reacting fuel 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 tend to reduce reactivity whereas sodium voids tend to increase reactivity. The fuel tube melting point is much greater than the temperature at which sodium voids can form within the fuel tubes. Formation of sodium voids in the fuel tubes will tend to blow the fixed fuel bundle core fuel rods into the fuel tube plenums causing 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 will cause a rapid increase in fuel temperature that might damage the fuel tubes due to excessive thermal stress. However, rapid withdrawal of the mobile fuel bundles is permitted during a reactor cool 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 cool shutdowns use separate full port hydraulic valves to enable rapid mobile fuel bundle withdrawal from the matrix of fixed fuel bundles.

The bottom of each octagonal fuel bundle plugs into a socket. The top of each octagonal fuel bundle is physically bolted to its four nearest neighbour octagonal fuel bundles to realize fixed fuel bundle matrix stability.

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 of fuel 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 cool shutdown.

The secondary sodium receives a circulation assist from electric induction pumps that are located on the secondary sodium return pipes from the steam generators. If there is a loss of secondary sodium level or secondary sodium pressure in a particular heat transport loop that entire heat transport loop is automatically shut down.

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 or too far insertion of a mobile fuel bundle which could potentially lead to fuel bundle thermal stress damage or fuel melting. 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 portion of the reactor into a sub-critical state.

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 fuel 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 fuel cannot stop the fission chain reaction at an acceptable fuel 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.

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.

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 150 degrees C.

Once the reactor reaches normal operating conditions the reactor thermal power is controlled by varying the secondary sodium flow rate. Under these circumstances the role of thermal expansion of the core fuel is to keep the fuel temperature at 520 degrees C.

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 below its design maximum to prevent fuel rod center line melting and/or damage to the fuel tubes.

There are three reactor shutdown states, warm shutdown, cool shutdown and cold shutdown.

A warm shutdown is achieved by setting the intermediate sodium induction pumps for about 8% of the normal full power flow. This is the normal minimum thermal power setting for the reactor. Due to lack of thermal load the primary liquid sodium temperature rises until the nuclear chain reaction stops. Fission product decay heat continues to be produced at about 8% of the reactor full power capacity. Steam continues to be generated at 11.25 MPa.

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 bundles 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.

In a cool shutdown the intermediate sodium induction pumps continue to operate, the steam pressure remains at 11.25 MPa and the primary sodium pool gradually cools to approach the steam generator water temperature of 320 degrees C.

A cold shutdown is like a cool shutdown except that the steam generators are vented to the atmosphere. The water temperature in the steam generators drops to 100 degrees C. There is enough natural circulation of intermediate sodium that intermediate sodium induction pump operation is not required. The primary sodium pool temperature gradually drops to approach the 100 degree C water temperature in the steam generators.

In a cold shutdown the primary safety concern is ongoing supply of sufficient clean water to the steam generators for removal of fission product decay heat.

Any reactor safety shutdown causes a total system cool shutdown. Any loss of induction pump power causes a further corresponding heat transport system cold shutdown.

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.

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.

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.

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

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.

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 go to cool 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 cool 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.

A liquid sodium flow obstruction will cause a local primary sodium flow decrease and hence a corresponding temperature increase 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.

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 fuel bundle materials should cause the fuel bundle reactivity to drop below criticality.

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.

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.

There are two completely redundant nuclear reaction shutdown systems. These systems operate by gravity withdrawal of every second mobile fuel bundle. Hence in total there are two independent nuclear reaction cool shutdown systems as well as the warm shut down system which normally maintains the reactor setpoint temperature.

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.

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 primary liquid sodium thermal contact with the intermediate heat exchange tube 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 3 m, so that the heat removal capacity of the intermediate heat exchangers 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 natural circulation of secondary liquid sodium to ensure safe removal of the fission product decay heat.
4) The secondary liquid sodium dumps its heat into steam generators. Hence on a reactor cold shutdown the pressure in the steam generators must be released so that the pumps that inject water into the steam generators in emergency conditions do not face a back pressure load.
5) There must be enough clean water in storage, preferrably 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 if possible condensing the resulting steam in adjacent natural draft cooling towers.

Heat is removed from the FNR via the intermediate heat exchange tube bundles. These heat exchange tubes normally operate at a temperature of up to 520 degrees C, have low pressure radioactive liquid sodium outside the tubes and high pressure non-radioactive sodium inside the tubes. Thus if there is an intermediate heat exchanger tube failure a limited volume of nonradioactive sodium flows into radioactive sodium, which is not a serious problem. If there is a steam generator tube failure non-radioactive sodium flows slowly into the water side of the steam generator tubes due to the non-radioactive sodium being kept at a slightly higher pressure than the steeam generator water/steam.

Another major constraining issue is the combined thermal stress and internal pressure stress in the heat exchange tubes which form the intermediate heat exchanger. In addition to internal pressure the intermediate heat exchanger can have a significant temperature differential across the heat exchange tube wall. This temperature differential can potentially lead to high thermal stress at the point where the cool secondary return sodium is first heated by the primary liquid sodium. This problem is minimized by keeping both the primary and secondary liquid sodium temperatures stratified.

The thermal mass of the sodium prevents the fuel tubes from being exposed to thermal shock relating to very rapid changes in net electricity grid load. However, the intermediate sodium loop components and the steam generator and turbo-generator are exposed to thermal expansion and contraction related to following rapid changes in net grid electricity load. Hence the heat exchange tubes in both the intermediate heat exchanger and the steam generator should be made from Inconel.

One of the issues with Inconel is long term creep. This issue is particularly important in the intermediate heat exchanger.

In the steam generator the material stress due to differential pressure across the tube wall is relatively small because the liquid sodium pressure is controlled to track the steam pressure. However, the thermal stress can be very large at the point where cool inlet water to the steam generator is first heated by warm liquid sodium that is on its way back to the induction pump and then the intermediate heat exchanger.

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 main chemical threat from a power FNR is the 4000 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 major 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, and a gas tight suspended metal ceiling. In the event of air penetration into the argon cover gas the reactor should immediately go to cold shut down and the heat contained in the primary sodium pool should be discarded to lower the primary 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 kerosene to prevent the liquid sodium oxidizing during subsequent 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 generators. It is important to have enough water in on-site tank storage to remove the fission product decay heat via latent heat of vaporization of water. Then the limiting factor is the maximum safe heat transfer capacity rate of the intermediate heat exchangers and the steam generators.

If there is a FNR roof failure it is essential to prevent the vented steam from condensing and falling onto the exposed liquid sodium surface. This issue highlights the importance of FNR enclosure ceiling integrity.

Quite apart from the potential release of Na2O and NaOH the big threat is melting of the fuel tubes leading to potential release of air borne plutonium and fission products. It is essential that the reactor be designed and sited such that a ceiling collapse in combination with an uncontrolled sodium pool fire cannot occur. In order to extinguish a sodium pool 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. The key issue is to not have any makeup fresh air flow.

Small sodium fires associated with pipe joint leaks 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 all three 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.

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 readily catching fire 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.

The history of liquid sodium cooled fast neutron reactors is repleat with delays related to sodium fires. The only practical way to make and operate a liquid sodium cooled power reactor is to design it so that sodium fires are small, well contained, easily extinguished and of little consequence.

This web page last updated August 15, 2020

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