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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 studying this web page students have already studied the web pages titled:

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Consider the core zone of an ideal FNR that has uniform fuel geometry throughout the core zone. The nuclear fuel is subject to thermal expansion. At some temperature To the neutron generation rate equals the neutron loss rate. If due to a fuel geometric change the neutron generation rate in the core zone rises slightly above the neutron loss rate in the core zone there will be fission heat generation and the core zone temperature will rise until the neutron generation rate decreases to equal the neutron loss rate. Then if the liquid sodium coolant is stationary there will be no fission heat injection and the core zone temperature will remain constant.

However, if liquid sodium with temperature T < To flows into the core zone the neutron generation rate in the core zone will rise which will cause a neutron surplus and hence local fission heat injection. This heat will cause sodium and fuel 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 zone the temperature set point determined 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 and fuel causing the core zone liquid sodium temperature to increase, which reduces the primary sodium natural circulation and turns off the nuclear chain reaction, causing the reactor's thermal power output to drop to zero.

If an external heat load removes heat from the liquid sodium the liquid sodium core zone temperature decreases which restarts both primary liquid sodium natural circulation and the nuclear chain reaction. The FNR attempts to maintain a constant coolant discharge temperature. The rate of heat extraction from the core zone fuel varies with primary sodium flow rate which varies with the elevation of the primary liquid sodium thermal stratification layer.

Thus when the movable 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 410 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 460 C. The fuel operating temperature setpoint of each active fuel bundle is controlled by the amount of movable fuel bundle insertion into the matrix of fixed fuel bundles. Withdrawing a movable fuel bundle from the matrix of fixed fuel bundles reduces the fuel temperature setpoint. When the setpoint is less than the primary liquid sodium coolant temperature the fission reactions stop.

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 exchanger bundles. Primary sodium with a temperature of about 410 degrees C emerges from the bottom of the intermediate heat exchanger bundles 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 coolant channels to about 460 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 primary liquid sodium inlet temperature reaches the FNR setpoint temperature 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 variable speed induction pumps to control the NaK flow rate through the intermediate heat exchange bundles.

Steam generator heat exchange tube sleeves are used to reduce thermal stress on the steam generator heat exchange tube portions that are immersed in steam generator water and hence are subject to the thermal stresses associated with 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 NaK supply temperature and the thermal load temperature as well as the NaK liquid 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 maximum safe rate of primary liquid sodium pool warmup is again limited by the thermal output power rating of the active fuel tubes.

If 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 and tolerance for blockage of two adjacent sodium cooling channels indicates that 460 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 10.8 m of primary sodium reach 460 degrees C the nuclear chain reaction stops. Hence with no thermal load the primary liquid sodium pool surface temperature stabilizes at 460 degrees C. However, there may still be fission product decay heat. If the average primary liquid sodium pool temperature continues to rise and exceeds 470 degrees C a cold shutdown is triggered.

Each movable fuel bundle can have its insertion position adjusted to allow precise setting of that fuel bundle's average 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 over 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 allows sufficient time for the fuel 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 movable 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 by programmed actuator position slew rate limits. Hence the insertion of the movable fuel bundles into the matrix of fixed fuel bundles is very slow and very carefully controlled. By contrast on loss of control power the withdrawal rate of the movable fuel bundles is relatively fast.

The hydraulic actuators must be rated for continuous use with liquid sodium at the highest possible liquid sodium operating temperature. To achieve the required temperature isolation the relevant Na control valves should be argon pressure actuated. 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 circulating primary liquid sodium temperature with respect to the fuel temperature. A rise in local primary sodium temperature causes local 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 to ensure that the local nuclear heat injection is not so large as to cause thermal stress failure of the fuel tubes. This issue will limit the maximum temperature difference between the top and bottom of the fuel tubes. If the load temperature difference is greater the return sodium must be warmed before it reaches the bottomof the fuel tubes.

Parameters of importance in practical FNR operation are the local movable 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 movable 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 reactor 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. At liquid sodium temperatures below 360 degrees C KOH will precipitate. At liquid sodium temperatures below 318 degrees C NaOH will precipitate. If not filtered out these precipitates could potentially cause flow and heat transfer problems.

To detect a 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 zone 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 cup primary sodium enclosure, a middle steel cup primary sodium enclosure and an outer steel cup 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 reliable removal of this heat. A FNR has numerous independent heat transport systems, a fraction 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 NaK. In order to meet this requirement the primary sodium surface level must not fall more than 5 m. This requirement effectively imposes geometrical constraints on the primary sodium pool wall design. There is also a requ9rement for continuing pumped nitrate salt or pumped heat transfer fluid circulation.

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 a dangerous 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 inside the fuel tubes. Formation of sodium voids in the fuel tubes tends to blow the fixed fuel bundle core fuel rods toward the fuel tube plenums causing a chain reaction shutdown. The reactor is designed so that too rapid insertion of the FNR movable 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 fuel melting and excessive thermal stress. However, rapid withdrawal of the movable fuel bundles is permitted during a reactor cool or cold shutdown.

To ensure control stability the movable fuel bundle insertion rate and withdrawal rate are both normally electronicly limited. Movable fuel bundle insertion control is not used for normal ongoing reactor power control. Reactor cold shutdowns use maximum rate movable fuel bundle withdrawal from the matrix of fixed fuel bundles.

The bottom of each fixed fuel bundle plugs into a socket. The top of each fixed fuel bundle is physically bolted to its four nearest neighbour fixed 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 movable 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 control valves. These valves operate by gravity and by argon pressure. The argon pressure control transducers are located in cool space outside the primary sodium pool enclosure.

The NaK receives a circulation assist from electric induction pumps that are located in the NaK return pipes from the NaK/salt heat exchangers. If there is a loss of NaK level in a particular heat transport loop that entire heat transport loop is automatically shut down and its contained NaK flows into its dump tank.

The movable fuel bundle positioning system is robust and extremely reliable and the rate of insertion of the movable bundles is both mechanically and electronicly limited.

A situation that must be avoided is too rapid or too deep insertion of a movable fuel bundle which could potentially lead to fuel bundle thermal stress damage or fuel melting. Hence the rate of insertion of the movable fuel bundles is limited by both hardware and software.

To avoid potential problems related to too rapid movable fuel bundle insertion orifices on the hydraulic actuator control valves limit the movable fuel bundle insertion rate. If the reactor monitoring system detects that a movable fuel bundle insertion is either too deep or too fast or if the gamma / neutron flux from a movable fuel bundle is too high or if the fuel bundle discharge temperature is too high the reactor safety system causes rapid withdrawal of the relevant movable fuel bundle. Simultaneously the hydraulic liquid sodium flow to the relevant insertion control actuator ports is turned off.

As an additional safety measure if the position of a movable fuel bundle fails to respond to position control signals or if the emitted gamma flux is too high or if the movable fuel bundle discharge temperature is too high then the reactor control computer automatically causes withdrawal of the surrounding nearest neighbor movable 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 fuel bundle liquid sodium discharge may take several seconds to respond. Hence there is a significant transportation delay between an increase in reactivity and the corresponding increase in liquid sodium temperature. To avoid reactor power oscillations it is important to avoid use of the discharge temperature for direct reactor power control. Note that the delayed neutrons introduce another 3 seconds of transportation delay into the reactor thermal power control loop.

The movable fuel bundle insertion rate is limited to prevent the reactor going critical on prompt neutrons before the liquid sodium temperature has had time to respond to the increased reactor power level. The movable fuel bundles 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 sensing system must have good resolution with negligible hysterisis. The corresponding movable fuel bundle power is immediately indicated by the strength of the gamma / neutron radiation propagating up the hollow portion of that fuel bundle's indicator tube. The fuel bundle liquid sodium discharge temperature should respond to a change in movable fuel bundle insertion with a multi-second time lag. If either a fuel bundle gamma / neutron signal or a fuel bundle discharge temperature is too large the movable 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 movable 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 movable fuel bundle have three valid states: increase movable fuel bundle insertion, do nothing, reduce movable fuel bundle insertion. Normally the actuator valves are in the do nothing state. If the insertionis oscillating the control parameters are wrong.

On loss of argon control pressure gravity and/or spring operation of the control valves causes an immediate movable fuel bundle withdrawal.

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.

If there is a primary sodium flow obstruction the affected fuel element temperature will rise but that core fuel element will still be in the neutron flux. Hence the fuel temperature will locally rise. Hence the local reactor thermal power will be degraded.

The FNR control system assumes that all the liquid sodium cooling channels in a fuel bundle are open. However, it is important that the liquid sodium be sufficiently filtered to eliminate particulate matter that over time might obstruct the liquid sodium cooling channels between fuel tubes. Each fuel bundle has sufficient cross flow to tolerate a few isolated blocked liquid sodium cooling channels but the the thermal output will degrade if there is 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 inverted pyramid shaped liquid sodium entrance filter.________ Behind each filter section is a common space which ensures proper fuel bundle operation even if half of the filter faces are 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 cool heat exchange surfaces causing a reduction in 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 movable fuel bundle fails to respond to control signals that movable fuel bundle can be shut down by withdrawing its 4 nearest neighbour movable fuel bundles. This reactor feature also allows implementation of two completely redundant reactor emergency shutdown systems.

On reactor turn-on the fuel bundle temperature setpoint must be raised gradually over about a half hour time period so that the fuel bundle discharge temperature does not exceed the fuel bundle inlet temperature by more than 40 degrees C.

Once the reactor reaches normal operating conditions the reactor thermal power is controlled by varying the NaK flow rate throuh the intermediate heat exchange bundles. Under these circumstances the role of thermal expansion of the core fuel is to keep the FNR discharge temperature at about 460 degrees C.

The reactor thermal output power is sensed via the NaK temperature differential and NaK 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 NaK flow rate 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 almost 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.

Movable fuel bundles are partially withdrawn to achieve a reactor cool shutdown. Under these circumstances eventually the primary liquid sodium pool will cool and argon in the 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 would be threatened by the decrease in cover gas argon pressure as the primary sodium temperature decreases. Note that on reactor warmup argon flowing into the bladder tanks from the primary sodium pool space must be cooled to protect the bladder material.

In a cool shutdown the NaK induction pumps continue to operate, the steam pressure slowly drops and the primary sodium pool gradually cools to approach 120 degrees C. The nitrate salt is transferred toits dump tanks and heat is rejected to the steam generators using a heat transfer fluid.

A cold shutdown is like a cool shutdown except that the pressure in the steam generator top manifolds is vented to the atmosphere. The water temperature in the steam generators drops to 100 degrees C. There is enough natural circulation of NaK that NaK induction pump operation is not required for fission product decay heat removal. The primary sodium pool temperature gradually drops to approach the 100 degree C water temperature in the steam generators.

In a cold shutdown the main safety concern is an ongoing supply of sufficient clean water to the steam generators for removal of fission product decay heat and maintenance of argon pressure in at least some of the NaK dump tanks and heat transfer fluid dump tanks to ensure enough NaK circulation for fission product decay heat removal.

Any serious reactor safety threat causes a total system cold 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 withdrawn.

If a fuel bundle discharge temperature gets too high, indicating fuel bundle overheating, that fuel bundle should be immediately withdrawn.

If an indicator tube top rises too high, indicating a potential movable fuel bundle positioning system problem that fuel bundle should be immediately withdrawn.

Failure of an indicator tube position to track its position setpoint:
should cause immediate movable fuel bundle withdrawal.

The vertical position of a movable 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 an overhead laser scanner. The fuel bundle gamma / neutron emission is sensed by an overhead radiation monitoring apparatus. The discharge temperature of each movable fuel bundle is indicated by the liquid sodium temperature inside the indicator tube, which is sensed by an overhead infrared camera. This entire electronics package is contained within a thermally isolated and cooled enclosure. Measures must be taken to prevent sodium vapor condensation on the surfaces of the viewing ports.

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

The fuel bundle discharge temperature distribution is monitored with an overhead IR scanner. In normal full power operation the discharge temperatures should be uniformly at 460 degrees C.

In normal operation the movable fuel bundle vertical position setpoint is adjusted so that at full rated reactor power the gamma outputs from all the interior active movable fuel bundles are identical. This state corresponds to uniform thermal power loading of the movable fuel bundles.

For safety purposes each movable fuel bundle must remain subcritical when it is fully inserted but the eight nearest neighbor movable fuel bundles are all withdrawn. This feature ensures an immediate safe reactor cold shutdown in the presence of a single movable fuel bundle 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 movable fuel bundle in the outer ring of movable fuel bundles, at least three of the eight nearest neighbor movable fuel bundles are already missing.

Reactor safety is further improved by requiring that the reactor will go to cool shut down if two movable fuel bundles jam. If two jammed movable fuel bundles are detected all the remaining movable fuel bundles should be immediately withdrawn to force a reactor cool shutdown.

A liquid sodium flow obstruction will cause a local primary sodium flow decrease and hence a local temperature increase up to To with a local decrease in gamma / neutron flux as compared to neighboring fuel bundles. If a relative decrease in gamma/neutron flux is detected that movable 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 and separated from other fuel bundles they are always sub-critical. When the fuel bundle assembly is formed with the movable fuel bundles withdrawn the reactor must always be sub-critical. When all of the movable 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.

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

In plan view the movable fuel bundles can be divided into two groups, a "red" group and a "black" group as on a checker board.

Each "red" movable fuel bundle has four surrounding "black" movable fuel bundles. Each "black" movable fuel bundle has four surrounding "red" movable fuel bundles. Neutrons in the core region scatter in three dimensions. Hence withdrawal of either all the "red" movable fuel bundles or all the "black" movable fuel bundles shuts down the entire reactor. Hence there are two independent reactor shutdown systems, one of which withdraws all the "red" movable fuel bundles and the other which withdraws all the "black" movable fuel bundles. For reactor service both groups of movable fuel bundles are withdrawn.

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. The insert port has anormally closed hydraulic fluid valve and the withdrawal port has a normally open hydraulic valve. Hence when argonpressure is removed these valves go to thir normal state. The normally open valves allow gravitationally pressurized Na to flow through the hydraulic motors which cause the movable fuel bundles to withdraw.

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 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 walls fail and the primary liquid sodium leaks into the space between the inner and most outside pool wall, the leakage into the space between the walls will not lower the primary liquid sodium level more than 4 m, so that at least (1 / 3) of 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 NaK induction pumps fail there must be enough natural circulation of NaK to ensure safe removal of the fission product decay heat.
4) The NaK transfers its heat to nitrate salt or heat transfer fluid and then 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 and so that the steam generator water temperature drops to 100 degrees C.
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 venting the resulting steam to the atmosphere via the 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 460 degrees C, have low pressure radioactive liquid sodium outside the tubes and 0.5 MPa non-radioactive NaK inside the tubes. Thus if there is an intermediate heat exchanger tube failure a limited volume of nonradioactive NaK flows into radioactive sodium, which is not a serious problem. If there is a steam generator tube failure water jets into the nitrate salt at a rate limited by the tube throat diameter. The steam raises the pressure in the nitrate salt at a rate determined by the steam / water inflow rate. The rise in salt pressure:
a) Stops the injection water pump;
b) Opens the lower shell drain valve;
c) Opens the steam generator pressure relief valve to the condenser;
d) Causes the roof mounted ball check to open releasing the salt / steam to the atmosphere.

Stopping the injection water pump and opening the lower shell drain valve stops further liquid water entering the nitrate salt. Then the remaining steam pressure drives water out of the shell and into the drain. As the steam pressure drops off, so does further water entry into the nitrate salt. Meanwhile the rate of rise of NaK pressure is sufficiently contained to prevent damage to the intermediate heat exchange bundle.

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 NaK is first heated by the primary liquid sodium. This problem is minimized by keeping both the primary and secondary liquid temperatures stratified. Then the temperature difference across the heat exchange tube wall is minimized.

The thermal mass of the primary sodium prevents the fuel tubes from being exposed to thermal shock relating to rapid changes in net electricity grid load. However, the intermediate NaK loop components, the nitrrate salt 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 the intermediate heat exchanger, the NaK/salt 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 steam generator where both the structural stress and the thermal stress can be very large at the point where cool inlet water to the steam generator is first heated by warm nitrate salt that is on its way back to the NaK/salt heat exchanger , induction pump and then the intermediate heat exchanger. This thermal stress is minimized by use of a external tube sleeves near the bottom of the heat exchange tubes.

The fuel bundles are repositioned and/or replaced using a polar overhead gantry crane and remote manipulation. Note that the air lock sizes 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 8.1 m to clear other fuel bundles 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.

NaK and Na FIRES:
The history of liquid sodium cooled fast neutron reactors is repleat with delays related to NaK fires. In a sodium cooled power reactor secondary NaK fires are a fact of life. The power plant must be designed so that any secondary NaK fires are small, well contained, easily extinguished and of little consequence. In this respect the NaK heat transport loops are of small capacity and are operated at a relatively low pressure which minimizes potential NaK leakage problems at flanged connections. The NaK is contained within concrete enclosed heat exchange galleries. In the event of a NaK fire the relevant heat transport loop will rapidly drain its NaK contents to its argon covered dump tank.

Small NaK fires associated with the NaK pipe joint leaks can be extinguished with sodium carbonate. These fires are deprived of fuel by draining the NaK into its dump tank.

The main fire 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 primary sodium will NEVER be exposed to flood water.

The other major chemical threat is a potential reaction between hot liquid sodium and air. To mitigate this threat the primary liquid sodium is covered by floating steel spheres, an argon cover atmosphere, a gas tight suspended metal ceiling and a rugged roof. 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 spontaneous combustion of sodium in air. As this heat is dumped stored argon molecules from bladders in the argon storage silos flows into the reactor space to maintain the 1 atmosphere pressure in the argon cover gas. About 300 yonnes of NaCl stored above the primarysodium pool shoulod rain down to formanair excluding crust supported by the buoyant stainless steel balls.

Once the liquid sodium temperature is down to about 120 degrees C the surface of the primary 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 primary liquid sodium to reduce its temperature to the point where kerosene can be safely used to prevent primary sodium oxidation. Until the heat is removed from the primary sodium argon and CO2 must be used to exclude oxygen from the primary liquid sodium surface. That heat extraction might easily take half an hour, depending on the available cooling capacity. Absent a sufficient thermal load 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 rate of the intermediate heat exchangers, NaK/salt 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 H2, Na2O and NaOH the big fire 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 primary sodium pool fire cannot occur. In order to extinguish a primary 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 prevent any makeup fresh air flow into the primary sodium pool cover gas.

The soil and bedrock around the liquid sodium pool must be sufficiently dry, dense and stable to safely contain the primary 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 important that the local fire department be trained that water should NEVER be used to fight a FNR fire. Inappropriate use of water carried by a fire truck could easily 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. A 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 exaggerated 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.

This web page last updated October 14, 2022.

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