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INTRODUCTION:
A FNR Power Plant has several major control systems:
a) Normal passive control of the FNR primary sodium pool temperature at 460 degrees C with supplementary monitoring.
b) Active electrical output power control via control of the nitrate salt flow rate which also provides indirect control of the NaK flow rate through the intermediate heat exchange bundles;
c) Preset control of the steam pressure fed to the turbine. This pressure indirectly sets the superheated water temperature in the steam generators which in turn sets the nitrate salt return temperature;
d) Turbine steam bypass to the condenser on turbine overspeed. This control system is electric to quickly prevent turbine overspeed on sudden loss of electrical load and to enable heat dumping to the condenser during system cool down.
e) System control as required for primary sodium pool cool down and warm up to enable periodic fuel changes and occasional intermediate heat exchange bundle changes. This system control must provide slow warmup of the primary sodium pool, slow warmup of the heat transport systems, slow warmup of steam turbines and must dependably dispose of fission product decay heat when the reactor is shut down.
f) Two independent emergency shutdown systems.
FNR THERMAL STABILITY:
A FNR normally acts as a constant temperature heat source. The heat output from the reactor normally makes the steam generators act as constant temperature constant pressure steam sources.
A FNR fuel assembly has a core zone abput 0.4 m high sandwitched between two adjacent blanket zones each 1.8 m high. The core zone is 10.0 metres in diameter flat face to flat face. The blanket zone is 1.2 m thick. The cooling zone is 0.6 m thick. Hence the fuel asembly is 13.6 m in diameter flat face to flat face.. The initial fissile fuel load (Pu-239) is located only in the core zone so the nuclear fissions occur and emit neutrons only in the core zone. On average, each nuclear fission releases 3.1 neutrons. The reactor is geometrically sized so that about half of the fisson neutrons exit the reactor core zone and are absorbed by the adjacent blanket and cooling zones.
Increasing the local average Pu concentration increases the local reactivity. Hence reducing the average Pu atom concentration via thermal expansion reduces the local reactivity which reduces the reactor's local equilibrium temperature.
Hence with no thermal load a FNR keeps its contained liquid sodium at a fixed temperature which is dependent upon its fuel geometry.
In a practical FNR the core zone temperature To at which the nuclear chain reaction shutdown occurs is adjusted by use of movable fuel bundles. Movable fuel bundle insertion or withdrawal with respect to the matrix of fixed fuel bundles changes the reactor reactivity. The reactivity should be zero at a core zone sodium discharge temperature of 460 degrees C and zero thermal load.
Note that the maximum permitted vertical travel of each movable fuel bundle is 1.1 m.
MOVABLE FUEL BUNDLES:
In normal reactor operation the movable fuel bundles insert into the fuel assembly through the assembly bottom. When the movable fuel bundles are all fully inserted into the matrix of fixed fuel bundles the reactor is at maximum reactivity. When the movable fuel bundles are all 1.1 m withdrawn from the matrix of fixed fuel bundles the chain reaction must be off at all accessible temperatures. This requirement limits the range of acceptable Pu-239 concentrations in the core fuel rods.
When the movable fuel bundles are partially inserted into the matrix of fixed fuel bundles there is a nearly open space under the fuel assembly that allows almost unimpeded horizontal and vertical liquid sodium circulation.
The FNR sodium pool temperature varies slowly due to the large sodium thermal mass. Hence the sodium thermal power and temperature response to any control action is slow.
It is dangerous to use a fuel geometry change to rapidly increase the sodium temperature. By so doing it is easy to overheat the fuel. In normal FNR operation the solid fuel geometry is kept fixed and the thermal power is regulated by thermal expansion of the fuel and to a smaller extent, thermal expansion of the fuel bundle steel and liquid sodium coolant. In practical FNR application the sodium discharge temperature is almost constant. The thermal power increases as the sodium inlet temperture to the fuel assembly decreases.
Each movable fuel bundle has a characteristic discharge temperature setpoint which is set by the amount of insertion of the movable fuel bundle into the matris of fixed fuel bundles.
FNR THERMAL POWER CONTROL STRATEGY:
Electrical output power control from a FNR is achieved by modulating the nitrate salt flow rate which in turn causes modulation of the steam flow rate out the steam generator pressure regulating valve. When the NaK flow rate through the intermediate heat exchange bundle is low the thermal output power is low so the steam flow rate is low. When the NaK flow rate through the intermediate heat exchange bundle is high the thermal output power is high and the steam flow rate is high. The steam discharge valve on the steam generator maintains the steam pressure inside the steam generator at 10 MPa which keeps the water in the steam generator at about 310 C.
The NaK return temperature to the intermediate heat exchange bundles has a fixed setpoint of about 330 degrees C which is maintained by modulating the NaK flow rate. Note that the change in temperature across both the nitrate salt loop and the NaK loop remains about 120 degrees C from low load to full load.
The change in sodium temperature across the fuel bundle is a function of the reactor power and varies from about 20 degrees C at low load to about 60 degrees C at full load. At full load the total sodium natural circulation through the fuel assembly is about 2X the total NaK pumped circulation.
This control methodology allows the FNR's electricity output to follow rapid changes in electricity load. However, changes in FNR thermal generation are slower. The thermal mass of the liquid sodium acts as a low pass filter.
MOVABLE FUEL BUNDLE INSERTION RATE LIMITATION:
Insertion of amovable fuel bundle has the effect of instantly raising the
FNR fuel setpoint temperature. Hence the fuel bundle will immediately get hot with respect to its surrounding coolant. After a small step insertion it is essential to allow the coolant temperature to catch up with the fuel temperature. Otherwise the fuel will overheat and melt along its center line.
As a step insertion of a movable fuel bundle is occuring the fission neutron flux increases in a big spike. Then as the coolant sodium approaches the fuel temperature this neutron spike diminishes to zero.
Monitoring this neutron emission is much easier and faster than monitoring small changes in FNR primary sodium temperature. However, the sodium temperature is the best indicator of maximum allowable movable fuel bundle insertion.
SHUTDOWN TYPES:
In liquid sodium cooled FNRs there are three different important shutdown states; a warm shutdown, a cool shutdown and a cold shutdown.
During any of these shutdowns fission product decay heat continues to be emitted.
In a warm shutdown nuculear fission stops but the sodium pool remains at its setpoint temperature, typically 460 degrees C. A warm shutdown will spontaneously occur whenever the thermal load is removed from the FNR.
In a cool shutdown nuclear fission stops and the primary sodium pond temperature falls to 120 C, to enable fuel bundle changes and intermediate heat exchange bundle replacement. In a cool shutdown the FNR movable fuel bundle actuators still work.
In a cold shutdown nuclear fission stops and the sodium pool temperature falls to below 98 C causing sodium freezing. During a cold shutdown the sodium is solid so the fuel bundle geometry cannot be changed until after the sodium has been remelted.
Sustained cold shutdowns are seldom used. During a cold shutdown the sodium is solid making replacement of fuel bundles or intermediate heat exchange bundles impossible. Remelting the sodium is time consuming and requires substantial amounts of externally supplied heat, usually provided via electric heating of a synthetic heat transfer fluid (HTF).
During normal reactor operation heat is transferred from the heat exchange galleries to the turbogenerator halls by molten nitrate salt. However, that method of heat transfer will not work at salt temperatures below 260 C due to high nitrate salt viscosity.
To realize both sodium melting and cool shutdowns heat is transferred into or out of the sodium via NaK and a synthetic heat transfer fluid (HTF) instead of via molten nitrate salt. Typically at least 2 of the 48 heat transfer circuits are reserved for this purpose.
SODIUM TEMPERATURE SETPOINT:
A major issue in FNR temperature control is proper positioning of the movable fuel bundles so that the chain reaction in each movable fuel bundle shuts down at the desired liquid sodium temperature. If a chain reaction shutdown temperature setpoint To is too high as compared to the coolant temperature the fuel rods will get very hot and could potentially melt and the fuel tubes could go through an alloy phase shift. The FNR relies on thermal expansion of the fuel, liquid sodium coolant and fuel bundle steel to provide smooth reactivity control. To prevent fuel rod overheating the movable fuel bundles should be inserted into the matrix of fixed fuel bundles only very slowly.
The strategy for initial FNR setup is to initially set the insertion depth of each movable fuel bundle based on that fuel bundle's emitted gamma radiation flux, which responds instantly to changes in fuel bundle insertion. Then the movable fuel bundle insertion depth setpoints can be fine tuned over time to achieve the desired fuel bundle discharge temperatures.
There is an approximately proportional relationship between a fuel bundle's emitted gamma flux and that bundle's instantaneous thermal power. However, there is a significant transportation time delay between a step change in gamma flux and the corresponding sensed change in fuel bundle liquid sodium discharge temperature. When the insertion depth of a movable fuel bundle is incrementally increased there is an initial high gamma flux which decreases as the liquid sodium in that fuel bundle approaches the average fuel temperature in that bundle. However, there is a significant transportation delay in the discharge temperature measurement apparatus.
The object is to normally operate all of the movable fuel bundles at the same liquid sodium discharge temperature. Then a sodium coolant flow obstruction will cause a local hot spot characterized by a localized lower gamma flux. Such an obstruction might be due to fuel tube distortion, fuel tube swelling and/or dirt entrapment.
Once the gamma flux profile has stabilized, indicating stable reactor operation, the movable fuel bundle insertion depths should be fine adjusted under a constant reactor thermal load to achieve uniform movable fuel bundle discharge temperatures across the reactor core zone. Note also that during the liquid sodium discharge temperature leveling the changes in movable fuel bundle insertion depths are very small.
The movable fuel bundle insertion depth setpoint adjustments are made slowly so that no part of the fuel rapidly rises in temperature beyond its safe temperature range and so that the thermal flux rating of the fuel tubes is not exceeded.
To ensure protection of the fuel tubes the reactor warm up time from a cool start should be several hours.
MOVABLE FUEL BUNDLE AND INDICATOR TUBE DETAIL:
Immediately above each movable fuel bundle is an attached indicator tube. The indicator tube projects above the surface of the primary liquid sodium. The positive buoyancy of the indicator tube keeps it vertical. The indicator tube shows the actual movable fuel bundle insertion depth. This indicator tube enables adjusting the local reactor reactivity via fine adjustment of the local core zone thickness.
The indicator tube conveys to overhead monitoring instrumentation the movable fuel bundle elevation, discharge temperature and gamma emission. The hollow indicator tube side walls provide buoyancy and permit gamma radiation to propagate up the tube while thermally insulating the central portion of the tube. The gamma radiation is sensed by an overhead gamma ray camera that can resolve the relative gamma flux from each indicator tube.
The elevation of the top of an indicator tube indicates the corresponding movable fuel bundle's insertion depth in the matrix of fixed fuel bundles. This elevation is measured by a laser range finder to an accuracy of better than +/- 1.5 mm. This range finder apparatus is mounted in an overhead temperature controlled chamber which has a mirror and radiation shielding to protect the apparatus electronics from gamma radiation and heat.
The central portion of the indicator tube contains liquid sodium at close to the movable fuel bundle discharge temperature. This liquid sodium emits a characteristic temperature dependent near IR spectrum. This thermally isolated column of liquid sodium is a good conductor of heat but still takes minutes to accurately follow changes in the movable fuel bundle discharge temperature.
Thus the movable fuel bundle liquid sodium discharge temperature, gamma ray emission and insertion depth are remotely monitored for each of the 464 movable fuel bundles.
MOVABLE FUEL BUNDLE INSERTION DEPTH SETTING:
Each movable fuel bundle is vertically positioned using a dedicated bidirectional hydraulic motor actuator driving a screw jack. The design of the motor actuators is such that absent any control signal each movable fuel bundle remains at its last set elevation.
Each movable fuel bundle has an insertion depth setpoint stored in software which the hydraulic motor actuator tries to achieve. The movable fuel bundle discharge temperature is adjusted by gradually changing the movable fuel bundle insertion depth setpoint while maintaining a constant reactor thermal load.
The relative instantaneous thermal power output of a movable fuel bundle is indicated by its gamma ray output.
The movable fuel bundle's liquid sodium discharge temperature is used to make small adjustments in the movable fuel bundle insertion depth setpoint. These adjustments must be made very slowly and over a limited range while the reactor is at constant power due to the inherent transportation delay in the movable fuel bundle liquid sodium discharge temperature measurement apparatus.
MOVABLE FUEL BUNDLE INSERTION SAFETY MEASURES:
For safety the maximum movable fuel bundle insertion rate is both electronically and physically limited. The issue is to prevent any movable fuel bundle being inserted into the matrix of fixed fuel bundles too quickly. Similarly, for control stability, in normal reactor operation the movable fuel bundle withdrawal rate is both electronically and physically limited.
A hydraulic-electric control loop sets and maintains each movable fuel bundle's insertion depth at its predetermined setpoint. There are differential alarms for any significant deviations from that setpoint. If an alarm corresponding to too rapid or too deep movable fuel bundle insertion trips the movable fuel bundle should be immediately fully withdrawn and locked out pending investigation of the cause of the alarm.
The ultimate movable fuel bundle insertion depth setpoint is established by fuel bundle discharge temperature measurement but the maximum insertion depth is constrained via both software and hardware. The movable fuel bundle insertion depth measurements respond instantaneously so they are well suited for instantaneous movable fuel bundle insertion depth control.
However, if a movable fuel bundle measured discharge temperature becomes too high in absolute terms (> 465 C) or too high in differential terms:
{[(discharge temperature) - (bottom temperature)] > 60 degrees C}
then there should be a latched alarm and an immediate movable fuel bundle withdrawal.
The primary liquid sodium average top surface temperature is measured using a back up infrared scanner.
The primary liquid sodium bottom temperature is measured with multiple fiber optic Bragg grating temperature sensors. If one does not agree with the others then reject its data and alarm.
For safety the automatic positioning system for each movable fuel bundle must be capable of being individually bypassed. However, an operator determined insertion depth setpoint must be range limited so as not to accidentally exceed the predetermined insertion depth maximum.
For safety, on loss of control power the available hydraulic motor actuator sodium pressure goes to zero which causes all the movable fuel bundles to remain at their last set positions.
SODIUM MELTING:
None of the other FNR control systems will function properly until after the sodium has melted, which occurs at about 99 degrees C. This melting is achieved by dedicating two HTF heat transfer circuits for sodium melting and for sodium cooling below 280 C. These two dedicated heat transfer circuits contain a synthetic heat transfer fluid (HTF) instead of nitrate salt. The heat transfer fluid is circulated through an electric tank heater. This heat transfer fluid will heat the corresponding NaK which when circulated will heat the sodium. Adjustment of the heat transfer fluid flow rate can be used to control the rate of heat transfer.
After the sodium has melted the FNR actuators can be used to partially insert the movable fuel bundles into the matrix of fixed fuel bundes, so that nuclear heat is available for further warming of the the sodium and the NaK. Once nuclear heat is available the heat transfer fluid can be thermally isolated from the hot sodium by transferring the corresponding NaK to its dump tanks.
FNR COOL START:
Once the sodium has melted the warmup sequence of a FNR can commence. The warmup rate from a cool shutdown must be slow. It is easy to damage fuel tubes by too rapid warmup. One indication of the heat flux is the difference between the fuel bundle liquid sodium discharge temperature and the liquid sodium bottom temperature in the sodium pool. The reactor is designed for this temperature difference to be up to 60 degrees C but no more.
For example, assume that at the commencement of the reactor warmup sequence the liquid sodium pool bottom temperature is 110 degrees C. Until the liquid sodium pool bottom temperature starts to rise the insertion depth of the movable fuel bundles must be limited to prevent the fuel bundle discharge temperature exceeding 170 degrees C. After the sodium pool bottom temperature rises the insertion depth setpoint of the movable fuel bundles can be slowly increased to maintain a movable fuel bundle discharge temperature 60 degrees C above the sodium pool bottom temperature. The movable fuel bundle insertion depth setpoint increase must stop when the movable fuel bundle discharge temperature reaches 300 degrees C.
At 300 degrees C the NaK of all the other heat transport circuits should be transferred from its dump tanks to its pipe loops. Turn on the corresponding NaK induction pumps. The sodium will warm this NaK up to 300 degrees C. The induction pump temperatures will rise to 300 degrees C.
Now drain the steam generators and use the electric heaters in the nitrate salt dump tanks to bring that nitrate salt in the dump tanks up to 300 degrees C.
Now transfer that molten nitrate salt to the nitrate salt pipe loops and turn on the nitrate salt pumps. Now the heat in the primary sodium pool will keep that nitrate salt liquid.
Now close the steam generator drain valves and enable the steam generator automatic water level control for all the high temperature rated heat transfer circuits. The steam generator water temperature will rise to about 300 degrees C but the steam generator pressure regulating valves, which should set for 10 MPa, should not open.
Now check that the NaK return temperature setpoint is 340 degrees C for all the high temperture rated heat transport circuits.
Now gradually raise the sodium discharge temperature setpoint to 460 degrees C.
Now slightly increase the nitrate salt flow rate to bleed steam into the turbines without causing turbine rotation. Continue this steam bleed until the turbine casing temperature approaches 400 degrees C.
Slowly raise the nitrate salt flow rate to icrease the power.
Synchronize the turbine to the line.
Now use electic power control to keep the nitrate salt flow rate sufficient to maintainminimumsteamflow through the turbine.
Now additional high pressure steam and electricity can be generated simply by increasing the nitrate salt flow rate.
Even with no external thermal load this primary sodium warmup process will likely take several hours.
Once the correct movable fuel bundle insertion depths have been established those insertion depths should be recorded as fixed setpoints and maintained using the measured height of the movable fuel bundle indicator tubes for sensing movable fuel bundle insertion depth.
SYSTEM WARM UP SUMMARY:
Assume that the system is initially in the state primary sodium melted, all heat transfer circuits have NaK, heat transfer fluid and nitrate salt in their respective dump tanks.
a)Slowly insert the movable fuel bundles to raise the primary sodium pool temperature to 300 degrees C;
b) Using compressed argon transfer the NaK for the high temperature loops to its pipe loops;
c) Start the NaK induction pumps.
d) Circulate the NaK to bring its temperature up to 300 C.
e) Ensure that the turbine steam bypass valve to the condenser is closed;
f) Drain the steam generators;
g) Set the NaK return temperature setpoint to 320 degrees C;
h) Using electricity melt the nitrate salt in its dump tanks;
i) Using compressed air transfer the nitrate salt to its pipe loops;
j) Start the nitrate salt pump;
k) Now the nitrate salt temperature should rise to 300 C;
l) Return the steam generators to automatic water level control;
m) Note that the vents on the nitrate salt loops and heat transfer fluid loops have ball checks, so that heat will not escape through the vents and oxygen entry is minimized. Also the heat will not excape via the steam generator because the nitrate salt temperature is less than 300 C.
EQUIPMENT COOL DOWN FOR SERVICE:
Assume that the initial condition is that the equipment has been operating producing electricity. The heat transfer circuit containingnitrate salt are active, the heat transfer circuits containing heat transfer fluid are shut down with thir fluids in dump tanks. The object is to cool the primary sodium pool down to 120 degrees C to permit fuel and/or intermediate heat exchange bundle service.
a) Withdraw movable fuel bundles to their 120 degree C position but continue generating electricity to lower the primary sodium pool temperature to 300 degrees C;
b) Open the main contactors.
c) Stop the nitrate salt pumps in the high temperature heat transport circuits;
d) Transfer the nitrate salt in the high temperature heat transport circuits to its dump tanks;
e) Turn off the NaK inductionpump;
f) Transfer the NaK in the high temperature heat transport circuits to its dump tanks.
g) Transfer the Nak in the low temperature heat transport circuits from its dump tanks to its pipes;
h) On the low temperature heat transport circuits open the steam turbine bypass valves to the turbogenerator condensers. That will have the effect of fixing the steam generator water temperature to 100 degrees C.
i) Lower the mixed NaK temperature setpoints on the low temperature heat transfer circuits to ~ 120 degrees C to limit the rate of heat transfer to the steam generators
j)Transfer heat transfer fluid from its dump tanks into the low temperature circuit pipe loops;
k) Turn on the heat transfer fluid circ pumps;
l) The circulating heat transfer fluid will gradually cool the NaK and hence the primary sodium, dumping the heat into the steam generator which dumps that heat into the condenser. Sustain this action for hours to dump the heat from the primary sodium pool.
m) When the NaK temperature reaches 120 degrees C the three way valve control should prevent unnecessary further heat loss.
n) Close the steam vent valve to the condenser.
o) Wait one week after step (a) to allow Na-24 decay.
p) Commence contemplated service work. Be aware that when the primary sodium pool temperature falls to 105 degrees C the nuclear reactions will need to restart to prevent primary sodium from freezing.
q) When service work is completed return the movable fuel bundles to their 120 C position. In doing so be careful about their insertion rate.
ONGOING CONTROL OVERVIEW:
The fuel assembly of a FNR normally operates in a "maintain the status quo" mode. In normal FNR operation the movable fuel bundle insertion depths remain constant and the movable fuel bundle insertion depths, gamma emissions and discharge temperatures are monitored looking for any significant deviation from their previously established nominal values.
In a FNR the reactor thermal power will passively adjust to maintain the sodium surface temperature set point. Hence to vary the reactor thermal output to follow a changing electrical load we modulate the nitrate salt flow rate and hence the NaK flow rate through the intermediate heat exchange bundles
The depth of thermal output power modulation is limited by the fission product decay heat output to about 12:1. For greater depth of output modulation it may be necessary to dump heat via the steam bypass valve to the condenser.
FNR THERMAL CONTROL STRATEGY:
Electrical output power control from a FNR is achieved by modulating the nitrate salt flow rate which indirectly sets the NaK flow rate through the intermediate heat exchange bundles which in turn causes modulation of the steam flow rate out the steam generator pressure regulating valve. When the NaK flow rate through the intermediate heat exchange bundle is low the thermal output power is low so the steam flow rate is low. When the NaK flow rate through the intermediate heat exchange bundle is high the thermal output power is high and the steam flow rate is high. The steam discharge valve on the steam generator maintains the steam pressure inside the steam generator at 10 MPa which keeps the water in the steam generator at about 310 C.
This control methodology allows the FNR's electricity output to follow rapid changes in electricity load. However, changes in FNR thermal generation are slower. The thermal mass of the liquid sodium acts as a low pass filter.
SUMMARY:
Two heat transport circuits containing organic heat transport fluid are dedicated to < 300 C energy transport tasks including:
a) Melting the contents of the primary sodium pool to achieve a primary sodium pool temperature of 120 degrees C;
b) Isolating the heat transport fluid from high temperature at primary sodium pool temperatures greater than 300 C;
c) Lowering the primary sodium pool temperature from 300 C to 120 C;
d) Freezing the primary sodium pool should that be necessary.
The remaining 46 heat transport circuits are dedicated to > 300 C energy transport.
ELECTRIC POWER CONTROL:
In normal FNR operation the FNR's fuel geometry normally does not change, regardless of its thermal and/or electrical load. The primary sodium surface temperature remains almost constant.
Varying the nitrate salt flow rate causes the NaK flow rate through the intermediate heat exchange bundles and hence the FNR's power output to change allowing it to be used for: generating constant electrical power, generating variable electric power to serve a power island, or for electric grid voltage stabilization.
The induction pump, which is fed from the bottom manifold of the NaK-nitrate salt heat exchanger, has a variable speed which maintains the NaK discharge temperature from the NaK-nitrate salt heat exchanger at a constant 330 degrees C. The NaK discharge temperature from the intermediate heat exchanger is nearly constant in the range 450 C to 460 C. Hence the NaK flow is proportional to the thermal load.
The NaK-nitrate salt heat exchanger maintains a constant temperature drop between the NaK and the nitrate salt. Due to conservation of energy in the NaK-nitrate salt heat exchanger the heat exchanger primary and secondary flows are proportional. Hence varying the secondary flow causes the primary flow to vary in the same manner.
Thus the temperature difference across the nitrate salt loop remains almost constant although the energy flux varies with the speed of the nitrate salt pump.
Changing the nitrate salt flow rate has the effect of controlling the entire heat transport circuit output power.
After the electricity generator is synchronized and is connected to the AC line comparison of the instantaneous power actually generated to the electric power setpoint provides an error signal that modifies nitrate salt flow which in turn controls the NaK flow through the intermediate heat exchange bundle. Note that a minimum electric power setpoint is required to protect the turbine from windage heating if the steam flow drops to zero.
The steam generator water temperature is fixed at 310 C (590 F) by the steam generator pressure regulating valve setpoint which is 10 MPa.
At full load the steam flow to the turbine is ___ but may drop to a fraction of that amount at very low loads such as during synchronization of the generator to the line at which time the steam generator pressure regulating valve is almost closed.
A significant electric power control issue is the thermal propagation delay time through the nitrate salt loop and the steam generator. When the output thermal power is changed by changing the nitrate salt flow rate there is a significant time delay before the steam flow from the steam generator to the steam turbine makes its corresponding flow change. In order to realize fast power turndown, as required on sudden loss of turbogenerator electrical load, the steam bypass valve to the turbo-generator condenser must rapidly open as soon as turbine overspeed is detected.
In the constant electrical power mode the FNR produces a constant amount of electrical power and grid stabilization issues are met by other generators. From the perspective of the reactor owner this mode is preferable because in this mode the temperature profile in the reactor and heat transport systems remains constant, which minimizes equipment wear due to thermal stress.
In the power island service mode the FNR electrical power output must modulate up and down with the load to maintain the desired island line frequency. This mode is essential for powering an isolated island. In this mode individual load steps need to be kept as small as reasonably practical. In order to obtain fast output power step response the steam bypass valve to the condenser might also need to be controlled.
In the grid stabilization mode the IESO specifies a nominal output power setpoint of 165 MWe and the FNR power output modulates up and down +/- 130 MWe to try to maintain the desired line frequency. This mode is helpful for following uncontrolled load. However, if there is parallel connected wind and solar generation the net load seen by the FNR will vary a lot, leading to long term thermal stress wear on the FNR related equiment.
In mid-winter the FNR is normally operated at its 300 MWe maximum electricity generation capacity so as to provide a constant 700 MWt of thermal output for the connected district heating system. Unused heat is rejected via the cooling towers.
During hot days in the summer the FNR may again be operated at its 300 MWe maximum capacity to meet the peak air conditioning electrical load. However, at other times the most effective use of the FNR might be for grid stabilization.
It is important that the revenue earned by a FNR include not only payment for dependable energy delivered but also payment for grid stabilization. Fundamentally a FNR needs a "take or pay" contract. Due to additional equipment wear in the grid stabilization mode the grid stabilization revenue rate should exceed the full power revenue rate.
STEAM:
In normal operation the FNR maintains a constant primary liquid sodium top surface temperature of 460 degrees C. At full load the nitrate salt discharge temperature fed to the steam generator may be as high as 440 degrees C. The steam fed to the turbine is hotter than the 310 C water temperature in the steam generator and hence is dry. Under heavy loads this dry steam discharge temperature will likely fall to about 400 degrees C.
Due to the saturation pressure-temperature relationship for water the steam generator pressure control valve in effect regulates the water temperature in the steam generator at about 310 C, which in turn, due to the large heat exchange area in the steam generator, sets the nitrate salt return temperature in the range 320 C to 330 C. The corresponding NaK return temperature to the FNR is about 340 degrees C. Hence the differential temperature across the intermediate heat exchange bundle is about:
450 C - 340 C = 110 C.
However, the corresponding primary sodium temperature differential is about
460 C - 400 C = 60 degrees C.
The flow of high pressure condenser water injected into the steam generator is locally controlled to maintain the desired water level in the steam generator.
POWER MODULATION:
It is anticipated that the FNR will drive up to:
(8 X 37.5 MWe = 300 MWe
of turbogenerators
The turbine condensers will be cooled by 16 independent natural draft dry cooling towers. Each cooling tower will support up to:
300 / 16 = 18.75 MWe
of electricity generation by rejecting up to:
(7 / 3) X 18.75 MWe = 43.75 MWt
of heat.
The determining issues with respect to the turbogenerator size choice are:
1) Is the turbine rotor to stator clearance sufficient? When a modulating turbine rapidly changes from low power to high power or vice versa there can be a change in inlet steam temperature of 20 degrees C. If the turbine is not rated to handle this abrupt change in steam temperature, differential thermal expansion between the rotor and turbime housing may cause the turbine rotor to rub against the housing, causing serious turbine damage. Hence it is essential that all the turbines be rated for the contemplated rapid steam temperature changes. This problem may be more acute with large turbines than with small turbines.
2) The FNR is normally limited by reglation to driving a maximum of 300 MWe of steam turbogeneration. The advantage of the multiple smaller units is that they are easier to transport and repair and provide inherent redundancy in depth. The advantage of larger units is that they may be slightly more efficient and may be less expensive to purchase on a total cost per installed kWe basis.
3) An important advantage of smaller turbogenerators is that they allow simple maintenance of station power, external water pumping and black start independent of the external electricity grid.
4) A disadvantage of smaller turbogenerators is that they often rely on gears to reduce a turbine rotation rate of about 10,000 RPM to 1800 RPM or 3600 RPM for 60 Hz electricity generation.
5) An important advantage of smaller generation units is that they lead to major cost savings in any commercial environment where electricity costs are set by monthly peak demand.
Each turbine should normally operate in the range 50% to 100% of its capacity. Thus at full system power all the turbogenerators run in parallel with the electricity load equally distributed amongst them. However, as the system output power drops below 50% of total rated system capacity it may be desirable to progressively disable some of the turbines to keep the remaining turbines in their more efficient operating regions. Assume that at full system power there are 8 identical turbogenerators operating in parallel.
a) If at 50% of system rated capacity 4 units are disabled the remaining 4 units will operate at 100% of their individual plate capacity to maintain the same output power.
b) Similarly as these 4 units operating in parallel each drop below 50% of their individual plate capacity the disabling of another 2 units causes the remaining 2 units to operate at 100% of their individual plate capacity to maintain the same output power.
c) Similarly as these 2 units drop below 50% of their individual plate capacity disabling of another unit causes the remaining unit to operate at 100% of its plate capacity to maintain the same output power.
d) The last step contains 1 unit that should each be rated for operation down to below 50% of its plate capacity so that the electric power output turndown range of the entire system is about 16:1.
Note that disabled turbogenerators are kept warm and remain line synchronized so that they can each develop full rated power as soon as their steam flow picks up. This is not a particularly thermally efficient way to operate, but the cost of the wasted steam is minimal in a nuclear power plant. The object may be to stabilize the electricity grid, not to minimize nuclear heat rejection.
Note that practical implementation of this multi-generator setup may require automatic resynchronization equipment for each turbo-generator to enable startup and shutdown of individual turbogenerators.
SYNCHRONIZATION:
As long as the external grid is functional all the turbogenerators can be individually synchronized to it. However, if the external grid goes down the turbogenerators should continue to operate islanded at a reduced power in accordance with an on-site frequency, phase and voltage reference. On restoral of the grid the on-site frequency, phase and voltage reference must be slowly brought into synchronization with the grid. Then to the extent that all the on-site turbogenerators are in synchronization with the on-site reference the entire station will be in synchronization with the grid.
Once the connection to the grid is re-established the grid becomes the frequency, phase and voltage reference and the generator power setpoints can be raised.
Hence an important control feature is that every turbogenerator must have the capability of being controlled to follow the on-site frequency, phase and voltage reference as an alternative to having these parameters established by the external grid.
FNR CRITICALITY:
All fission type nuclear reactors rely on delayed neutrons for power control. The rate of prompt neutron generation must be less than the rate of neutron absorption in order to provide smooth stable power control. The reference Delayed Neutrons indicates that thermal and fast fission produce the following neutron counts per fission:
| ISOTOPE | NEUTRON ENERGY | SIGMA | PROMPT NEUTRONS / FISSION | DELAYED NEUTRONS / FISSION |
|---|---|---|---|---|
| U-235 | 0.025 eV | 585 b | 2.42 | 0.0162 |
| U-235 | 2 MeV | 1.27 b | 2.63 | 0.0165 |
| Pu-239 | 0.025 eV | 747 b | 2.87 | 0.0065 |
| Pu-239 | 2 MeV | 1.93 b | 3.16 | 0.0067 |
For thermal neutron fission of U-235 the fraction:
(delayed neutrons) / (total neutrons)
= (.0162) / (.0162 + 2.42)
= (.0162 / 2.4362)
= .00665
For fast neutron fission of Pu-239 the fraction:
(delayed neutrons) / (total neutrons)
= (.0067) / (.0067 + 3.16)
= (.0067 / 3.1667)
= .00212
The significance of this data is that a fast neutron reactor (FNR) operating with Pu-239 fuel will transition from normal operation to exponential neutron growth with (1 / 3) the change in reactivity that makes a reactor operating with U-235 transition from from normal operation to exponential neutron growth. Hence in a FNR it is imperative to have a rigid fuel geometry and to avoid a rapid drop in coolant inlet temperature Ti below its normal steady state value.
There is a further complication in a large power FNR that unless there is individual zone reactivity trimming the reactivity will not be uniform across the reactor due to both fuel and geometry variations. All points on the reactor the prompt neutron generation rate must exceed the neutron absorption rate.
If a single movable fuel bundle was used and the most reactive point in the core zone was made safely subcritical on prompt neutrons the least reactive points in the core zone would not be producing sufficient thermal power. The solution to this problem is to divide the core zone into several hundred control sub-zones each of which has its own temperature setpoint trimming adjustment. Then the temperature across the reactor can be made uniform at a particular fixed reactor thermal load by adjusting the movable fuel bundle insertion of each sub-zone to produce the same discharge temperature and the same gamma emission as all the other sub-zones.
The present FNR design has:
464 movable fuel bundles each forming a control zone.
This data has further significance. The ratio of:
(fast neutron velocity) / (thermal neutron velocity)
= [(2 X 10^6 eV) / (.025 ev)]^0.5
= [80 X 10^6]^0.5
= 8.944 X 10^3
The ratio of:
(Pu-239 fast neutron fission cross section) / (U-235 thermal neutron cross section)
= (1.93 b / 585 b)
= 0.0033
The time between successive fissions is:
[1 / (Vn Sigma Nf)]
where:
Vn = neutron velocity;
Sigma = fission cross section;
Nf = fissionable atom concentration
For fast neutrons:
Mn Vn^2 / 2 = 2 X 10^6 eV X 1.6 X 10^-19 J / eV
and
Mn = 1.67 X 10^-27 kg
and for Pu-239 in the fast neutron spectrum:
Sigma = 1.93 X 10^-28 m^2
Hence:
Vn = {[2 X 2 X 10^6 eV X 1.6 X 10^-19 J / eV] / [1.67 X 10^-27 kg]}^0.5
= {6.4 X 10^-13 J / 1.67 X 10^-27 kg}^0.5
= 1.958 X 10^7 m / s
In a FNR core zone the average fissionable atom concentration is about 5% of pure Plutonium's atomic concentration. Hence the Pu atomic concentration in the core zone is:
0.05 X 19 g / cm^3 X (6.023 X 10^23 atoms / 239 gm) X 10^6 cm^3 / m^3
= 0.0239 X 10^29 atoms / m^3
= 2.394 X 10^27 atoms / m^3
The corresponding time T between successive fissions in a FNR is:
T = 1 / (Vn Sigmaf Nf)
= 1 / (1.958 X 10^7 m / s X 1.93 X 10^-28 m^2 X 2.394 X 10^27 atoms / m^3)
= 0.1105 X 10^-6 s
Note that the fissionable atom concentration Nf is temperature dependent. As the temperature increases Nf decreases.
When the rate of prompt neutron production equals the rate of neutron absorption the delayed neutrons are excess and the neutron population growth per fission step is:
1.00212
PRIMARY SODIUM STRATIFICATION OVERVIEW:
In normal operation the FNR attempts to maintain a liquid sodium temperature in the immediate proximity of the core fuel rods at about 460 deg C. The hot primary sodium will naturally circulate up past the fuel tubes. The rate of this circulation is set by the difference in elevation between the liquid sodium thermal transition level and the elevation of the core fuel rods. At full reactor power this elevation difference is:
6 m - 1.8 m - 0.4 m = 3.8 m
As the thermal stratification layer moves down this elevation difference decreases which reduces the natural circulation flow of primary liquid sodium through the reactor decreasing the average available reactor thermal power output.
In effect the FNR behaves as a constant temperature source with a variable flow rate. That constant temperature is 460 degrees C to keep the peak fuel centerline temperature below 560 degrees C. The fuel tubes are designed for a 8 degree C temperature drop across the fuel tube wall at maximum rated thermal load. The intermediate heat exchange bundles are designed for a 10 degree C temperature drop across the heat exchange tube walls at maximum rated load. The sodium / salt heat exchanger bundles are designed for a 10 degree C temperature drop at full load. The steam generator, which has a pressure control valve, acts as a constant temperature load. That temperature is chosen to be 320 degrees C to avoid precipitation of solid NaOH at 318 C on the secondary sodium side of the steam generator heat exchange tubes. It is difficult to keep NaOH completely out of the secondary liquid sodium.
Under full load conditions the average primary sodium discharge temperature is 460 degrees C, the NaK discharge temperature is 450 degrees C, the nitrate salt discharge temperature is 440 degrees C, the steam discharge temperature is 400 degrees C, the steam generator water temperature is 310 degrees C, the nitrate salt return temperature is 320 degrees C, the NaK return temperature is 330 degrees C and the primary sodium return temperature is 340 degrees C. Thus at full load conditions the primary sodium differential temperature is:
460 C - 340 C = 120 C
This temperature differential contributes to the primary sodium natural circulation.
At 50% load the nitrate salt return temperature drops to about 315C, the NaK return temperature remains at about 330 C and the primary sodium return temperature is about 340 C giving a sodium differential temperature of about:
460 C - 340 C = 120 C
This sodium temperature differential maintains the sodium natural circulation flow rate which mitigates the drop in reactor power.
FNR THERMAL STEP RESPONSE:
In order for the average FNR thermal power generation to change from zero to maximum power output the liquid sodium temperature transition layer has to drop by about 3.8 m. At full load the corresponding decrease in primary liquid sodium temperature over that change in stratification layer elevation is:
460 C - 340 C = 120 C. Hence the thermal energy that must be extracted from the liquid sodium pool to achieve this change is:
(Pi X (10 m)^2 X 3.8 m) X (927 kg / m^3) X (1.276 kJ / kg-C) X (120 C)
= 169,451,305 kJ
= 169,451.3 MWt-s
This heat is immediately available for spinning reserve electricity generation but the reactor may take as long as:
2[169,451.3) MWt-s / 1000 MWt]
= 338.9 s to ramp from minimum to maximum thermal power.
Hence the FNR's nuclear thermal output power has step response time of about 5.65 minutes.
FNR BEHAVIOR WHEN THE NaK INDUCTION PUMPS ARE OFF:
A turbogenerator is taken off line by reducing its output power setpoint to zero by reducing its pumped nitrate salt flow which reduces the NaK flow through the intermediate heat exchanger. As a result the steam generator PRV almost closes.
When the relevant NaK loop induction pump is turned off there will still be natural sodium circulation through the NaK circuit. Assuming that the three way valve is calling for heat this natural circulation will continue to transfer heat to the nitrate salt which will continue to boil water in the steam generator. When the pressure in the steam generator exceeds the PRV setpoint some steam will still flow to the turbine. If the corresponding turbine is not available the steam should be routed to the condenser. This process will maintain nitrate salt temperature. Otherwise the nitrate salt should be transferred to its dump tank to prevent nitrate salt freezing in the salt loop pipes.
The steam power flowing to the turbine is increased by increasing the NaK flow through the intermediate heat exchanger.
In effect natural circulation of NaK maintains a thermal load. It is necessary to maintain sufficient total NaK circulation to remove fission product decay heat as well as to prevent the nitrate salt freezing.
SENSOR OR CONTROL FAILURE:
In a typical FNR there are 464 movable fuel bundles each which reports discharge temperature, gamma flux and fuel bundle insertion depth. Sooner or later one of these sensor systems will fail and give an erroneous reading or an erroneous control signal. That erroneous reading or erroneous control signal must not lead to over insertion of the movable fuel bundle and consequent fuel bundle damage or prompt criticality. Thus if the reported fuel bundle insertion depth fails to track its setpoint or if the fuel bundle gamma ray output exceeds its normal range then a latched alarm must be triggered and the movable fuel bundle immediately withdrawn and locked in the withdrawn state.
Similarly if the reported movable fuel bundle insertion depth exceeds a fixed predetermined setpoint a latched alarm must be triggered and the movable fuel bundle must be immediately withdrawn and locked in the withdrawn state.
This web page last updated November 4, 2023.
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