h FNR CONTROL

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XYLENE POWER LTD.

FNR CONTROL

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

NORMAL FNR OPERATION:
The fuel assembly of a FNR normally operates in a "maintain the status quo" mode. The movable fuel bundle insertion depths are controlled to keep them at their previously established insertion depth setpoints and the actual moveable fuel bundle insertion depths, gamma emissions and discharge temperatures are monitored looking for any significant deviation from their previously established nominal values.

The FNR's fuel geometry normally does not change, regardless of its load.

By suitably varying the secondary sodium flow rate a FNR can be used for generating a constant electrical power, generating variable power to serve a power island, or for a grid stabilization.

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 nearly constant, which minimizes equipment wear due to thermal stress.

In the power island service mode the FNR electrical power output modulates up and down with the lod to maintain the desired island voltage and 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 the grid stabilization mode the IESO specifies a nominal output power setpoint of 165 MWe and the FNR power output modulates up and down +/- 135 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 thermal stress wear in the FNR related equiment.

In mid-winter the FNR is normally operated at its 300 MWe maximum capacity so as to provide a constant 700 MWt of thermal output for the connected district heating system. Unused heat is rejected via the cooliing 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.

Thus it is important that the revenue earned by a FNR include not only payment for energy delivered but also payment for grid stabilization. Due to additional equipment wear in the grid stabilization mode the grid stabilization revenue rate should exceed the full power revenue rate.
 

REACTOR TEMPERATURE SETPOINT CONTROL:
In normal reactor operation the movable fuel bundles insert into the matrix of fixed fuel bundles from the below. When all the movable fuel bundles are fully inserted into the fixed fuel bundle matrix the reactor is at maximum reactivity. When all the movable fuel bundles are 1.2 m withdrawn from the fixed fuel bundle matrix the chain reaction must be off. The fuel bundle geometry limits the range of acceptable Pu concentrations in the core fuel rods.

From a power control perspective each movable fuel bundle and its surrounding active fixed fuel bundles can almost be regarded as an independent FNR. Each movable fuel bundle has its own discharge temperature setpoint which is set by the amount of insertion of the movable fuel bundle into the fixed fuel bundle matrix. Under cold shutdown conditions gravity causes all the movable fuel bundles to withdraw 1.2 m with respect to the fixed fuel bundle matrix.

When the movable active fuel bundles are fully inserted into the fixed fuel bundle matrix there is a ~ 1.5 m high nearly open space under the fuel bundle assembly that allows almost unimpeded horizontal and vertical liquid sodium circulation.
 

FNR Criticality:
All 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 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-2350.025 eV585 b2.420.0162
U-2352 MeV1.27 b2.630.0165
Pu-2390.025 eV747 b2.870.0065
Pu-2392 MeV1.93 b3.160.0067

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 rapid drops 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 mobile 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

Hence the neutron population doubles in about 300 fission steps. Hence prompt neutron criticality must be avoided because even a small level of prompt criticality will quickly blow the reactor fuel assembly apart.
 

CONTROL SUMMARY:
The most practical way to rapidly change FNR power output is to modulate the secondary sodium flow rate. In a FNR the nuclear thermal power will automatically adjust to maintain the primary sodium coolant temperature set point. However, the rate of change of nuclear heat output will be reduced by the thermal mass of the primary liquid sodium. The heat delivered to the steam generator is approximately proportional to:
(secondary sodium flow) X (temperature difference between the primary sodium and the steam generator water).
Hence to track a rapidly varying load we modulate the secondary sodium flow rate.

Thermal power modulation is limited by the fission product decay heat output to about 12:1. For greater depth of output modulation it is necessary to dump heat.

Note that the steam generator requires measures to reduce thermal stress at points where liquid sodium heats liquid water. The secondary sodium temperature differential may be 100 degrees C but the temperature differential across the steam generator tubes below the water level should be under 20 degrees C. One way of reaching this objective is to keep the liquid water that is in contact with the heat exchange tubes at its boiling point so that the heat exchange tubes under the water surface are covered with steam bubbles. However, that means that incoming water must be preheated before it contacts the exposed heat exchange tubes. It is contemplated that the lower portions of the heat exchange tubes in the steam generators will be sleeved to reduce thermal stress.

By keeping a layer of steam between the heat exchange tubes and the water in the base of the steam generator the local heat flux through the tube wall is reduced. It is necessary to keep the temperature differential across each heat exchange tube wall under 20 degrees C to limit wall stress.

The FNR primary sodium temperature varies slowly due to the large primary sodium thermal mass. Hence the primary sodium temperature response to control action is slow.

It is dangerous to use fuel geometry change to rapidly increase the primary sodium temperature. By so doing it is easy to overheat the fuel or make the reactor prompt critical. In normal operation the solid fuel geometry is kept fixed and the primary sodium temperature is regulated by thermal expansion of the fuel and to a smaller extent, thermal expansion of the steel and liquid sodium. A cool shutdown is a fuel geometry step change which lowers the primary liquid sodium temperature set point to near ambient temperature.
 

CONTROL OVERVIEW:
In normal operation the FNR attempts to maintain a liquid sodium temperature in the immediate proximity of the core fuel rods of about 500 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 stratification layer and the elevation of the core fuel rods. At full reactor power this elevation difference is:
6 m - 1.8 m = 4.2 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 500 degrees C to keep the inside of the fuel tube material below 515 degrees C. The fuel tubes are designed for a 15 degree C temperature drop across the fuel tube wall at maximum rated thermal load. The intermediate heat exchanger and the steam generator are designed for a 10 degree C temperature drop across the heat exchange tube walls at maximum rated thermal 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 primary sodium discharge temperature is 490 degrees C, the secondary sodium discharge temperature is 480 degrees C, the steam generator water temperature is 320 degrees C, the secondary sodium 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:
490 C - 340 C = 150 C

This temperature differential drives the primary sodium natural circulation.

The steam pressure regulating valve maintains a constant water temperature and pressure in the steam generator. The full load steam flow is a result of adequate natural circulation of primary liquid sodium through the reactor with a 150 degree C primary sodium temperature differential.

The electricity generation is controlled by controlling the secondary sodium flow rate which controls the steam flow fed to the turbine. After the electricity generator is synchronized and connected to the line comparison of the instantaneous power actually generated to the power setpoint provides an error signal that controls the secondary sodium flow. The steam generator water temperature is 320 C (608 F) and the corresponding saturated steam pressure is 1637.3 psi (11.25 MPa).

The steam input to the turbine is at a lower pressure than the steam in the steam generator. At full load the turbine input pressure is almost 11.25 MPa but may drop to almost zero 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.

In normal operation the FNR maintains a constant primary liquid sodium top surface temperature of 490 degrees C. Depending on the thermal load the secondary liquid sodium discharge temperature fed to the steam generator will be about 480 degrees C. The steam fed to the turbine is hotter than the 320 C water temperature in the steam generator and hence is dry. Under heavy loads this steam discharge temperature will likely fall to about 440 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 320 C which in turn, due to the large heat exchange area in the steam generator, sets the secondary liquid sodium return temperature in the range 320 C to 330 C. By indirectly controlling the secondary liquid sodium return temperature this control loop also maintains the desired primary liquid sodium temperature profile in the FNR.

The flow of condenser water injected into the steam generator is controlled to maintain the desired water level in the steam generator.
 

FNR THERMAL STEP RESPONSE:
In order for the average FNR thermal power generation to change from zero to maximum power output the bottom of the primary liquid sodium stratification layer has to rise by about 4.2 m. At full load the corresponding decrease in primary liquid sodium temperature over that change in stratification layer elevation is:
490 C - 340 C = 150 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 4.2 m) X (927 kg / m^3) X (1.276 kJ / kg-K) X (150 K)
= 232,235,466 kJ
= 232,235.4 MWt-s

This heat is immediately available for spinning reserve electricity generation but the reactor may take as long as:
2[232,235.4) MWt-s / 1000 MWt]
= 464 s to ramp from minimum to maximum thermal power.

Hence the FNR's nuclear thermal output power has step response time of about 7.7 minutes.
 

FNR BEHAVIOR WHEN THE SECONDARY SODIUM INDUCTION PUMPS ARE OFF:
A turbogenerator is taken off line by reducing its output power setpoint to zero by reducing its pumped secondary sodium flows. As a result the steam generator PRV almost closes.

When the relevant secondary sodium loop induction pumps are turned off there will still be natural sodium circulation through the secondary sodium circuit. This natural circulation 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 secondary sodium should be transferred to its dump tank to stop steam generation.

The steam power flowing to the turbine is increased by increasing the secondary sodium flow.

In effect natural circulation of secondary sodium maintains a thermal load. It is necessary to maintain sufficient total intermediate sodium circulation to remove fission product decay heat.
 

POWER MODULATION:
It is anticipated that the FNR will drive up to:
(8 X 3 X 5.4 MWe) + (8 X 4 X 5.4 MWe) = 302.4 MWe
of turbogenerators

The turbine condensers will be cooled by 16 independent natural draft dry cooling towers. Each cooling tower will support up to 19 MWe of electricity generation by rejecting up to:
(7 / 3) X 19 MWe = 45 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 40 C to 50 C. If the turbine is not rated to handle this abrupt change in steam temperature, differential thermal expansion between the rotor and stator may cause the turbine rotor to rub against the stator, causing serious tubine 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 total 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 kW 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 usually rely on gears to reduce a turbine rotation rate of about 10,000 RPM to 3600 RPM for 60 Hz electricity generation.

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 75% of total rated system capacity it may be necessary 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 16 identical turbogenerators operating in parallel.
a) if at 75% of system rated capacity 4 units are disabled the remaining 12 units will operate at 100% of their individual design capacity to maintain the same output power.
b) Similarly as these 12 units operating in parallel each drop below 67% of their individual design capacity then another 4 units are disabled so that the remaining 8 units operate at 100% of their individual design capacity to maintain the same output power.
c) Similarly as these 8 units operating in parallel each drop below 50% of their individual design capacity the disabling of another 4 units causes the remaining 4 units to operate at 100% of their individual design capacity to maintain the same output power.
d) Similarly as these 4 units drop below 50% of their individual design capacity disabling of another 2 units causes the remaining 2 units to operate at 100% of their individual design capacity to maintain the same output power.
e) The last step contains 2 turbines that should each be rated for operation down to below 50% of their individual design capacity so that the readily controllable 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 is to stabilize the electricity grid, not to minimize nuclear heat dissipation.

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.
 

RESYNCHRONIZATION:
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 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. One way to achieve this objective is for one turbogenerator to act as master and all the others to act as slaves. However, for system reliability the unit that is the master must be reassignable.
 

FNR THERMAL STABILITY:
A FNR is in essence a constant temperature heat source.

A FNR has one core zone 0.0 to 0.7 m high sandwitched between adjacent blanket zones each 1.8 m high. The core zone is 10 metres in diameter. 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. Each nuclear fission releases 3.1 neutrons. The reactor is geometrically sized so that about half of the fisson neutrons exit the reactor core and are absorbed in the adjacent blankets.

Increasing the local average Pu density increases the local reactivity. Hence reducing the average Pu density via thermal expansion reduces the local reactivity which reduces the reactor's local adiabatic equilibrium temperature.

Hence an adiabatic FNR core regulates its contained liquid sodium at a fixed temperature which is dependent upon its geometry.

In a practical FNR the temperature at which the nuclear chain reaction shutdown occurs in the core zone is adjusted by use of movable fuel bundles which adjust the reactivity versus position in the core zone. Typically the core zone discharge temperature set point is 490 deegrees C.

Note that the maximum permitted vertical travel of a movable fuel bundle is 1.2 m.
 

FNR THERMAL CONTROL STRATEGY:
Electrical output power control from a FNR is achieved by modulating the secondary sodium flow rate which in turn causes modulation of the steam flow rate out the steam generator pressure regulating valve. When the secondary sodium flow rate is low the thermal output power is low so the steam flow rate is low. When the secondary liquid sodium flow rate 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 11.25 MPa which keeps the water in the steam generator at about 320 C.

This control methodology allows the FNR's electricity output to follow rapid changes in electricity load. However, changes in FNR thermal output are slower. The thermal mass of the liquid sodium acts as a low pass filter.

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 core zone shuts down at the desired primary liquid sodium temperature. If a chain reaction shutdown temperature is set too high the fuel tubes could go through an alloy phase shift and the fuel could potentially melt. If a movable fuel bundle is inserted too far into the fixed fuel bundle matrix the local reactivity could be so large as to cause prompt neutron criticality. The reactor relies on gradual thermal expansion of the fuel and liquid sodium for smooth reactivity control. For safety the movable fuel bundles should be inserted very slowly.

The obvious way to control the reactor is to initially set the movable fuel bundle insertion depth setpoint based on that bundle's emitted gamma radiation flux, which responds instantly. Then the movable fuel bundle insertion setpoint can be fine tuned over time to achieve the desired fuel bundle discharge temperature.

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 a movable fuel bundle is incrementally inserted there is an initial high gamma flux which decreases as the bulk liquid sodium reaches its equilibrium temperature. Then there is an additional thermal transportation delay in the temperature measurement apparatus.

The object is to operate all of the movable fuel bundles at the same liquid sodium discharge temperature. A flow obstruction will cause a lower gamma flux for the same fuel bundle discharge temperature. Such an obstruction may be due to variable fuel tube swelling and/or variable dirt entrapment.

Once the gamma flux has stabilized indicating stable reactor operation the movable fuel bundle discharge temperatures should be made uniform across the reactor core by successive fine adjustments of the movable fuel bundle insertion depth setpoints. Note that the fuel bundle liquid sodium discharge temperature response time is long compared to the gamma response time. Note also that the liquid sodium discharge temperature leveling movable fuel bundle insertion depth setpoint adjustments are very small. The initial gamma level based movable fuel bundle insertion depth setpoint adjustment is made slowly so that no part of the reactor rapidly rises in temperature beyond its safe thermal control range and so that the thermal flux rating of the fuel tubes is not exceeded and so that prompt neutron criticality is not approached. To ensure protection of the fuel tubes the reactor warm up time from a cold start should be over one hour.
 

MOVABLE FUEL BUNDLE AND INDICATOR TUBE DETAIL:
Each movable fuel bundle has an attached indicator tube. The indicator tube projects above the surface of the liquid sodium. The positive buoyancy of the indicator tube keeps it nearly vertical. A movable fuel bundle provides a means of adjusting the effective reactor local core zone thickness.

Immediately above each movable fuel bundle is the indicator tube which conveys the movable fuel bundle vertical position, discharge temperature and gamma emission to overhead instrumentation. The hollow indicator tube walls permit gamma radiation to propagate up the tube while thermally insulating the center part of the tube. This gamma radiation is sensed by an overhead gamma ray camera that can resolve gamma flux from individual indicator tubes.

The height of the top of the indicator tube indicates the movable fuel bundle's insertion depth. This height is measured by a laser range finder to an accuracy of +/- 1.5 mm. This range finder apparatus is mounted in an overhead temperature controlled chamber which has a mirror and shielding to keep the electronics shielded from gamma radiation.

Thus the movable fuel bundle liquid sodium discharge temperature, gamma ray emission and insertion depth are all remotely monitored for each of the 464 movable fuel bundles.
 

MOVABLE FUEL BUNDLE INSERTION DEPTH POSITIONING:
For each movable fuel bundle the reactor control system constantly monitors its insertion depth, its gamma flux and the resulting liquid sodium discharge temperature.

Each movable fuel bundle is vertically positioned using a dedicated liquid sodium hydraulic actuator.

Each movable fuel bundle has an insertion depth setpoint stored in software which the hydraulic positioning system tries to maintain. The mobile fuel bundle discharge temperature is adjusted by gradually changing the movable fuel bundle insertion depth setpoint.

The instantaneous thermal power 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 full power due to the transportation delay in the fuel bundle liquid sodium discharge temperature measurement apparatus.
 

FNR COLD START:
The cold starting sequence of a FNR is critical because it is easy to crack the fuel tubes during a too rapid cold start. Remember that one of the reactor power limits is set by the heat flux rating of the fuel tubes. When the primary liquid sodium is cold the fuel tubes are easily damaged by excessive heat flux. 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 primary sodium pool. The reactor is designed for this temperature difference to be up to 150 degrees C but no more.

For example, assume that at the commencement of the reactor turn-on 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 active fuel bundle control portions must be limited to prevent the fuel bundle discharge temperatures exceeding 260 degrees C. As the primary liquid sodium pool bottom temperature starts to rise the insertion depth setpoint of the movable fuel bundles can be slowly increased to maintain a movable fuel bundle discharge temperature 150 degrees C above the primary sodium pool bottom temperature. The movable fuel bundle insertion depth setpoint increase must stop when the movable fuel bundle discharge temperature reaches 490 degrees C.

Even with no external thermal load this primary sodium warmup process will likely take over an hour.

Once the correct movable fuel bundle insertion depths have been established those insertion depths can 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.
 

SAFETY MEASURES:
For safety the maximum movable fuel bundle insertion rate is both electronically and mechanically limited. The issue is that under no circumstances can any portion of the reactor be driven into potential fuel melting or prompt neutron criticality by too rapid insertion of a movable fuel bundle. Similarly, for control stability in normal operation the movable fuel bundle withdrawal rate is similarly both electronically and mechanically limited. However, in either the cool or cold shutdown modes no such limitation on the movable fuel bundle withdrawal rate applies.

A hydraulic-electronic control loop maintains the movable fuel bundle insertion depth at its software determined setpoint. There are differential alarms for deviations from that setpoint that indicate a problem. If an alarm corresponding to too rapid or too deep movable fuel bundle insertion trips the movable fuel bundle is instantly withdrawn and locked out pending investigation of the cause of the alarm.

The movable fuel bundle insertion depth setpoint is established by fuel bundle discharge temperature measurement but the maximum insertion depth is constrained via software and hardware. The movable fuel bundle insertion depth measurements respond instantaneously so they are best suited for instantaneous movable fuel bundle insertion depth control.

However, if a movable fuel bundle measured discharge temperature becomes too high in absolute terms (> 500 C) or too high in differential terms:
{[(discharge temperature) - (bottom temperature)] > 155 degrees C}
then there should be a latched alarm and an immediate movable fuel bundle withdrawal.

The primary liquid sodium top surface temperature is measured using a back up remote 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 exceed the predetermined upper limit.

For safety on loss of control power the hydraulic sodium pressure goes to zero which causes all the movable fuel bundles to withdraw. The rate of movable fuel bundle re-insertion is limited under all circumstances to prevent thermal over stressing of the fuel tubes and to ensure against prompt neutron criticality.
 

SENSOR OR CONTROL FAILURE:
In a typical FNR there are 464 movable fuel bundles each with a discharge temperature sensor, a gamma flux sensor and an insertion depth measurement apparatus. Sooner or later one of these sensor circuits or one of the control circuits 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 March 17,2021.

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