Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links


XYLENE POWER LTD.

FNR CONTROL

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

CONTROL OVERVIEW:
The FNR behaves as a constant temperature source, That constant temperature is 440 degrees C to keep the inside of the fuel tube material below 455 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 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 440 degrees C, the secondary sodium discharge temperature is 430 degrees C, the load 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 maximum load conditions the primary sodium differential temperature is:
440 C - 340 C = 100 C

It is not practical to operate at a lower maximum load primary sodium differential temperature due to the differential temperature requirements for primary sodium natural circulation.

It is desirable if possible to maintain a constant water temperature and pressure in the steam generator. To do so we must have adequate natural circulation of primary liquid sodium through the reactor with a 100 degree C primary sodium temperature differential.

A FNR operates in the "normal" mode of the reactor following the turbine. The electricity generation is controlled by controlling the secondary sodium flow rate which controls the steam flow fed to the turbine. After the 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 typically about 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 440 degrees C. Depending on the thermal load the secondary liquid sodium discharge temperature fed to the steam generator will be in the range 430 C to 440 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 410 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 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 9.0 m. At full load the corresponding decrease in liquid sodium temperature over that change in stratification layer elevation is 440C - 340 C = 100 C. Hence the thermal energy that must be extracted from the liquid sodium pool to achieve this change is:
(18.4 m X 25.4 m X 9.0 m) X (927 kg / m^3) X (1.276 kJ / kg-K) X (100 K)
= 497,535,940 kJ
= 497,535.940 MWt-s

This heat is immediately available but the chain reaction may take as long as:
2[497,535.940 MWt-s / 735 MWt]
= 1344 s to ramp from off to maximum power.

Hence the FNR's chain reaction can lags the real time electricity load by as much as:
1354 s / (60 s / minute) = 22.56 minutes
 

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

When the secondary sodium induction pump is 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 excess steam must be directly bypassed to the condenser. If the turbine is available this bleed steam flow can be run through the turbine.

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

In effect natural circulation of intermediate sodium forces a thermal load with little or no accompanying electricity generation. This issue diminishes the overall system thermal efficiency when some turbogenerators are off line. It may be necessary to carefully consider the best part load operating point of the turbines in light of this issue.
 

POWER MODULATION:
It is anticipated that the FNR will drive
16 X 46 MWt turbines

The turbine condensers will be cooled by at least 2 natural draft closed cycle cooling towers.

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 can in principle support 16 X 15.3 MWe steam turbogenerators operating in parallel. The advantage of the 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 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.

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 most efficient operating regions. Assume that at full system power there are 32 identical turbogenerators operating in parallel.
a) if at 75% of system rated capacity 8 turbines are disabled the remaining 24 turbines will operate at 100% of their individual design capacity to maintain the same output power.
b) Similarly as these 24 turbines operating in parallel each drop below 75% of their individual design capacity then another 6 turbines are disabled so that the remaining 18 turbines operate at 100% of their individual design capacity to maintain the same output power.
c) Similarly as these 18 turbines operating in parallel each drop below 66% of their individual design capacity the disabling of another 4 turbines causes the remaining 14 turbines to operate at 100% of their individual design capacity to maintain the same output power.
d) Similarly as these 14 turbines drop below 75% of their individual design capacity disabling of another 4 turbines causes the remaining 10 turbines to operate at 100% of their individual design capacity to maintain the same output power.
e) Similarly as these 10 turbines operating in parallel each drop below 60% of their design capacity disabling of another 4 turbines causes the last 6 remaining turbines to operate at 100% of their individual design capacity to maintain the same output power.
f) Similarly as these 6 turbines operating in parallel each drop below 66%% of their design capacity disabling of another 2 turbines causes the last 4 remaining turbines to operate at 100% of their individual design capacity to maintain the same output power.
g) The last 4 turbines should be rated for operation down to below 50% of their individual desgn capacity so that the readily controllable electric power output turndown range of the entire system is about 32:1.

Note that practical implementation of this control strategy may require automatic resynchronization equipment for each turbo-generator.
 

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 leader and all the others to act as followers. However, for system reliability the unit that is the leader must be reassignable.
 

FNR THERMAL STABILITY:
The FNR has one core zone 0.35 m high sandwitched between adjacent blanket zones each 1.200 m high. The core zone is 10.4 metres in diameter. The fissile fuel (Pu-239) is located only in the core zones 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 emitted neutrons exit the core zone and are absorbed in the adjacent the blanket zones.

Increasing the average Pu density increases the fraction of emitted neutrons that are captured in the core zone and hence increases reactivity. Conversely, reducing the average Pu density via thermal expansion reduces reactivity which reduces the reactor's thermal power output.

Hence a FNR regulates its liquid sodium coolant at a fixed temperature which is dependent upon its geometry, fuel load and active fuel bundle control portion insertion depth.

By modulation of the secondary liquid sodium coolant flow this phenomena can be used to control the thermal output power of a liquid sodium cooled fast neutron reactor. The temperature at which the nuclear chain reaction shutdown occurs in the core zone is adjusted by use of active fuel bundle control portions which adjust the reactivity in the core zone. Typically the core zone discharge temperature set point is 440 C.

The relative fuel bundle insertion positions are shown below.

________FIX

Note that the maximum permitted vertical travel of an active fuel bundle control portion is about 1.0 m.
 

FNR THERMAL CONTROL STRATEGY:
Output power control is achieved by modulating the intermediate sodium flow rate which in turn causes modulation of the steam flow rate out the steam generator pressure regulating valve. When the intermediate sodium flow rate is low the thermal output power is low so the steam flow rate is low. When the intermediate 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.

A major issue in FNR temperature control is proper positioning of the active fuel bundle control portions so that the chain reaction in each active fuel bundle core zone shuts down at the desired primary liquid sodium temperature. If a shutdown temperature is set too high the fuel could potentially melt. If an active fuel bundle control portion is inserted too far the fuel and adjacent liquid sodium will become too hot almost instantaneously. The reactor relies on thermal expansion of liquid sodium for criticality temperature control. For safety the active fuel bundle control portion should be inserted very slowly so that the liquid sodium temperature does not overshoot the desired setpoint. The obvious way to control the reactor is to initially set the active fuel bundle control portion insertion depth setpoint based on that bundle's emitted gamma flux, which responds instantly. Then the active fuel bundle control portion 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 indicated change in fuel bundle liquid sodium discharge temperature. When a control bundle is incrementally inserted there is an initial high gamma flux which decreases as the 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 the fuel bundles at the same liquid sodium discharge temperature. For the same ongoing gamma flux per bundle the liquid sodium discharge temperature can vary due to variable primary sodium flow channel obstruction. Such obstruction may be due to variable fuel tube swelling and dirt entrapment.

Once the gamma flux has stabilized the active fuel bundle discharge temperatures should be made uniform accross the reactor core by successive fine adjustments of the active fuel bundle control portion insertion depth setpoint. Note that the fuel bundle liquid sodium discharge temperature response time is long compared to the gamma resposnse time. Note also that the liquid sodium discharge temperature leveling active fuel bundle control portion insetion depth setpoint adjustments are small. The initial gamma level based active fuel bundle control portion 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. To protect the fuel tubes the reactor warm up time from a cold start should be over one hour.
 

ACTIVE FUEL BUNDLE CONTROL PORTION AND INDICATOR TUBE DETAIL:
Each active fuel bundle control portion has an attached indicator tube. The indicator tube projects above the surface of the liquid sodium. The positive buoyancy of the indicator tube helps it remain vertical. An active fuel bundle control portion functions by varying the average Pu concentration in the core zone of its fuel bundle.

Immediately above each fuel bundle is the closed bottom indicator tube which contains an internal pond of liquid mercury. The temperature of the internal liquid mercury pond sets the mercury vapor pressure inside the indicator tube in the temperature range of interest (300 deg C to 500 deg C). At the top of the indicator tube is a bourdon tube that changes its shape in response to the variable mercury vapor pressure inside the indicator tube. The bourdon tube must be of a material that retains its spring properties at up to 500 deg C and does not react with sodium or hot mercury vapor. (eg molybdenum?) The changing shape of the bourdon tube in response to the changing mercury vapor pressure changes the direction of a reflected laser light spot, which change indicates the active fuel bundle liquid sodium discharge temperature.

The hollow indicator tube permits gamma radiation to propagate up the tube. This gamma radiation is sensed by an overhead gamma ray camera that can resolve flux from individual indicator tubes.

The height of the top of the indicator tube indicates the active fuel bundle control portion 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 that uses a mirror to keep it shielded from gamma radiation.

Thus the active fuel bundle liquid sodium discharge active fuel bundle gamma ray emission and the active fuel bundle control portion insertion depth are all remotely monitored for each of the 532 active fuel bundles.
 

ACTIVE FUEL BUNDLE CONTROL PORTION POSITIONING:
For each active fuel bundle the reactor control system constantly monitors the insertion depth of its control portion and the resulting fuel bundle liquid sodium discharge temperature.

Each active fuel bundle control portion is vertically positioned using a dedicated liquid sodium hydraulic actuator.

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

The thermal power of the fuel bundle is indicated by the temperature difference between the active fuel bundle discharge temperature and the liquid sodium bottom temperature.

The active fuel bundle's liquid sodium discharge temperature is used to make small adjustments in the active fuel bundle control portion 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 the reactor power limit is set by the heat flux rating of the fuel tubes. The primary liquid sodium can be cold but the fuel tubes are 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 100 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 210 degrees C. As the primary liquid sodium pool bottom temperature starts to rise the insertion depth setpoint of the active fuel bundle control portions can be slowly increased to maintain an active fuel bundle discharge temperature 100 degrees C above the pool bottom temperature. The insertion depth setpoint increase must stop when the active fuel bundle discharge temperature reaches 440 degrees C.

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

Once the correct active fuel bundle control portion insertion depths have been established those insertion depths can be recorded as fixed setpoints and maintained using the measured height of the active fuel bundle indicator tubes for sensing active fuel bundle control portion insertion depth.
 

SAFETY MEASURES:
A hydraulic control loop maintains the active fuel bundle insertion depth at its software determined setpoint. There are differential alarms from that setpoint that advise the operator of a problem. If the differential alarm corresponding to too deep insertion trips the control portion is instantly withdrawn pending investigation of the cause of the alarm.

The active fuel bundle control portion insertion depth setpoint is established by temperature measurements but the insertion depth can be constrained by active fuel bundle control portion insertion depth measurements and a fixed maximum permitted insertion depth. The control portion insertion depth measurements respond almost instantaneously so they are best suited to acting as limits on the control portion insertion depth that occur as a result of temperature measurements.

However, if the active fuel bundle measured discharge temperature becomes too high in absolute terms (> 450 C) or too high in differential terms {[(discharge temperature) - (bottom temperature)] > 110 degrees C} then there should be a latched alarm and an immediate control portion 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 doe not agree with the others then reject its data and alarm.

For safety the automatic positioning system for each active fuel bundle control portion must be capable of being individually bypassed. However, an operator determined insertion depth setpoint must be clamped to 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 active fuel bundle control portions to withdraw. The rate of active fuel bundle control portion re-insertion is limited under all circumstances to prevent thermal over stressing of the fuel tubes.
 

SENSOR OR CONTROL FAILURE:
In a typical FNR there are 532 active fuel bundles each with a discharge temperature sensor and an insertion depth measurement apparatus. Sooner ater one of these 1064 sensor circuits or one of the 532 control circuits will fail and give an erroneous reading or an erroneous control signal. We do not want that erroneous reading or erroneous control signal to lead to over insertion of the fuel bundle and consequent fuel bundle damage. Thus if the reported fuel bundle insertion depth fails to track its setpoint or if the fuel bundle discharge temperature goes outside its normal range then a latched alarm must be triggered and the fuel bundle control portion immediately withdrawn and locked in the withdrawn state.

Similarly if the reported insertion depth exceeds a fixed predetermined setpoint a latched alarm must be triggered and the fuel bundle control portion immediately withdrawn and locked in the withdrawn state.

Similarly if the gamma ray output of a particular active fuel bundle exceeds a fixed upper limit a latched alarm must be triggered and the fuel bundle control portion immediately withdrawn and locked in the withdrawn state.
 

This web page last updated January 14, 2018

Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links