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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 445 degrees C. The steam generator acts as a constant temperature load. That temperature is chosen to be 320 degrees C to avoid precipitation of solid NaOH at 318 C under low load conditions. At low load conditions the primary sodium discharge temperature is 445 degrees C, the primary sodium return temperature is 320 degrees C, the secondary sodium discharge temperature is 445 degrees C and the secondary sodium return temperature is 320 degrees C. Thus at low load conditions the primary sodium differential temperature is:
445 C - 320 C = 125 C

Under high load conditions the primary sodium discharge temperature is 445 degrees C, the secondary sodium discharge temperaute is 430 degrees C, the load water temperature is 320 degrees C, the secondary sodium return temperature is 335 degrees C and the primary sodium return temperature is 350 degrees C. Thus at high load conditions the primary sodium differential temperature is:
445 C - 350 C = 95 C

In principle the primary sodium differential temperature at high loads can be increased by programming the steam generator pressure to decrease as the load increases. At low loads the water temperature is programmed to be 320 C. At high loads the water temperature is programmed to be 305 C. Then at high loads the primary sodium dischare temperature is 445 C, the secondary sodium discharge temperature is 430 C, the secondary sodium return temperature is 320 C and the primary sodium return temperature is 335 C. Then the primary differential temperature is:
445 C - 335 C = 110 C

However, dropping from a water temperature of 320 C to a water temperature of 305 C with increasing load corresponds to a significant pressure drop and a major increase in turbine size. 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 95 degree C temperature drop.

A FNR operates in the "normal" mode of the reactor following the turbine. The electricity generation is controlled by controlling the steam flow fed to the turbine. After the generator is synchronized to the line 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 that generates steam which feeds the turbine. 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 445 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 445 C. The steam fed to the turbine is hotter than the water temperature in the steam generator and hence is dry. Under heavy loads this steam temperature will likely fall to about 410 C.

Due to the saturation pressure-temperature relationship for water this control loop 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 335 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 geneator 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 4.8 m. At full load the corresponding decrease in liquid sodium temperature over that change in elevation is 445C - 350 C = 95 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 4.8 m) X (927 kg / m^3) X (1.276 kJ / kg-K) X (95 K)
= 252,084,876 kJ
= 252,084.876 MWt-s

At a system thermal power level of 1000 MWt the reactor thermal response delay is:
252,084.876 MWt-s / 1000 MWt
= 252 s

Hence the reactor primary liquid sodium pool thermal response lags the real time electricity load by about:
252 s / (60 s / minute) = 4.2 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 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 60 MWt turbines

The turbine condensers will be cooled by at least 2 natural draft 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 60 MWt 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, critical cooling power 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 16 identical turbogenerators operating in parallel.
a) if at 75% of system rated capacity 4 turbines are disabled the remaining 12 turbines will operate at 100% of their individual design capacity to maintain the same output power.
b) Similarly as these 12 turbines operating in parallel each drop below 75% of their individual design capacity then another 3 turbines are disabled so that the remaining 9 turbines operate at 100% of their individual design capacity to maintain the same output power.
c) Similarly as these 9 turbines operating in parallel each drop below 66% of their individual design capacity the disabling of another 2 turbines causes the remaining 7 turbines to operate at 100% of their individual design capacity to maintain the same output power.
d) Similarly as these 7 turbines drop below 75% of their individual design capacity disabling of another 2 turbines causes the remaining 5 turbines to operate at 100% of their individual design capacity to maintain the same output power.
e) Similarly as these 5 turbines operating in parallel each drop below 60% of their design capacity disabling of another 2 turbines causes the last 3 remaining turbines to operate at 100% of their individual design capacity to maintain the same output power.
f) Similarly as these 3 turbines operating in parallel each drop below 66%% of their design capacity disabling of another turbine causes the last 2 remaining turbines to operate at 100% of their individual design capacity to maintain the same output power.
g) The last 2 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.
 

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 fuel bundle insertion position.

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 shudown occurs in the core zone is adjusted by use of control fuel bundle portions which adjust the reactivity in the core zone. Typically the core zone discharge temperature set point is 445 C.

The relative fuel bundle insertion positions are shown below.

________FIX

Note that the maximum permitted vertical travel of a control bundle 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 control bundles 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 a control bundle 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 control bundle 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 control bundle position based on that bundle's emitted gamma flux, which responds instantly. Then the control bundle insertion 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 fuel mix, fuel age and 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 fuel bundle discharge temperatures should be made uniform accross the reactor core by successive fine adjustments of the control bundle positions. 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 control bundle insertion adjustments are small compared to the initial gamma level control rod adjustment. The initial gamma level control bundle insertion 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 at least one half hour.
 

CONTROL ROD AND INDICATOR TUBE DETAIL:
Each control bundle 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. A control bundle 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 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 metal diaphragm or bourdon tube that changes its shape in response to the variable mercury vapor pressure inside the indicator tube. The diaphragm or 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 diaphragm or bourdon tube in response to the changing mercury vapor pressure changes the size, shape or direction of a reflected laser light spot, which change indicates the liquid sodium temperature at the fuel bundle discharge.

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 gamma radiation from individual indicator tubes.

The height of the top of the indicator tube indicates the control bundle vertical position. This height is measured by a laser range finder to an accuracy of +/- 1.5 mm. This range finder apparatus is mounted in a temperature controlled chamber that is shielded from gamma radiation and operates via a mirror located above the reactor.

Thus the fuel bundle liquid sodium discharge temperature, fuel bundle gamma ray emission and the control bundle vertical position are all remotely monitored for each of the 532 active fuel bundles.
 

CONTROL BUNDLE POSITIONING:
For each active fuel bundle the reactor control system constantly monitors the vertical position of its control bundle and the resulting fuel bundle gamma emission and fuel bundle liquid sodium discharge temperature.

Each control bundle is vertically positioned using a dedicated liquid sodium hydraulic pump which adds or extracts liquid sodium from the corresponding support tube piston chamber.

Each control bundle has a vertical position setpoint stored in software which the control bundle bundle positioning system tries to maintain. The fuel bundle discharge temperature is adjusted by gradually changing the control bundle vertical position setpoint.

The thermal power of the core zone is indicated by its emitted gamma flux.

The fuel bundle's liquid sodium discharge temperature is used to make small adjustments in the control bundle vertical position setpoint. These adjustments must be made very slowly and over a limited range due to the transportation delay in the fuel bundle liquid sodium discharge temperature measurement apparatus.
 

SAFETY MEASURES:
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 a fiber optic Bragg grating temperature sensor.

For safety in depth each reactor should have 100% redundant control bundle bundle position, gamma ray output and fuel bundle liquid sodium discharge temperature sensing systems.

For safety the positioning system of each control bundle must be capable of being individually bypassed.

For safety on loss of control power the hydraulic sodium pressure goes to zero which causes control bundles to withdraw. The rate of control bundle insertion is limited under all circumstances to prevent undesired nuclear reaction thermal transients.
 

This web page last updated February 21, 2017

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