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

A Fast Neutron Reactor (FNR) produces heat at up to about 460 degrees C. The most practical way to convert that heat into electricity is by using the heat to produce high pressure steam. The steam then expands while turning a steam turbine which in turn drives a line synchronous generator that produces 60 Hz 3 Phase electric power.

A FNR Power Plant has four major control systems:
a) Normal passive control of the FNR primary sodium pool temperature with supplementary monitoring.
b) Active electrical output power control via control of the NaK flow through the intermediate heat exchange bundles and the nitrate salt flow rate;
c) Preset control of the steam pressure fed to the turbine and the turbine steam bypass to the condenser. 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.
d) 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 also dependably dispose of fission product decay heat when the reactor is shut down.

This web page is primarily concerned with matters (b) and (c) above relating to control of the FNR to meet the needs of the electricity system.

The design fundamentals of steam turbines have not changed significantly during the last 100 years. However, most steam turbines are intended for operation with fossil fuel fired boilers whereas the temperatures and pressures available from liquid sodium cooled FNRs are somewhat different.

Energy Conversion reference.
Operation and Maintenance Schedule of a Steam Turbine Power Plant

Issues related to the practical design of liquid sodium cooled FNRs effectively set the steam pressure delivered to the turbine at 10.0 MPa (corresponding to a saturated steam temperature of 310 degrees C to 320 degrees C) and set the dry steam temperature between about 390 degreees C at full load rising to about 430 degrees C at 10% of full load. The steam mass flow rate is variable depending on the electric power required at a particular moment in time. At full load the steam thermal power is 125 MWt.

The turbogenerator feed steam conditions are 125 MWt, 10 MPa, 400 degrees C. As set out later herein, the condenser must be capable of continuous absorption of the full steam thermal energy.

The plan is to modulate the NaK flow rate through the intermediate heat exchanger to set the system power and to modulate the nitrate salt pumping rate to try to hold the steam temperature at the input to the steam turbine as constant as possible over the load range 25% to 100%.

We must also be concerned about turbine over speed if under full load operation the generator suddenly disconnects from the grid or an islanded electrical load. It is necessary to continuously monitor the turbine RPM and if the RPM becomes too high it is necessary to immediately bypass most of the high pressure steam to the condenser, and to stop the NaK circulation through the intermediate heat exchanger. In normal operation the electric power generation is set by the NaK circulation through the intermediate heat exchanger but sudden stopping of this circulation will not stop thermal energy flow through the extended nitrate salt pipes fast enough to protect the turbogenerator from overspeed if the generator disconnects from the grid or has a sudden loss of islanded electrical load. Note that in these circumstances and without turbine steam bypass to the condenser an impulse turbine will tend to over speed by a factor of two.


3) An efficient turbo-generator typically consists of a high pressure turbine and a low pressure turbine connected on either end of a synchronous electricity generator. Assuming that the AC electricity grid is energized the generator will act as a motor causing the turbine to rotate at a constant speed irrespective of the steam flow. The RPM is a function of the line frequency (60 HZ) and the manner in which the generator is wound. For practical reasons related to generator design the usual generator rotation rate is 1800 RPM. The function of the steam flow in a turbine is to exert tangential force on the turbine blades which causes torque on the turbine shafts which cause torque on the generator shaft causing the generator to act as an energy source feeding the electricity grid.

4) A steam generator is fitted with a pressure regulating valve to act as a constant pressure variable mass flow rate steam source. A steam generator connected to a FNR operates at an internal pressure of about 10 MPa. Hence at full load steam flows from the steam generator to the high pressure steam turbine input at a nominal pressure of about 10 MPa. However, as the reactor thermal power is reduced the internal pressure in the steam generators remains the same but the steam mass flow is reduced which causes the steam pressure in the turbine(s) to fall.

5) During the steam expansion there is some loss of steam temperature. In order to prevent premature steam condensation that can potentially cause erosion of the low pressure turbine blades, some heat is usually added to the steam between the high pressure and the low pressure turbine by an interstage heating coil. This extra heat is obtained from the steam generator.

6) After passing through the high pressure turbine the steam further expands in the low pressure turbine. The steam pressure drops by about a factor of 4.4% _____in each turbine stage. The high pressure turbine reduces the steam pressure from 10 MPa to 1.0 MPa. In the low pressure turbine the steam pressure falls from about 1 MPa to about 0.10 MPa.

7) From the highest pressure turbine inlet to the lowest pressure turbine discharge the steam expands about 100 fold in volume. Since the mass flow is uniform the turbine casing radius must increase about 10 fold.

8) The discharged steam then flows past a recuperator coil. The function of the recuperator coil is to transfer as much heat as possible from the low pressure steam to the high pressure condensate water being fed to the steam generator. Remember that at steady state operation the mass flow of low pressure steam and the mass flow of high pressure condensate water are equal.

9) The remaining steam/water from the turbine low pressure steam discharge flows into the condenser where the steam is fully condensed dumping as much thermal energy as possible into a heat sink. For maximum electricity generation the heat sink should be cold lake or ocean water. However, under other circumstances the heat sink is a district heating loop or a cooling tower. The result is a partial vacuum in the condenser.

10) The liquid condensate is then pumped from the low pressure water accumulation inside the condenser (typically at 0.04 MPa) to 10 MPa, the feedwater pressure to the steam generator. This feedwater injection pump is a major parasitic load on the system.

11) The high pressure liquid condensate flows through the recuperator coil mentioned above which raises the condensate temperature part way to the steam generator water temperature.

12) This heated condensate is mixed with steam generator bottom water which raises the steam generator feed water temperature to about 300 degrees C. This preheating of the feed water injected into the steam generator is necessary to minimize thermal stress within the steam generator.

13) The steam generator feed water flows into the base of the steam generator shell. Flowing nitrate salt in the steam generator tubes raises the steam generator water temperature to 310 degrees C.

14) Flowing nitrate salt in the steam generator tubes then adds latent heat of vaporization to the steam generator water to convert it to saturated steam at 310 degrees C.

15) Flowing nitrate salt in the steam generator tubes then adds sensible heat to the steam to raise the steam temperature near the top of the steam generator to 390 deg C to 430 deg C, depending on the steam and nitrate salt mass flow rates.

16) The steam flows out of the steam generator to the high pressure steam turbine steam inlet via a pressure regulating valve which maintains the steam pressure in the steam generator at 10 MPa. This pressure regulating valve sets the saturated steam temperature in the steam generator at 310 degrees C, which in turn sets the liquid water temperature in the steam generator at 310 degrees C.

17) This steam generator liquid water temperature together with a maximum 11 deg C drop across the steam generator tube wall, ensures that the nitrate salt discharge temperature from the steam generator is in the range 320 deg C to 330 deg C.

18) This nitrate salt discharge temperature is also sufficient to ensure that there is no deposition of solid NaOH on heat exchange surfaces in the NaK-salt heat exchanger. NaOH melts at 318 degrees C. This nitrate salt discharge temperature is also sufficient to prevent the nitrate salt freezing.

19) The secondary sodium discharge temperature, together with the counterflow design of the intermediate heat exchange and a maximum 10 degree C drop across the intermediate heat exchange bundle tube wall, ensures that the primary sodium return temperature to the reactor fuel assembly is about 400 degrees C.

20) This return temperature of 340 degrees C to the reactor fuel assembly together with the 460 degree C discharge temperature from the reactor to the intermediate heat exchanger provides the temperature difference necessary to realize the natural primary sodium circulation flow rate required to make the entire heat exchange process operate as intended.

21) The turbine power is modulated by modulating the NaK flow rate through the intermediate heat exchange bundles. Note that the total tube cross sectional area in the intermediate heat exchanger, in the NaK-salt heat exchanger and in the steam generator must be sufficient to remove by natural circulation 1.8 MWt of heat with the NaK induction pump unpowered. In these circumstances some power will still be required to operate the nitrate salt circulation pump and the condensate injection pump. There should be a significant reservoir of stored water (such as the volume of a district heating pipe loop) for emergency cooling via injection into the condenser. Typically the condenser is below grade level so little pumping power is required to achieve emergency cooling sufficient for removal of fission product decay heat.

Note that the normal reactor shutdown sequence is to keep the reactor in a warm shutdown state until fission product decay heat subsides. This strategy keeps the nitrate salt liquid and provides power for the various fluid circulation pumps which enable continuing fission product heat removal.

22) Each of the eight turbine halls will contain six independent steam generators with their steam outputs parallel connected to drive a single steam turbogenerator. Each such turbogenerator must be rated for continuous operation at 37.5 MWe. Thus the total system rating is:
8 X (37.5 MWe) = 300 MWe.

Before the generator breaker is closed the generator must be both frequency and phase synchronized to the external AC grid. This action is accomplished by setting the flow rate of the secondary sodium induction pumps near minimum so the steam flow to the turbine is near minimum and then adjusting the steam bypas valve to achieve the desired synchronization at which time thegenerator breaker is closed. Then the steam bypass valve is closed and the speed of the induction pumps is increased until the generator reaches the desired power level.

If a NPP is operating at full power and then suddenly loses load immediate preprogrammed action is necessary to prevent the steam turbine over speeding and destroying itself. The immediate response is to open the steam bypass valve and dump the steam to the condenser. However, that action will soon damage the condenser unless the flow of thermal energy is stopped. To stop the flow of thermal energy all the NaK flow through the intermediate heat exchanger must be stopped and the thermal energy contained in the heat transport loops must be rapidly dissipated.

Even if the molten nitrate salt pumps are stopped the pressure head that maintains the salt flow will be maintained for some time. That pressure head will continue to cause more hot nitrate salt to be delivered to the steam generators. Hence the condenser must be sufficiently oversized that it can continuously absorb the entire full power steam flow.

However, there is an additional problem. With no load the turbine will speed up and then slow down, possibly leading to loss of house power, in which case the NPP might be out of service for days until a grid black start sequence is implemented.

Hence under no load conditions the steam bypass valve must be controlled by the turbine RPM. Then just enough steam should be dumped so as to still maintain house power. Then you find that the condenser is being destroyed because every occasion this happens the condenser goes through a rapid 300 degree C temperature step.

Then you seek to mitigate that problem by spraying cold water into the surplus steam. However, those water droplets hit the condenser like machine gun bullets. Hence it is necessary to provide a replaceable baffle arrangement to protect the condenser tubes.

There is yet another problem that the loss of turbo-generator load may be unrelated to an NPP problem but instead caused by a remote grid problem. Now the issue is much more complex because instead of serving just the NPP house load it is also necessary to serve the variable local area load. Now the computer control logic and valve control system have to be an order of magnitude more complex because in the few initial ms of the incident the computer must determine the cause of the loss of load, the extent of the loss of load and the appropriate control action. That involves automatic interfacing with the protection system of the surrounding municipal utilities. Failure to do so will cause a remote grid fault to trip off the NPP which will then black out the surrounding community, which will then force a black start sequence.

Depending on the size of the surrounding community there may be no easy black start methodology. It may be necessary to get a remote transmission problem fixed so the NPP can be synchronized to external generation which then allows local community restart.

You might then find that 25 years ago some engineer devised a fix for this problem but for lack of funds at the time the fix was never widely implemented. Meanwhile everyone who fully understood the scope of the blackstart issues retired.

You then discover that in the interim fossil generation, which was previously relied upon for difficult grid black start, no longer exists. You then look for the black start procedure and find that it was written 25 years ago when grid circumstances were significantly different and that procedure may be impossible to execute today. Moreover, due to system reliability there has not been a blackout in 18 years and no existing employee on duty knows the black start procedure.

Then some well intentioned person tries to devise a black start procedure on the fly, makes a mistake, damages equipment and then you are worse off than when you started.

Let me summarize by saying that what started out to be a simple measure to protect a steam turbine from over speed and self destruction can easily become a major problem. In part that problem is the result of replacing fossil generation by clean generation without decision makers appreciating the depth of the consequences. In part the problem is a result of not heeding expert advice that when such generation replacements take place there are control consequences on parallel connected nuclear reactors that may be totally unappreciated by electricity system management, much less the general public or the politicians. In part the problem evolved because when the old NPPs were originally designed there was always alternative fossil fuel generation available for grid black start.

There is no appreciation that while it is theoretically possible to provide black start that capability often either does not exist in the as built equipment or is subject to all kinds of operating constraints. In this respect the variable power control of FNRs via its three way valves provides major advantages.

This web page last updated May 8, 2022

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