XYLENE POWER LTD.

FNR STEAM TURBINE

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

NUCLEAR ELECTRICITY:
A Fast Neutron Reactor (FNR) produces heat at up to about 460 degrees C. That heat is used to produce steam with a pressure of about 10 MPa, a saturation temperture of about 315 degrees C and a discharge temperature of about 400 degrees C. This steam is expanded through a steam turbine to produce rotational mechanical power which in turn directly drives a line synchronous electricity generator to produce 60 Hz 3 Phase AC electric power.

A possible alternative to steam is supercritical CO2 as outlined in:
Closed Cycle Gas Turbine for Power Generation

This web page addresses basic matters relating to steam turbines. The design fundamentals of steam turbines have not changed significantly during the last 100 years. However, most present steam turbines are intended for operation with fossil fuel fired boilers. The steam temperatures and pressures available from liquid sodium cooled FNRs are somewhat different.

REFERENCES:
Energy Conversion
and
Operation and Maintenance Schedule of a Steam Turbine Power Plant
and
THERMAL POWER PLANT
and
Steam Turbine Basics
and
Steam Turbine Fundamentals

Potential suppliers of steam turbo generators in the power range 19 MWe to 38 MWe include:
Peter Brotherhood Steam Turbine Generators
and
Howden Steam Turbines
and
Buffalo Turbines

Note that Peter Brotherhood has recently been purchased by Howden.

Issues related to the practical design and operation of liquid sodium cooled power FNRs effectively set the steam pressure delivered to the turbine at 10.0 MPa to 11.25 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 450 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 maximum steam thermal power available for each of the eight turbine halls.
 

IMPULSE TURBINE CONCEPTUAL DESIGN:
An impulse turbine consists of a series of rotating wheels with perimeter blades sharing a common shaft. The turbine wheels are separated by fixed baffles such that the region between baffles is at a nearly constant pressure and there is a one nozzle pressure drop across each baffle. Each fixed baffle has a central hole that permits the shared shaft to rotate. The shaft to fixed baffle clearance is minimized to minimize steam leakage through the turbine adjacent to the shaft. Similarly the clearance between the rotating turbine blades and the turbine housing is minimized, subject to constraints imposed by the material thermal coefficients of expansion.

There are strict limits on the maximum rate of change of steam temperature. The problem is that even with precise matching of thermal coefficients of expansion, on a rising steam temperature due to their lower mass per unit of surface area the turbine blades expand faster than the turbine casing and on a falling steam temperature due to their lower mass per unit of surface area the turbine baffles contract faster than the turbine shaft.

Thus steam turbine design requires a compromize between turbine efficiency and rate of change of steam temperature and hence rate of change of shaft power. Steam turbines which are designed for following rapid net electrical load changes are generally less efficient than steam turbines that are intended to be operated at flat load.

Each baffle enclosed bladed turbine wheel is referred to as a stage. Steam flows between successive turbine stages via nozzles. These nozzles convert the interstage differential gas pressure into gas kinetic energy. The nozzle discharge directs the steam at the moving turbine blades. The steam flow vector through the turbine wheels at the blades is nearly tangential to the blade motion but elsewhere is nearly axial.

In the space between adjacent baffles the gas pressure is nearly constant. The working gas (steam) enters the high pressure end of the turbine housing via nearly tangential nozzles to impact the blades. For optimum momentum transfer the tangential velocity of the steam should be about twice the tangential velocity of the turbine blades. At each stage the steam looses its tangential momentum component. The steam acquires new tangential momentum in the next nozzle. After passing through a succession of stages steam exits from the low pressure end of the turbine housing. This steam flows into a condenser where it is condensed causing at a partial vacuum.

As the steam progresses axially through the turbine it expands, leading to a requirement for turbine wheel and housing diameters to increase along the turbine axis.

Steam that leaks through the turbine either through gaps between the turbine shaft and baffles or through gaps between the turbine blades and the turbine casing does no useful work and hence contributes to turbine inefficiency.

Note that an impulse turbine is designed for maximum efficiency at a particular steam feed pressure and steam mass flow. In a nuclear power plant we are seldom concerned about turbine part load efficiency. However, we are concerned about synchronizing the generator to the grid. As the steam mass flow reduces the steam nozzle velocity will drop until at reduced steam flow the nozzle velocity is barely above the turbine wheel tangential velocity. In this condition the turbine is barely able to maintain grid synchronous speed with no load. That is the condition for synchronization of the generator to the grid.

We must also be concerned about turbine over speed if under full load operation the generator suddenly disconnects from the grid or from 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 flow rate 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 turbo generator from overspeed if there is sudden loss of grid frequency or loss of islanded electrical load. Note that with no load and without turbine steam bypass to the condenser an impulse turbine will tend to over speed by a factor of two. That amount of overspeed will increase turbine material stress by a factor of four, which will likely cause serious equipment damage.
 

EQUIPMENT MODULARITY:
Typically the 37.5 MWe turbogenerator consists of the following major components:
a) A low pressure turbine;
b) A high pressure turbine;
c) Steam reheater;
d) Generator rotor;
e) Generator Stator;
f) Main contactor;
g) Two sets of torque transmission couplings;
h) Equipment base;
i) Condenser.

The size and weight of each component must be compatible with truck transport and installaion in the available 26 m long space using a mobile crane parked on the adjacent roadway or laneway. The equipment installation is via a removable roof section. The electrical equipment, including the synchronous generator, is installed above grade level. The condenser and steam generators are installed below grade level.
 

THEORETICAL TURBINE EFFICIENCY:
The ideal gas equation at gas state i is:
Pi Vi = Ni R Ti
where:
Pi = absolute pressure
Vi = gas volume
Ni = number of gas moles or molecules
R = ideal gas constant
Ti = absolute temperature

Note that Pi Vi is the energy content of the gas at state i. If the gas changes from state a to state b in a turbine the change in contained gas energy is:
[Pa Va - Pb Vb] = [Na R Ta - Nb R Tb]

In turbine no gas molecules are lost so that:
Na = Nb = N
which gives:
[Pa Va - Pb Vb] = N R [Ta - Tb]

The amount of energy that must be added to a confined liquid to achieve the initial gas state is:
Pa Va

The amount of energy that must be rejected at the final gas state in order to achieve the phase change necessary to return the working fluid to the liquid state is:
Pb Vb

Hence the maximum possible efficiency of a steam turbine is given by:
[Pa Va - Pb Vb] / Pa Va = N R [Ta - Tb] / N R Ta
= [Ta - Tb] / Ta
= which is the famous Carnot limit formula.

Asssume that:
Ta = 390 degrees C = 663 degrees K
Tb = 100 degrees C = 373 degrees K

Then the theoretical maximum turbine efficiency is:
(663 - 373) / 663 = 0.437
 

IMPULSE TURBINE CONCEPTUAL DESIGN:
An impulse turbine consists of a series of rotating wheels with perimeter blades mounted on a common shaft. The turbine wheels are separated by fixed baffles such that the region between successive baffles is at a nearly constant pressure and there is a pressure drop across each baffle. Each fixed baffle has a central hole that permits the shaft to rotate but the shaft to fixed baffle clearance is minimized to minimize steam leakage through these axial holes.

Each baffle enclosed bladed turbine wheel is referred to as a stage. Steam flows from a higher pressure stage to the next lower pressure stage via nozzles. These nozzles convert the interstage differential steam pressure into gas kinetic energy. and direct the steam tangentially at the blades of the lower pressure stage. Thus the steam flow through the turbine is tangential at the blades but is axial between successive stages.

In the space between adjacent baffles the steam pressure is nearly constant. The steam enters the high pressure end of the turbine housing and moves from stage to stage losing pressure at each stage. The tangential velocity of the steam is about twice the tangential velocity of the blades. Hence at each stage the steam looses its tangential momentum component. The steam acquires new tangential momentum in its next nozzle. Steam exits at the low pressure end of the turbine housing.

Note that this impulse turbine is designed for maximum efficiency at a particular steam feed pressure and steam mass flow. In a nuclear power plant we are seldom concerned about turbine part load efficiency. However, we are concerned about synchronizing the generator to the grid at low steam flow. As the steam mass flow reduces the steam nozzle velocity will drop until at reduced steam flow the nozzle velocity is barely above the turbine wheel tangential velocity. In this condition the turbine is barely able to maintain synchronous speed with no load. That is the condition for synchronization of the generator to the grid.
 

TURBINE WHEEL BLADE MAXIMUM TANGENTIAL SPEED:
The diameter of a turbine wheel is limited by its material yield strength at the working gas operating temperature. Consider a rotating ring with:
R = radius
Ri = inside radius
Ro = outside radius
T = axial thickness
Rhow = density of wheel material
W = angular frequency
Sh = material hoop stress
Phi = angle around the turbine axis
dM = element of mass
dA = element of area

Consider a ring with radial thickness dR:
dA = R dPhi T

Centrifugal force:
dF = dM V^2 / R
= (dA dR Rhow)(R W)^2 / R
= (R dPhi T dR Rhow)(R W)^2 / R
= (dPhi T dR Rhow)(R W)^2

The corresponding radial force at radius Ri on the ring angle dPhi is:
F = Integral from Ro to Ri of: (dPhi T Rhow)(R W)^2 (-dR)
= (dPhi T Rhow W^2 (Ro^3 - Ri^3) / 3

Effective internal pressure at Ri:
= F / A
= dPhi T Rhow W^2 (Ro^3 - Ri^3) / 3 Ri dPhi T
= Rhow W^2 (Ro^3 - Ri^3) / 3 Ri

Note that this internal stress goes to infinity as Ri approaches zero. Hence Ri > 0.

Now drill a round hole in the center of the disk so that Ri > 0.

Hoop stress:
Sh = [Rhow W^2 (Ro^3 - Ri^3) / 3 Ri] 2 Ri T / [2 (Ro - Ri) T]
= [Rhow W^2 (Ro^3 - Ri^3) / 3 ] / (Ro - Ri)

For Ri << Ro:
Sh = Rhow W^2 Ro^2 / 3

Rearranging this equation gives:
(Ro W)^2 = [3 Sh / Rhow]
or
(Ro W) = [3 Sh / Rhow]^0.5
which imposes an upper limit on tangential velocity of a thin rotating disc with a central hole.

For typical steel the maximum working value of Sh is:
Sh = 10,000 lb / inch^2
= 10,000 lb X .454 kg / lb X 9.8 m / s^2 / (1 inch X 0.0254 m / inch)^2
= 6.89627 X 10^7 kg m / s^2 m^2
= 6.89627 X 10^7 kg / s^2 m

For typical steel:
Rhow = 8.74 X 10^3 kg / m^3

Hence:
(Ro W) = [3 Sh / Rhow]^0.5
= [(3 X 6.89627 X 10^7 kg / s^2 m) / (8.74 X 10^3 kg / m^3)]^0.5
= 153.85 m / s

At 1800 RPM:
W = 2 Pi (30 rev / s) = 188.4 radians / s

Hence at 1800 RPM the maximum value of Ro is given by:
Ro = (Ro W) / W = (153.8 m / s) / (188.4 radians / s)
0.816 m

In a real turbine the material may be marginally stronger but that additional strength is off set by the rotating mass of the turbine blades.
 

INTERSTAGE DIFFERENTIAL PRESSURE:
Now consider a gas nozzle connecting region "x" with pressure Px to region "y" with pressure Py where Px > Py. Define: At = nozzle throat area
Vt = gas velocity in nozzle throat
Rhoy = gas density in region y

Power = Force X velocity
= (Px - Py) At Vt

Power = kinetic energy / unit time
= (dM / dt) Vt^2 / 2
= At Vt Rhoy Vt^2 / 2

Equate the two expressions for power to get:
(Px - Py) At Vt = At Vt Rhoy Vt^2 / 2
or
(Px - Py) = Rhoy Vt^2 / 2

There is another important equation which says that for good momentum transfer between the working gas and the turbine blades:
Vt = (2 Ro W)

Hence for each stage of the turbine:
(Px - Py) = Rhoy Vt^2 / 2
= Rhoy (2 Ro W)^2 / 2
= Rhoy 2 (Ro W)^2

Recall that the turbine wheel material strength limitation gave the maximum value of (Ro W)^2 as:
(Ro W)^2 = [3 Sh / Rhow]

Hence:
(Px - Py) = Rhoy 2 (Ro W)^2
= Rhoy 2 (3 Sh / Rhow)
= Rhoy 6 (Sh / Rhow)

This formula effectively limits the maximum pressure drop between successive turbine stages.
 

NUMERICAL EXAMPLE:
Assume 1800 RPM:
W = (2 Pi radians / rev) (30 rev / s)
= 60 Pi radians / s

Sh = 10,000 lb / inch^2 X (.454 kg X 9.8 m / s^2) / lb X (1 inch / 0.0254 m)^2
= 6.896 X10^7 kg /m s^2

Rhow = 7,870 kg / m^3

Ro = (1 / W)[3 Sh / Rhow]^0.5
= (1 s / 60 Pi) [3 (6.896 X10^7 kg /m s^2) / (7,870 kg / m^3)]^0.5
= 0.86 m

(Px - Py) = Rhoy 6 (Sh / Rhow)
= Rhoy 6 [(6.896 X10^7 kg /m s^2) / (7,870 kg / m^3)]
= Rhoy 6 [8762.3] m^2 / s^2

At 273 degrees K, 101 kPa Rhoo = 18 g / 22.4 lit
= .018 kg / 22.4 X 10^-3 m^3
= 18 kg /22.4 m^3

At 320 deg C:
Rhoy = (18 kg / 22.4 m^3) [273 / (273 + 320)][Py / 101,000 Pa]
= 0.36994 [Py / 101,000 Pa] kg / m^3

Thus:
(Px - Py) = Rhoy 6 [8762.3] m^2 / s^2
= 0.36994 [Py / 101,000 Pa] kg / m^3 {6 [8762.3] m^2 / s^2}

Thus:
Px = Py {1 + 0.1925 kg / m s^2 Pa}
where:
1 Pa = 1 kg m / s^2 m^2
giving:
Px = Py {1.1925}
or
Py / Px = 1 / 1.1925
= 0.8385

Assume that the fractional interstage pressure drops are equal. Then:
(Py / Px)^2 = .7032070637
(Py / Px)^4 = 0.4945001745
(Py / Px)^8 = 0.2445304226
(Py / Px)^16 = 0.0597951276
(Py / Px)^28 = 0.001230447

Hence realizing maximum energy recovery involves at least 28 turbine stages. Typically they are in two sets each of over 14 stages, one set being the high pressure turbine and the other set being the low pressure turbine.

Thus an impulse type steam turbine consists of over 28 rotating bladed wheels separated by fixed baffles. The baffles direct the gas flow through nozzles to nearly tangentially impact the blades of the next turbine wheel. Each fixed baffle must withstand the differential pressure drop across it.

A turbine housing with uniform wheel spacing must gradually expand in diameter to allow for the increasing steam volume. An increase in diameter affects the tangential speed which affects the optimum interstage pressure drop. Typically there is a high pressure turbine and a low pressure turbine, as the housing thickness and diameter requirements are different.
 

EFFICIENCY:
1) When steam is expanded adiabatically through a turbine the pressure drops and the volume increases resulting in work being done.
2) However, if an adiabatic expansion is used the turbine's steam discharge temperature is too high for good system thermal efficiency.
3) The best way of increasing the system thermal efficiency is to do a non-adiabatic steam expansion where as the steam moves parallel to the turbine axis part of its contained heat is transferred first to pressurized makeup water and ten to unpressurized makeup water. The result is lower energy content steam at the turbine discharge.
4) This heat transfer has to be carefully designed to prevent premature condensation within the turbine.
5) The lower temperature steam discharge results in less sensible heat being dumped to the heat sink via the condenser.
6) The lower mass flow of steam in the turbine discharge results in less latent heat being dumped to the heat sink via the condenser.
7) The complexity of this makeup water heating system makes it only ecomomically practical on relatively large steam turbines.
8) From the perspective of SMRs we need to find a supplier of smaller steam turbines that have this feature. While this feature seems to be relatively available at the 300 MWe level, it does not seem to be readily available at lower powers such as 40 MWe where it would be highly desirable.
9) Maybe somebody makes such a steam turbine for submarines???
10) This apparent unavailability of a suitable high efficiency lower power steam turbine that works at 1800 RPM or 3600 RPM may be one of the SMR deployment challenges.
 

May 3, 2023 Note from Paul Acchione:
The thermodynamic efficiency improves when you use extraction steam feedwater heaters because the steam extracted from the turbine and used to preheat the feedwater does not pass through the condenser and so its latent heat of condensation is not lost to the power cycle. The condensed steam is added to the feedwater flow stream (at a lower pressure point in the feedwater path) and returned to the boiler. For turbines that have main steam reheat of the lower pressure steam entering the LP turbines, the condensed steam is pumped up to the boilers using reheater drains pumps. Typically this reheater drains flow is less than 8% of the main steam flow to the HP turbines.

Feedwater heaters and reheating of LP steam were clever improvements to the basic Rankin cycle. The number of extraction points and heaters is limited by practical considerations. As you mentioned, smaller size turbines do not have the same number of heaters as large machines due to economic considerations.
 

The turbogenerator feed steam conditions for a 38 MWe turgogenerator are 125 MWt, 10 MPa, 400 degrees C. In order to operate in turbine steam bypass, the turbine condenser must be capable of continuous absorption of the full steam thermal energy.
 

SUMMARY OF STEAM TURBINE OPERATION:
1) The purpose of a turbine is to capture mechanical energy from expansion of steam in a manner suitable for efficient generation of electrical energy.

2) Most large power stations use axial turbines. However, axial turbines have limitations.
a) For efficiency the clearance between the turbine blades and the turbine casing must be small. However, on rapid power up the casing will be cool while the turbine wheel is warm, potentially leading to the turbine blades rubbing against the casing, which is a disaster. Hence if the turbine must track rapid changes in the electricity grid there is loss of steady state efficiency due to steam leakage through the gaps between the outer perimeters of the turbine blades and the turbine casing;
b) A turbine uses a reduction in working gas pressure to linearly accelerate the working gas through a nozzle. The working gas transfers its kinetic energy to the turbine blades. Simultaneous conservation of both energy and momentum indicates that energy transfer from the working gas to the turbine blade is most efficient when the turbine blade tangential velocity is about half of the working gas velocity.
c) In a practical steam turbine the maximum blade tangential velocity is limited by high temperature material strength issues. At a particular stage the blade tangential velocity is held nearly constant by the nearly constant AC line frequency and the turbine wheel diameter;
d) Hence to achieve turbine efficiency the maximum working gas nozzle velocity is limited to twice the tangential blade velocity;
e) This limit on maximum working gas velocity limits the gas pressure drop across each nozzle;
f) Hence a practical turbine has a series of stages, which share a common drive shaft;
g) As the working gas pressure drops the working gas volume increases, so the turbine housing size increases as the gas flows axially from stage to stage;
h) The increase in housing diameter changes the turbine wheel diameter which changes the interstage pressure.
i) The gas volume at each stage is proportional to the housing inside diameter squared and the stage thickness. Hence a for equally spaced stages a 10 fold reduction in gas pressure requires a 3.16 fold increase in turbine housing inside diameter.
j) Typically in practice there is an overall 200 fold decrease in gas presssure which is achieved via two series connected turbines referred to as the high pressure turbine and the low pressure turbine.

3) An efficient turbo-generator 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 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.
 

STEAM TURBINE LINE SYNCHRONIZATION:
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.
 

STEAM TURBINE OVER SPEED PREVENTION:
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.
 

BLACKSTART ISSUES:
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.
 

SUMMARY:
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 variable NaK pumping rate provides major advantages.
 

This web page last updated May 2, 2023