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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 500 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 two stage steam turbine which in turn directly drives a line synchronous generator which produces 60 Hz 3 Phase electric power.

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

Issues related to the practicxal design of liquid sodium cooled FNRs effectively set the steam pressure delivered to the turbine at 11.25 MPa (corresponding to a saturated steam temperature of 320 degrees C) and set the dry steam temperature between about 450 degreees C at full load rising to about 530 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.

The purpose of this web page is to explain the operation of each of the multiple two stage steam turbines connected to a FNR.
 

BASIC STEAM TURBINE OPERATION:
1) A turbine consists of blades attached to the perimeter of circular rotating wheels connected coaxially to 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, generally 900 RPM, 1800 RPM or 3600 RPM. 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 and turbine engineering 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 shaft which causes the generator to act as an energy source feeding the electricity grid.

2) A boiler or 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 11.25 MPa. Hence steam flows from the steam generator to the turbine high pressure steam input at a pressure of about 11.25 MPa.

3) This steam expands through pressure reducing nozzles into the high pressure turbine casing. This steam expansion decreases the steam pressure 10X to about 1.0 MPa and due to the ideal gas law equation:
P V = N R T
where:
P = pressure
V = volume
N = number of moles
R = ideal gas conatant
T = absolute temperature
increases the steam volume about 10X. This increase in steam volume gives the steam a high velocity at the nozzle discharge. The high velocity steam impacts the high pressure turbine blades tangentially causing rotational torque. This blade impact also greatly reduces the steam linear velocity.

4) Note that in order to efficiently transfer energy from the steam flow to the moving turbine blades the steam velocity at the nozzle discharge must be about 2X the tangential velocity of the turbine blades. Since the turbine blade velocity is fixed by the generator RPM and by the turbine wheel radius, there is an important geometric relationship between these various parameters that determines steam turbine geometry. An important practical issue in steam turbines is to minimize the clearance between the turbine blades and the turbine casing without allowing the clearance to shrink to zero due to differential thermal expansion.

5)In reality the steam is not an ideal gas and in the expansion process there is some loss of steam temperature. In order to maintain the desired steam temperature, which prevents premature steam condensation that can potentially cause erosion of the low pressure turbine blades, some heat is added to the steam by an interstage heating coil while the steam is at a pressure of 1.0 MPa. This extra heat is obtained from the steam generator.

6) The steam now expands through a second nozzle which feeds the low pressure turbine. The steam pressure drops from about 1.0 MPa in the high pressure turbine casing to less than 0.10 MPa in the low pressure turbine casing. Again the steam volume increases about 10X. Again the pressure drop causes the steam to be accelerated to a high velocity. This steam impacts the blades of the low pressure turbine causing additional rotational torque.

7) From the high pressure turbine inlet to the low pressure turbine discharge the steam expands about 100 fold in volume. Since the mass flow is uniform parallel to the turbine shaft the turbine casing cross sectional area must increase 100 fold, implying that the turbine casing radius must increase about 10 fold. In practise this radial increase is achieved by two turbine stages, each of which increases in radius by more than 3 fold.

8) The low pressure 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 low pressure turbine discharge flows into the condenser where the steam is fully condensed dumping as much thermal energy as possible to a heat sink. Ideally the heat sink is cold lake or ocean water. When such near ideal heat sinks are not available the heat sink is usually a cooling tower. The result is a partial vacuum in the condenser.

10) The liquid condensate is then pumped from the low pressure accumulation inside the condenser (typically at 0.04 MPa) to 11.25 MPa, the feedwater pressure to the steam generator. This feedwater injection pump is an important parasitic load on the system.

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

12) This heated condensate is mixed with recirculated steam generator bottom water which further raises the mixture temperature to at least 300 degrees C. This preheating of the condensate water feed to 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. Flowing intermediate sodium in the steam generator tubes raises the steam generator water temperature to 320 degrees C.

14) Flowing intermediate sodium in the steam generator tubes then adds latent heat of vaporization to the steam generator water to convert it to saturated steam at 320 degrees C.

15) Flowing intermediate sodium 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 450 deg C to 500 deg C, depending on the steam and intermediate sodium mass flow rate.

16) The steam flows out of the steam generator to the turbine steam inlet via a pressure regulating valve which maintains the pressure in the steam generator at 11.25 MPa. This pressure regulating valve sets the saturated steam temperature in the steam generator at 320 degrees C, which in turn sets the maximum liquid water temperature in the steam generator at 320 degrees C.

17) This steam generator liquid water temperature together with a maximum 10 deg C drop across the steam generator tube wall, ensures that the intermediate liquid sodium discharge temperature from the steam generator is in the range 320 deg C to 330 deg C.

18) This intermediate sodium discharge temperature is sufficient to ensure that there is no deposition of solid NaOH on heat exchange surfaces. NaOH melts at 318 degrees C.

19) This intermediate sodium discharge temperature, together with the balanced counterflow design of the intermediate heat exchanger and a maximum 10 degree C drop across the intermediate heat exchanger tube wall, ensures that the primary sodium return temperature to the reactor is less than 340 degrees C.

20) This return temperature of 340 degrees C to the reactor together with the 490 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 intermediate sodium flow rate. Note that the total tube cross sectional area in the intermediate heat exchanger and in the steam generator must be sufficient to remove by natural circulation 100 MWt of heat with the intermediate sodium induction pump unpowered, and with steam from the steam generator directly vented to the atmosphere to minimize the load on the condensate injection pump. In these circumstances some minimum power will still be required to operate the condensate injection pump. There should be a significant reservoir of stored water for emergency cooling via injection into the base of the condenser. Typically the base of the condenser is close to grade level so no additional pumping is required to achieve emergency cooling sufficient for removal of fission product decay heat.

22) The current thinking is that each of the eight turbine halls will contain two Siemens SST-200 steam turbines. These turbines (15 MWe and 20 MWe) operate at up to 14,000 RPM. These turbines are rated for steam at up to 120 bar and up to 540 degrees C. These turbines will drive alternators that produce high frequency (400 Hz) 3 phase AC. The high frequency AC will be down converted into 60 Hz AC using a rectifier-inverter or cycloverter. The nominal shaft RPM will be:
(400 Hz / 60 Hz) X 1800 RPM = 12,000 RPM

The 60 Hz AC output frequency and phase will be varied to simulate the inertial behavior of a synchronous generator.

23) An alternative turbo-generator solution is the 23 MWe turbogenerator available from Buffalo Turbines. Click on the following links to obtain the dimensions and specifications of this turbo-generator. The intent is to install two such Buffalo Turbine units in parallel in each turbine hall such that the two units share the same condenser tube pull space allowance.
 

This web page last updated June 9, 2020

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