WO2003043025A1

WO2003043025A1 - Nuclear power plant with a gas cooled reactor

Info

Publication number: WO2003043025A1
Application number: PCT/IB2002/004254
Authority: WO
Inventors: Pieter Johannes Lodewyk Venter
Original assignee: Pebble Bed Modular Reactor (Proprietary) Limited
Priority date: 2001-11-16
Filing date: 2002-10-15
Publication date: 2003-05-22

Abstract

The invention provides a nuclear power plant which includes a high temperature gas cooled reactor and a power conversion unit which, together with the reactor, forms a closed loop power generation circuit. The plant further includes a primary pressure boundary which defines a pressurised enclosure. The power conversion unit includes a power turbine positioned within the primary pressure boundary of the plant. An electrical generator which is positioned outside the primary pressure boundary and drive means which extends through the primary pressure boundary and drivingly connects the power turbine to the generator.

Description

NUCLEAR POWER PLANT WITH A GAS COOLED REACTOR

THIS INVENTION relates to a nuclear power plant.

According to the invention there is provided a nuclear power plant which includes a high temperature gas cooled reactor; a power conversion unit which, together with the reactor, forms 3 closed loop power generation circuit; and a primary pressure boundary which defines a pressurised enclosure, the power conversion unit including a power turbine positioned within the primary pressure boundary of the plant, an electrical generator which is positioned outside the primary pressure boundary and drive means which extends through the primary pressure boundary and drivingly connects the power turbine to the generator.

The various components of the power conversion unit, with the exception of the generator, may be positioned within the primary pressure boundary.

A gas, typically helium, is contained within the primary pressure boundary and is maintained under pressure. Helium may be used as the working fluid in the power generation circuit and within the primary pressure boundary to pressurise the enclosure defined by the primary pressure boundary. The pressure of the helium within the primary pressure boundary may be maintained at a pressure corresponding to the maximum working pressure of helium in the power generation circuit so that should a leak form in the power generation circuit, helium will pass from the primary pressure boundary into the lower pressure region of the power generation circuit.

The power conversion unit may include a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, a pre-cooler positioned upstream of the low pressure compressor, an intercooler positioned between the low pressure compressor and the high pressure compressor, and a recuperator, the recuperator having a hot or low pressure side connected in series between an outlet of the power turbine and an inlet of the pre-cooler and a cold or high pressure side connected in series between an outlet of the high pressure compressor and an inlet of the reactor, the primary pressure boundary defining a flow path whereby the high pressure compressor and the recuperator are connected in flow communication.

The reactor may be of the pebble bed type making use of spherical fuel elements.

The drive means may include a turbine shaft which extends through the primary pressure boundary and which is drivingly connected to the generator with a seal arrangement being provided at the interface between the drive shaft and the primary pressure boundary to inhibit the leakage of gas from within the primary pressure boundary. The seal arrangement may include a dry gas seal at the shaft/primary pressure boundary interface. The seal arrangement may use helium as a lubricant and/or coolant.

The turbine shaft may be coupled directly to a shaft of the generator.

Instead, the turbine shaft may be drivingly connected to the shaft of the generator through a gearbox.

The shaft of the generator may extend vertically and be supported by an oil lubricated thrust bearing positioned outside the primary pressure boundary. In a preferred embodiment of the invention, the shafts of the power turbine and generator are vertically aligned.

The generator may be an air cooled generator.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings: Figure 1 shows a schematic flow diagram of a nuclear power plant in accordance with the invention;

Figure 2 shows schematically the connection between a power turbine and a generator in accordance with one embodiment of the invention;

Figure 3 shows schematically the connection between a power turbine and a generator in accordance with another embodiment of the invention; and

Figure 4 shows a schematic layout of the nuclear power plant in accordance with the invention. In Figure 1 of the drawings, reference numeral 10 refers generally to part of a nuclear power plant in accordance with the invention.

The nuclear power plant 10 includes a closed loop power generation circuit, generally indicated by reference numeral 12. The power generation circuit 12 includes a nuclear reactor 14 and a power conversion unit, generally indicated by reference numeral 16. The plant uses helium as the working fluid.

The power conversion unit 16 includes a high pressure turbine 18, a low pressure turbine 20, a power turbine 22, a counterflow recuperator 24, a pre-cooler 26, a low pressure compressor 28, an intercooler 30 and a high pressure compressor 32.

The reactor 14 is a pebble bed reactor making use of spherical fuel elements. The reactor 14 has a working fluid inlet 14J and a working fluid outlet 14.2.

The high pressure turbine 18 is drivingly connected to the high pressure compressor 32 and has an upstream side or inlet 18.1 and a downstream side outlet 18.2, the inlet 18J being connected to the outlet 14.2 of the reactor 14.

The low pressure turbine 20 is drivingly connected to the low pressure compressor 28 and has an upstream side or inlet 20.1 and a downstream side or outlet 20.2. The inlet 20J is connected to the outlet 18.2 of the high pressure turbine 18. The nuclear power plant 10 includes a generator, generally indicated by reference numeral 34 to which the power turbine 22 is drivingly connected, as described in more detail herebelow. The power turbine 22 includes an upstream side or inlet 22J and a downstream side or outlet 22.2. The inlet 22J of the power turbine 22 is connected to the outlet 20.2 of the low pressure turbine 20.

The recuperator 24 has a hot or low pressure side 36 and a cold or high pressure side 38. The low pressure side of the recuperator 36 has an inlet 36J and an outlet 36.2. The inlet 36J of the low pressure side is connected to the outlet 22.2 of the power turbine 22.

The pre-cooler 26 is a helium to water heat exchanger and includes a helium inlet 26J and a helium outlet 26.2. The inlet 26J of the pre-cooler 26 is connected to the outlet 36.2 of the low pressure side 34 of the recuperator 24.

The low pressure compressor 28 has an upstream side or inlet 28.1 and a downstream side outlet 28.2. The inlet 28J of the low pressure compressor 28 is connected to the helium outlet 26.2 of the pre-cooler 26.

The intercooler 30 is a helium to water heat exchanger and includes a helium inlet 30J and a helium outlet 30.2. The helium inlet 30.1 is connected to the outlet 28.2 of the low pressure compressor 28.

The high pressure compressor 32 includes an upstream side or inlet 32J and a downstream side or outlet 32.2. The inlet 32.1 of the high pressure compressor 32 is connected to the helium outlet 30.2 of the intercooler 30. The outlet 32.2 of the high pressure compressor 32 is connected to an inlet 38J of the high pressure side of the recuperator 24. An outlet 38.2 of the high pressure side of the recuperator 24 is connected to the inlet 14.1 of the reactor 14.

The nuclear power plant 10 includes a start-up blower system, generally indicated by reference numeral 40, connected between the outlet 36.2 of the low pressure side 36 of the recuperator 24 and the inlet 26.1 of the pre-cooler 26.

The start-up blower system 40 includes a normally open start-up blower system in-line valve 42 which is connected in line between the outlet 36.2 of the low pressure side of the recuperator and the inlet 26J of the pre- cooler 26.

A low pressure compressor recirculation line 48 extends from a position between the outlet or downstream side 28.2 of the low pressure compressor 28 and the inlet 30J of the intercooler 30 to a position between the start-up blower system 40 and the inlet 26.1 of the pre-cooler 26. A normally closed low pressure recirculation valve 50 is mounted in the low pressure compressor recirculation line 48.

A high pressure compressor recirculation line 52 extends from a position between the outlet or downstream side 32.2 of the high pressure compressorand the inlet 38J of the high pressure side 38 of the recuperator 24 to a position between the outlet or downstream side 28.2 of the low pressure compressor 28 and the inlet 30.1 of the intercooler 30. A normally closed high pressure recirculation valve 53 is mounted in the high pressure compressor recirculation line 52. A recuperator bypass line 54 extends from a position upstream of the inlet 38J of the high pressure side 38 of the recuperator 24 to a position downstream of the outlet 38.2 of the high pressure side 38 of the recuperator

24. A normally closed recuperator bypass valve 56 is mounted in the recuperator bypass line 54.

The plant 10 includes a high pressure coolant valve 58 and a low pressure coolant valve 60. The high pressure coolant valve 58 is configured when open, to provide a bypass of helium from the high pressure side or outlet 32.2 of the high pressure compressor 32 to the inlet or low pressure side 20J of the low pressure turbine 20. The low pressure coolant valve 60 is configured, when open, to provide a bypass of helium from the high pressure side or outlet 32.2 of the high pressure compressor 32 to the inlet 22.1 of the power turbine 22.

The plant 10 further includes a helium inventory control system, generally indicated by reference numeral 61 , whereby the inventory of working fluid in the power generation circuit 12 can be regulated.

The power generation circuit is configured to operate on a Brayton cycle as the thermodynamic conversion cycle.

Reference is now also made to Figure 4 of the drawings, in which the same reference numerals used above are used to represent the same parts. The plant 10 includes means defining a primary pressure boundary 70 within which, as can be seen in Figure 1 of the drawings, the components of the power generation circuit are contained. The primary pressure boundary 70 is defined by a shell 71 which comprises a plurality of modules within which the components of the power generation circuit are housed, the modules being connected together in a fluid tight manner.

As can best be seen in Figure 4 of the drawings, the primary pressure boundary 70 defines a flow path 73 which, as indicated by arrows 75, connects the high pressure compressor 32 in flow communication with the recuperator 24. As a result, the primary pressure boundary 70 in fact forms part of the power generation circuit and helium contained therein is at a pressure which corresponds to the maximum pressure within the power generation circuit as determined by the pressure of the helium discharged from the high pressure compressor 32. An advantage with this arrangement is that should a leak form in the partition 79 between the high pressure and low pressure regions of the power generation circuit, helium will typically flow from the primary pressure boundary through the leak into the lower pressure region of the power generation circuit where the leak has formed.

The generator 34 is positioned outside the primary pressure boundary

70. More particularly, with reference to Figure 2 of the drawings, the power turbine 22 has a shaft 72 which extends through the primary pressure boundary 70. The generator 32 has a shaft 74. The shafts 72, 74 are vertically in register and drivingly connected to one another. If desired, the shaft 74 can be supported on an oil lubricated thrust bearing 76 which is positioned outside the primary pressure boundary 70.

it will be appreciated that, in the embodiment shown in Figure 2, the shafts 72, 74 are coupled together and accordingly rotate at the same speed.

Reference is now made to Figure 3 of the drawings in which, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. The main difference between the embodiment of the invention as shown in Figure 3 of the drawings and that shown in Figure 2 of the drawings is that, in case of the embodiment shown in Figure 3 of the drawings, the shaft 72 is drivingly connected to the shaft 74 via a gearbox 80. In the embodiment shown, the gearbox 80 is in the form of a planetary gearbox. It will be appreciated, however, that any speed reduction arrangement could be used.

A seal arrangement, generally indicated by reference numeral 82 is provided at the interface between the shaft 72 and the primary pressure boundary 70. The seal arrangement 82 typically comprises a dry gas seal. This seal functions on the principle of aerodynamic lift-off of the seal faces. When relative rotation between the seal faces occurs, they part due to gas entering specially designed aerodynamic recesses. The recesses are of such a design so as to cause a local increase in pressure between the two seal faces, and a subsequent lifting force. Spring forces oppose this parting force, and act to close the faces again. Equilibrium is thus reached leaving a constant gap in the order of microns. A small helium gas leak occurs over the sealing faces and serves as lubricant and coolant. Helium lost through the seal is recovered from a containment zone. The recovered helium is purified and re-introduced into the primary pressure boundary.

If the generator were to be contained within the primary pressure boundary, then a different form of thrust bearing would be required since no lubricants such as oil can be used inside the primary pressure boundary since they would contaminate the helium. This would entail the use of, for example, an electromagnetic thrust bearing and possibly a pressure balancing system on the turbine which leads to a substantial increase in the complexity of the arrangement. Accordingly, an advantage of the present invention is that an oil lubricated thrust bearing can be used. These bearings are capable of supporting substantially higher thrust loads than an electromagnetic bearing. In addition, they are substantially less expensive. Further, maintenance of the bearing is facilitated which will result in a greater degree of reliability in the system. In addition, by positioning the generator outside the primary pressure boundary, a conventional design air-cooled generator can be utilized. This will simplify the generator set-up and also eliminate the need for primary pressure boundary penetrations for cabling and the like. Another advantage will be reduced windage losses, as cooling of the generator will take place at a reduced pressure compared to pressures in the primary pressure boundary. In addition, positioning the generator outside the primary pressure boundary will substantially improve the accessibility to the generator and therefor significantly improve maintenance access.

Further, the introduction of a gearbox will result in a higher turbine speed and a lower generator speed. The higher turbine speed paves the way for a much smaller power turbine and associated equipment. Except for the obvious advantages associated with a smaller turbine (eg reliability, cost, practicality) a major advantage is the lower leak rates associated with the smaller diameters of rotating equipment resulting in increased system efficiency. The other equipment associated with the turbine, such as the gas seal and electromagnetic bearings will also be smaller, resulting in increased reliability and less risk. The use of a gearbox will furthermore provide for more flexibility in terms of generator selection. Conversion froma high speed to a low speed generator, or from a 50 Hz to 60 Hz generator can be achieved by changing the gear ratio, without having to alter the turbine. Further, the incorporation of a thrust-transmitting gearbox enables the use of an oil lubricated thrust bearing on the output shaft of the gear box. In this case it will be on the low speed side of the gearbox. The lower speed will result in substantially lower thrust bearing losses and increased efficiency, as compared to the situation where there is no gearbox and the thrust bearing is positioned on the single high speed shaft.

Claims

1. A nuclear power plant which includes a high temperature gas cooled reactor; a power conversion unit which, together with the reactor, forms a closed loop power generation circuit; and a primary pressure boundary which defines a pressurised enclosure, the power conversion unit including a power turbine positioned within the primary pressure boundary of the plant, an electrical generator which is positioned outside the primary pressure boundary and drive means which extends through the primary pressure boundary and drivingly connects the power turbine to the generator.

2. A nuclear power plant as claimed in claim 1 , in which the various components of the power conversion unit, with the exception of the generator, are positioned within the enclosure defined by the primary pressure boundary.

3. A nuclear power plant as claimed in claim 1 or claim 2, in which helium is used as the working fluid in the power generation circuit and within the primary pressure boundary.

4. A nuclear power plant as claimed in claim 3, in which the pressure of the helium within the primary pressure boundary is maintained at a pressure corresponding to the maximum working pressure of helium in the power generation circuit.

5. A nuclear power plant as claimed in any one of the preceding claims, in which the power conversion unit includes a high pressure turbine drivingly connected to a high pressure compressor, a low pressure turbine drivingly connected to a low pressure compressor, a pre- cooler positioned upstream of the low pressure compressor, an intercooler positioned between the low pressure compressor and the high pressure compressor, and a recuperator, the recuperator having a hot or low pressure side connected in series between an outlet of the power turbine and an inlet of the pre-cooler and a cold or high pressure side connected in series between an outlet of the high pressure compressor and an inlet of the reactor, the primary pressure boundary defining a flow path whereby the high pressure compressor and the recuperator are connected in flow communication.

6. A nuclear power plant as claimed in any one of the preceding claims, in which the reactor is of the pebble bed type making use of spherical fuel elements.

7. A nuclear power plant as claimed in any one of the preceding claims, in which the drive means includes a turbine shaft which extends through the primary pressure boundary and which is drivingly connected to the generator with a seal arrangement being provided at the interface between the drive shaft and the primary pressure boundary to inhibit leakage of gas from within the primary pressure boundary.

8. A nuclear power plant as claimed in claim 7, in which the seal arrangement includes a dry gas seal at the shaft / primary pressure boundary interface.

9. A nuclear power plant as claimed in claim 8, in which the seal arrangement uses helium as a lubricant.

10. A nuclear power plant as claimed in any one of claims 7 to 9 inclusive, in which the turbine shaft is coupled directly to a shaft of the generator.

11. A nuclear power plant as claimed in any one of claims 7 to 9, inclusive, in which the turbine shaft is drivingly connected to a shaft of the generator through a gearbox.

12. A nuclear power plant as claimed in claim 10 or claim 11 , in which the shaft of the generator extends vertically and is supported by an oil lubricated thrust bearing positioned outside the primary pressure boundary.

13. A nuclear power plant as claimed in any one of claims 10 to 12, inclusive, in which the shafts of the power turbine and generator are vertically aligned.

14. A nuclear power plant as claimed in any one of the preceding claims, in which the generator is an air cooled generator.

15. A nuclear power plant as claimed in claim 1 , substantially as described and as illustrated herein.

16. A new nuclear power plant substantially as described herein.