Patent Number: 041586042
Section: description

Referring to FIG. 1, there is shown the reactor 10 having a helium cooled core 12 disposed within a concrete pressure vessel 14. Also disposed within the concrete pressure vessel are four main boilers 16 (of which only one is shown in FIG. 1) each main boiler 16 having a reheater 18 associated therewith. The main boilers 16 and associated reheaters 18 are connected to a large multi-cylinder steam turbine, generally referenced 20, arranged to drive a 666 MW alternator 22. Four auxiliary boilers 24 (again only one of which is shown in FIG. 1) are also disposed within the pressure vessel 14 and are connected to two auxiliary steam turbines 26 (of which only one is shown) each arranged to drive a 60 MW alternator 28. Referring to FIG. 2 there is agin shown one main boiler 16 and one auxiliary boiler 24. Each main boiler 16 is disposed within a vertical cylindrical channel or pod 50 defined within the wall thickness of the pressure vessel 14. The main boiler 16 comprises an economizer and evaporator section 32 and a super-heater section 34. The reheater 18 is disposed below the super-heater section 34. Above the main boiler 16 is a reactor coolant circulating pump 36 arranged to draw helium coolant upwardly through the boiler from the bottom of the core 12. The auxiliary boiler 24 is disposed in a pod 38 similarly defined in the wall thickness of the pressure vessel 14, the boiler comprising economizer, evaporator and superheater water/steam tube sections generally referenced 40. Disposed above the auxiliary boiler 24 is a reactor coolant circulating pump 42 arranged to draw reactor coolant downwardly (as explained below the reference to FIG. 4) through the tube sections 40 from the bottom of the core 12. Both the coolant pump 36 and the coolant pump 42 are arranged to discharge cool coolant into a plenum space 44 above the core 12. Thus coolant flow through the core 12 occurs downwardly and refuelling, control rod and other apparatus penetrating the top of the pressure vessel 14 is exposed only to reactor coolant at the lowest circuit temperature. Referring to FIG. 3 it will be seen that there are four pods 30 and four pods 38 disposed alternately around the core 12. It will be noticed that the pods 38 which house the auxiliary boilers are smaller in diameter than the pods 30 which house the main boilers. Referring to FIG. 4, in which one auxiliary boiler 24 is shown in greater detail, it will be seen that the tube sections constitute a generally annular formation supported at its inner periphery by a support spine 46 extending from the floor of the pod 38 and at two axially spaced points on its outer periphery by annular structures 48 which are attached to the pod walls. Within the support spine 46 is a coolant duct 50 having a closed lower end and whose upper end communicates with the coolant circulator 42. In operation of the auxiliary boiler reactor coolant flows from the bottom of the core 12, into the pod 38, down through the tube sections 40, radially inwardly into the bottom of the duct 50, and upwardly through the duct 50 to pass through the circulator 42 to the plenum 44. Water is fed from below the pressure vessel 14 through feed tail pipes (not shown) to the lower end of the tube sections 40 and steam passes from the upper end thereof downwardly via superheater tail pipes between the support spine 46 and the duct 50 and out through the bottom of the pressure vessel 14. Thus the direction of water flow through the tube sections 40 is upward and so the auxiliary boiler will remain stable even when operated at relatively low water flowrates. With all three turbines 20, 26 and alternators 22, 28 in operation a useful load of approximately 750 MW can be supported, the difference between this output and the installed capacity of 786 MW (666+60+60) being consumed by various internal power station services. The thermal output capacity of the reactor core 12 is suitably uprated to provide for such an output. Alternatively the main turbine can be run alone to support a load of up to 666 MW. Where the electricity demand on the reactor varies over a total range of rather less than 120 MW, the main boilers and turbine can be operated under constant load, and the auxiliary boilers and one or both of the auxiliary turbines can be operated under varying load, thus avoiding the subjection of the large main turbine to thermal stresses caused by rapid load changes. After emergency or normal shut-down of the core 12 and the main boilers and turbine it is possible to maintain operation of the auxiliary boilers and turbine in order to support at least some essential services necessary for the reactor and associated plant. The auxiliary boilers can operate in this manner by abstracting residual heat from the reactor core, and thus serve an additional purpose in acting as a core heat sink. Whereas the steam conditions at outlet from each main boiler superheater section 34 are 2350 p.s.i.a. at 1000.degree. F. and from each reheater 18 600 p.s.i.a. at 1000.degree. F., the corresponding conditions at outlet from each auxiliary boiler 24 are 915 p.s.i.a. at 900.degree. F. Thus the length of time over which the auxiliary boilers can be run on residual core heat is considerable and it is possible to make a worthwhile capital financial saving by reducing the amount of normally idle emergency generating plant provided for the reactor. The auxiliary boilers can also be operated at an early stage in reactor start-up in order to support at least some essential reactor services.