Patent Number: 041586042
Section: summary

This invention relates to power generating plant and has especial application to boiler systems for nuclear power stations. In order to cope with peak demands for electricity there is an increasing requirement for a power station having auxiliary generating capacity over and above the normal maximum continuous rating of the station. Such auxiliary capacity can be provided by gas turbine or diesel engine powered plant, but it would be convenient to associate at least some auxiliary generating plant with the heat source already present in the station. Further, where only large steam turbines are employed in a power station a degree of inflexibility in accommodating rapid changes in electricity demand exists because such turbines suffer considerable thermal stresses if subjected to rapid load changes. A further disadvantage lies in the fact that steam turbines are inefficient at low load so that it is undesirable to run a large steam turbine at low load in order to meet a considerably reduced demand for electricity. Finally, with some nuclear reactors, and in particular with the so-called High Temperature Reactor, it is desirable that the predominant flow of reactor coolant through the reactor core be downward. To simplify the reactor coolant circuit it is therefore convenient to pass the reactor coolant upwardly through associated boilers and so, in order to achieve counterflow within the boiler, water and steam must flow downwardly through the boiler. Owing to waterside instability such water downflow boilers could be difficult to operate at low flow rates which may be occasioned either by low load running of a large steam turbine to which they are connected, or by emergency use of such boilers as a heat sink for the reactor core. According to the invention there is provided power generating plant comprising a heat source, at least one main boiler arranged to be heated by heat from the heat source, and at least one main steam turbine arranged to be driven by steam generated in the said main boiler, wherein there is further provided at least one further boiler, of lower capacity than the said main boiler and also arranged to be heated by heat from the heat source, and at least one further steam turbine of lower capacity than the main steam turbine and arranged to be driven by steam generated in the said further boiler. Where the boiler system is used to supply steam to turbines which are each connected to a generator, the said further or auxiliary boiler(s) may be operated to enable the said further or auxiliary turbine(s) and generator(s) to provide auxiliary generating capacity over and above the normal maximum continuous rating of the main turbine and generator. Further, whilst the main boiler(s) are being operated to enable the main turbine to run under substantially constant load, the auxiliary boiler(s) may be operated to enable the auxiliary turbine(s), being smaller and thus more tolerant of rapid load changes, to run under varying load in order to accommodate changes in demand for electricity. The common heat source may advantageously be the core of a nuclear reactor, the boilers being disposed in a heat exchange circuit including the core such that reactor coolant fluid can flow through the core to abstract heat therefrom, and then through the boilers in which the heat is used to generate steam. Conveniently each boiler is disposed in a vertical channel or `pod` defined within the wall thickness of the reactor pressure vessel. To avoid operating the main boiler(s) at low flowrate when supporting a very considerably reduced load, it is possible to shut the main boiler(s) and turbine down and carry the load solely on the auxiliary boiler(s) and turbine(s). Thus the problem, mentioned above, of instability in a main boiler operating at low flowrate can be avoided since, in supporting the same load, the auxiliary boiler(s), being of smaller size, will operate at a considerably higher flowrate. Further, because of the smaller size of the auxiliary boiler(s), it is comparatively easy to arrange the heat exchange circuit such that reactor coolant flows downwardly through the auxiliary boiler(s), so enabling water flow therein to be upward. In order to maintain essential services in a power station during an emergency involving loss of normal power supplies, it is usual to provide emergency generating facilities. Such facilities are usually completely idle during normal running of the station. However, where a boiler system in accordance with the invention is associated with a nuclear heat source it becomes possible to dispense with at least part of the emergency generating facilities since, after emergency or even normal shut-down of the reactor core and the main boiler(s), the auxiliary boiler(s) can be operated to enable the auxiliary turbine(s) and generator(s) to run under at least part load. The auxiliary boiler(s) can continue operating in this manner for some considerable time simply by abstracting residual heat from the reactor core. This operating time can be maximised by designing the auxiliary boilers to produce steam at an appreciably lower temperature and pressure than the main boilers. To enable such continued operation of the auxiliary boiler(s) to occur a water feed and condensing system separate from that for the main boiler(s) is provided. It will be appreciated that a further advantage accrues from the use in such circumstances of a boiler system in accordance with the invention, in that, after shut-down of the reactor core, the core is effectively cooled by the auxiliary boiler(s) which act(s) as a core heat sink. The auxiliary boiler(s) can also be used for power generation during reactor and main boiler start-up.