Patent Number: 039740293
Section: summary

This invention relates to gas-cooled nuclear reactors, that is to say reactors of the kind in which thermal energy is arranged to be extracted from the reactor core by the passage therethrough of a gaseous coolant, commonly helium, for subsequent transfer to one or more energy converting units. The invention is especially, though not exclusively, concerned with nuclear reactors of this kind in which the gaseous coolant is used directly as the working fluid for one or more closed cycle gas turbines providing the main power output from the reactor. According to the invention a gas-cooled nuclear reactor has a main circulatory system for the gaseous coolant incorporating one or more main energy converting units, such as gas turbines, and an auxiliary circulatory system for the gaseous coolant incorporating at least one steam generating boiler arranged to be heated by the coolant after its passage through the reactor core to provide steam for driving an auxiliary steam turbine. In the case of a reactor in which the main energy conversion is effected by one or more gas turbines such an arrangement enables the reactor to be started up utilising the cooling effect of the auxiliary circulatory system independently of the gas turbine cycle start-up procedure. This is an advantage because the large powers required to drive the compressors of gas turbine plant give rise to problems on starting, and special provisions have to be made to ensure that the turbine rotor attains a speed sufficient to allow the turbine drive to be self-sustaining, so that by separating the starting of the reactor and gas turbine systems the complete installation start-up procedure can be considerably simplified. In addition long term heat removal, either after a reactor trip or under normal reactor shut-down, can be considerably eased by shutting down the gas turbine and removing heat by the auxiliary circulatory system. The steam turbine associated with the or each said auxiliary boiler is preferably arranged to run continuously in operation of the reactor and may be used to drive an auxiliary generator for providing electrical power for reactor services and ancillary equipment, any surplus power being exported. In a typical embodiment a plurality of gas turbine units could be disposed around the reactor core, either in chambers in the thickness of a pressure vessel wall surrounding the reactor core or in the space between a primary pressure vessel accommodating the reactor core and a secondary pressure vessel which defines an enclosed space containing at least part of the wall of the primary pressure vessel in the manner described in co-pending U.S. patent application Ser. No. 222947, the associated auxiliary boilers preferably also being similarly disposed around the reactor core between the gas turbine units. In the case of high temperature reactors especially, the temperature of the gas leaving the reactor core may be too high for satisfactory operation of the auxiliary boiler or boilers, in which case means may be provided for cooling the gas between the reactor core and the boiler or boilers; for example the arrangement may include means for mixing the gas emerging from the reactor core with gas at a suitably lower temperature. This may be achieved, for example, by arranging for some of the gas leaving a boiler of the auxiliary circulatory system to by-pass the reactor core, and subsequently to mix with the heated gas leaving the core to effect the cooling thereof. However other sources of cooler gas may be employed, for example from points inside the gas turbine system. For a high temperature reactor the thermal capacity of the auxiliary circulatory/steam turbine system may be about 16% to 20% of the thermal capacity of the core.