Patent Number: 040574652
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to nuclear reactors and more particularly to an auxiliary heat removal system for use in gas-cooled nuclear reactors. A nuclear reactor produces heat by the fissioning of a fissile material which is fabricated into fuel elements and assembled into a nuclear core. In gas-cooled nuclear reactors, the heat produced by the fissile materials is transferred to an inert gas such as helium or argon, which is then circulated, typically, through turbines, heat exchangers, and compressors before being returned to the nuclear core. The power output of the turbines is then converted into electrical power. The use of a nuclear reactor as a heat source introduces the need for a cool-down system to remove, residual heat. A nuclear reactor does not cease generating heat immediately upon being shut down. Delayed neutrons continue to cause fission to occur for a period of time after shutdown. For a longer period of time, dependent upon the reactor characteristics and prior operating history, the products resulting from the fission of the fuel continue to decay, and in decaying release energy. Additionally, the fuel elements themselves are often operated at temperatures above the level at which structural damage would occur if cooling were not provided. These abovementioned factors mandate the requirement that adequate cooling be provided for the nuclear core after the reactor is shut down, to remove the residual heat retained in the core. Typically, cool-down under normal conditions, such as for refueling, is achieved through the use of the normal circulatory system. The turbines, heat exchangers, and compressors continue operations until the desired temperature level of the nuclear core is attained. In the unlikely event of a emergency condition, it may not be possible for the normal circulatory system to remove the heat generated by the nuclear core. For example, one of the emergency conditions which is postulated, although its probability is extremely small, is the complete failure of the normal circulatory system to operate. In the event of such an occurrence, a separate system must be provided to remove the heat generated by the nuclear core after it has been shut down. In the prior art, at least two different systems have been proposed to remove the residual heat retained after reactor shut-down. One system involves installing a redundant system to provide circulation whenever the normal circulatory system is inoperable. This emergency system generally is dependent upon an external power source and external controls. As such, these external controls and the external power source must be operable under all possible emergency conditions, a difficult and expensive requirement. Another method utilized in the prior art is to remove the residual heat through convection circulation of the reactor coolant. Although this system removes the dependence upon an external power source, it has disadvantages. The natural convection circulation may not be adequate to remove sufficient heat from the nuclear core. Additionally, the use of the natural convection circulation system is dependent upon the reactor gravitational orientation. SUMMARY OF THE INVENTION The above-mentioned problems of the prior art are eliminated by this invention by providing an auxiliary heat removal system for use with gas-cooled nuclear reactors to remove residual heat generated by the nuclear reactor core without the necessity of an external power source of the use of convection flow. The hot gaseous reactor coolant is removed from the core and supplied to a gas turbine, through which the hot gas is expanded. From the gas turbine, the coolant is supplied to a means for removing heat from the coolant, where the gaseous coolant transfers its heat to a second coolant; flows to a compressing means; is compressed and is resupplied to the nuclear core. The gas turbine is mechanically coupled to, and drives both the means for compressing the gaseous coolant, and a pump which is utilized for circulating the second coolant. A pneumatic starter provides starting of the gas turbine upon the occurrence of an emergency condition. As such, the auxiliary heat removal system utilizes the residual heat produced by the reactor core to power the system for cooling the reactor core, and is not dependent upon an external power source.