This invention relates generally to nuclear reactors and more particularly to a system for controlling the environment of environmentally isolated areas in a nuclear reactor installation without the release of process gas or radioactivity to the external atmosphere.
A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and situated within a nuclear core. The nuclear core is located in a reactor vessel, and for safety considerations, the reactor vessel is frequently situated in an enclosed area which is situated in a containment building. The top of the vessel is sealed by a cover known generally as a head. Through this head pass numerous penetrations for the operation, control and surveillance of the reactor core and its attendant systems.
Power and supply equipment for the head penetrations are located above the pressure vessel and head. Additional machinery for the removal of the head, and fuel handling and transfer machinery are also located above the head. To prevent the escape of any radio-activity when refueling operations are being undertaken, this area is enclosed and shielded, and is known generally as the refueling hot cell.
During reactor operation, the environment in this hot cell may be either air or inert nitrogen. However, during refueling operations and fuel transfer, the environment is this hot cell must be an inert gas such as nitrogen. Conversely, during regular maintenance operations, the hot cell must have a breatheable atmosphere, mainly an air environment. Therefore, there are times when the environment in the refueling hot cell must be changed from air to nitrogen or from nitrogen to air.
Prior practice in the field when changing from an air to nitrogen environment has been to use a bleed and feed method. In this method, pure nitrogen is fed into the hot cell while a nitrogen rich air-nitrogen mixture is discharged from it. The bleed and feed method continues until the desired nitrogen concentration is reached. The reversed process is used when changing from nitrogen to air, with pure oxygen being fed into the cell while the nitrogen-oxygen is being discharged from it.
Shortcoming to this method, however, are that large amounts of bled gas are discharged to the environment and this gas can contain radioactive fission gases such as xenon and krypton. A once through gaseous radioactive waste disposal facility specifically for monitoring, processing, and removal of the fission gases from enclosed areas is seldom provided because of the required size and expense. The non-fission gases prochessed through such a facility, if it were used, would eventually be discharged to the atmosphere.
Environmentally and economically, this bleed and feed method is undesirable. Considerable quantities of nitrogen and oxygen are used in such procedure, and are expelled to the atmosphere during the changeover. Additionally, the discharge stream from gaseous radioactive waste disposal system may contain trace, but objectionable, amounts of radioactivity.