Patent Number: 048287601
Section: summary

BACKGROUND OF THE INVENTION The present invention broadly relates to a method of cleaning a spent fuel assembly which has been removed from service in a nuclear reactor. It particularly relates to a method of cleaning such a spent fuel assembly contaminated with a radioactive alkali metal and in which the fuel assembly comprises a spent fissile material selected from the group consisting of oxides and carbides of uranium and/or plutonium or a metallic uranium/plutonium alloy. The typical breeder reactor utilizes an alkali metal as a coolant. In operation of the reactor a primary coolant is circulated via a pump through a core of fuel assemblies and then through a heat exchanger from which thermal energy is extracted by indirect heat exchange with a secondary coolant. The primary coolant picks up a certain amount of radioactivity in passing through the core. All components in contact with this primary coolant also become contaminated with the radioactive constituents. Thus, any component in contact with such coolant after removal from service must be cleaned, not only to remove the chemically highly reactive coolant but also the radioactive coolant. All fuel handling of spent core assemblies must be done remotely in an inert gas filled fuel handling cell or facility such as a fuel storage building or housing, until the spent core assemblies are cleaned of sodium. Thereafter, they may be handled remotely in an air filled fuel handling storage facility or stored under water. One method utilized for cleaning such components is to wash them with an organic solvent such as alcohol or wash them with water. The disadvantage of these methods is that they produce a large volume of liquid, low-level radioactive waste with its attendant storage and disposal problems. Another technique utilized is to heat the contaminated parts to a sufficient temperature to volatilize the alkali metal and the radioactive constituents. It also has been proposed to place such contaminated components in a chamber maintained under a vacuum to removal alkali metal vapors and radioactive constituents. A fuel assembly removed from a breeder reactor presents a particularly unique problem. For example, during service the cladding of some of the individual fuel pins comprising the fuel assembly could have been cracked or ruptured such that some of the alkali metal will be within the cladding of the individual pins. In addition, the spent fuel assembly, after removal from service, will continue to generate some heat generally referred to as decay heat. Thus, if the fuel assembly was placed in a vacuum the individual pins could generate sufficient heat to rupture the cladding of those pins which were still intact with the release of more radioactive material. Clearly, the use of external heating would be equally inapplicable. Obviously there is a need for a procedure uniquely adapted to the cleaning of a spent fuel assembly which has been in contact with an alkali metal coolant. OBJECTS OF THE INVENTION It is an object of the invention to provide a cleaning and decontamination method particularly adapted for cleaning spent fuel assemblies. It is another object of the invention to provide a method for cleaning such fuel assemblies which is both economical and efficient in removing contaminants. It is another object of the invention to provide such a cleaning method which can accommodate increases in temperature resulting from decay heat of the fuel assemblies. It is another object of the invention to provide a fuel assembly cleaning method which utilizes a combination of heat and vacuum. Still another object of the invention is to provide such a method which also produces an offgas stream free of alkali metal and radioactive contaminants such that the offgas may be reused for additional cleaning. These and other objects of the invention will be more apparent from the drawings and following detailed description. SUMMARY OF THE INVENTION The present invention provides a method of decontaminating or cleaning a fuel assembly contaminated with a radioactive alkali metal. Typically, a fuel assembly comprises a plurality of elongated pressurized metallic pins or tubes containing a spent fissile material under pressure. The present invention is particularly suitable for fuel assemblies containing a fissible material selected from the group consisting of carbides and oxides of uranium and/or plutonium and which are going to be reprocessed. The method comprises a plurality of sequential steps. First, the fuel assembly is placed in a sealed chamber. A heated inert gas is passed through the chamber to heat the fuel assembly to a temperature sufficient to cause evaporation of the alkali metal but not so high as to affect the structural integrity of the individual metal pins. Thereafter the chamber is evacuated to a pressure of about less than 0.05 and preferably less than 0.005 mm of mercury and maintained at that pressure until the temperature of the fuel assembly increases to a level which could affect the structural integrity of the metal pin, typically about 1000.degree. F. Af that time a cool, inert gas is introduced through the chamber and the fuel assemblies to reduce the temperature of the fuel assembly back to a safe level, typically about 800.degree. C. The vacuum and cooling steps are repeated as often as required to insure removal of substantially all of the radioactive alkali metal. The decontaminated fuel assemblies may then be removed from the chamber and are suitable for water storage or shipment to a chemical reprocessing center. In accordance with certain preferred embodiments of the invention the inert gas utilized is argon and the gases removed from the sealed chamber are passed in indirect heat exchange relationship with a coolant for the condensation and removal of any vaporized alkali metal contained therein. Typically, the coolant will be one which is inert with respect to the alkali metal, for example, alcohol or other organic liquids. In accordance with another preferred embodiment, the gas removed from the chamber when it is being evacuated is also passed through a cryogenic trap to insure substantially complete removal of any remaining radioactive alkali metal from the gas. Typically, the alkali metal will be either sodium or a mixture of sodium and potassium. The method of the present invention is particularly applicable for use in a fuel storage building which is maintained in an inert atmosphere and generally is located adjacent to the reactor from which the fuel assemblies are removed. In such application the inert gas within the facility may be used for the heating and cooling, and after treatment for removal of sodium or other alkali metal, may be introduced into an existing gas cleaning facility which is provided within such storage buildings.