Materials for fast experimental reactor 'Joyo'

The fast breeder reactor ''Joyo'' is sodium-cooling type, and the mixed oxide of plutonium and uranium is used as the fuel. The purposes are to obtain the technological experience concerning fast breeder reactors through the design, construction and operation, and to utilize it for the irradiation test to develop materials and fuel. Joyo attained the criticality on April 24, 1977. The construction of Joyo, especially the elements composing the core, are described. As for the material of fuel assemblies, mostly stainless steel is used because the high-temperature strength is important under the operating condition of FBRs, the corrosion resistance is excellent in sodium, and the neutron absorption cross section is not serious with fast neutrons. The manufacturing technologies have been developed for cladding tubes, wrapper tubes, and end plugs. Also the inspection techniques have been developed, such as ultrasonic flaw detection and eddy current flaw detection. The high-temperature strength test and the irradiation test on cladding tubes were carried out. The specifications were established for cladding tubes and other parts as the results of trial manufacture, the development of inspection techniques, and various tests. The improvement of core design can be expected as the irradiation data on the materials used accumulate gradually. (Kako, I.)