Preoperational test of the fast experimental reactor 'JOYO'

The fast experimental reactor JOYO has attained the criticality on April 24, 1977. The conceptual designing of this reactor had started in 1964, and the construction started in 1970. The test operation of the reactor has been performed since 1975, and the functional and performance tests were carried out for two years. Then, its criticality was achieved on April 24, 1977. The pressure vessel of the reactor is a double-walled stainless steel vessel with plate thickness of 25 mm. Nitrogen gas is filled in the gap of the vessel. A rotating plug is set on the top of the reactor, which rotates eccentrically. The core fuel assembly is composed of 91 fuel elements filled with mixed uranium-plutonium oxide pellets and depleted uranium oxide pellets. The functional tests performed are the grip test of the fuel handler, the simulated core test, the test of the fuel exchanger and the test of cooling system, performed at the room temperature. The pre-heating tests were carried out before sodium filling. The system tests in sodium began in February, 1976. The results of tests of the circulation pumps showed good performance characteristics. The safety check-up of the cooling system showed good results. The performance tests performed are the criticality test and the low output characteristics test. The initial criticality was achieved with 64 fuel assemblies. The initial core is composed of 70 assemblies. The results of measurement of output power distribution was in good agreement with the expected values. (Kato, T.)