Study on cooling characteristics for spent fuel in direct water pool storage system

Fujii, Tadashi; not registered; not registered; Sakai, Takaaki; not registered; Oki, Yoshihisa; not registered

The conceptual design study of the large-scale sodium-cooled reactor is in progress in the "Feasibility Study on Commercialized Fast Reactor Cycle Systems (F/S)". The direct water pool storage system is being examined as a candidate concept to simplify the fuel handling facility for the sodium-cooled reactor. In this concept, the decay heat of a fuel subassembly is relatively higher (18kW which is about 4.5 times of the Ex-vessel Storage Tank system in the demonstration FBR). Therefore, the information about the cooling characteristics of the fuel subassembly are lacking in cases of submergence process at a normal operation and cooling water injection process from upper part of the subassembly at a transfer accident. Accordingly, the understanding of the cooling characteristics of the fuel subassembly in higher decay heat condition was pointed out as one of the thermal hydraulic problems which influence the realization of the plant concept. Using the single heater pin equipment, fundamental tests were conducted with the parameters of the thermal conditions of a fuel pin, the outlet shapes of it, the submergence speeds and so on. Then, following basic data were acquired to be reflected in the actual plant design. (1)Cooling modes of the normally submergence tests and water injection tests were identified by visualization of the boiling behavior in the test section and the temperature change of the heater pin. (2)The initial temperature of the heater surface and the blockage size of the outlet of test section were dominating factors to the cooling completion time. (3)Maximum temperature rise of the heater surface was about 4K in normally submergence tests and 6K in water injection tests, respectively. Therefore, the heater was well cooled without significant temperature rise.(4)In the normally submergence tests, the pressure of the upper part of the test section did not exceed the lower part pressure and a water level rise in the test section was not obstructed ...