Character and manufacturing problems of transport cask for spent nuclear fuels: fire resistance

If some accident occurs during the transport of radioactive substances and the cask for transport is broken, the large damage to the lives and properties of people is expected. Therefore ''The safety standard concerning the transport of radioactive substances'' was established in Jan. 1975 in Japan. The thermal problems being taken into consideration in the design, manufacture and transport of the B-type casks for transport prescribed in the standard are the elimination of decaying heat at the time of normal transport and the fire resistance at the time of fire. The former requires the outward dissipation of generated heat, while the latter requires the prevention of inward heat transfer, and these requirements are utterly contradictory. The research and development concerning the casks for transport satisfying such requirements have been carried out in various countries. When the heat generation is large, helium gas coolant is filled in the cavity. For the cooling from outside, heat dissipating fins are provided on the outside surface, and sometimes forced cooling is adopted. As for the conditions of fire accidents, 8000C for 30 min is prescribed in IAEA rule, supposing the fires following vehicle collisions. It is required to maintain such soundness of casks that the deterioration of shielding capability or the leak of contents do not occur after the fire test. The melting of shielding lead is one of the criteria for judging the fire resistance. The fire resistance of neutron shielding has been tested also. (Kako, I.)