Performance of the Transport Casks and Cladding Tubes for Fresh MOX fuel against External Water Pressure

In Japan, for the time being, up to commercial commissioning of fast breeder reactors, MOX fuel is planned to be utilized in light water reactors in Japan. The plutonium recovered in overseas reprocessing plant is basically used for fabrication of MOX fuel in Europe and MOX fuel is supposed to be transported to Japan by sea. There is a special safety standard called the INF Code at IMO (International Maritime Organization) which pertains to the structure and systems of the sea transport ship. In Japan, the Director-General of the Maritime Technology and Safety Bureau, Ministry of Transport has founded a technical standard that reflects the INF Code. It is stipulated in these safety standards in addition to normal standards, that the ship has a securing system, automatic collision preventive system, etc. for the aspect of systems, and has a structure protected from collision, running aground, etc. for the structural aspect. Therefore, it is considered that there is little possibility of an abnormal accident such as a collision with another ship, sinking in the sea, etc. The probability of the submergence of ships is found in the literature1)2). Sprung, et al.(1998), which estimated the probability of extremely severe accidents that a charter freighter might experience during a voyage from Charleston, South Carolina, directly to Cherbourg, France. Because the collision-only accident analyzed is the least severe accident expected to cause package failure, while the double failure, collision-plus-fire accident, is probably about as severe an accident as is at all credible, the range of these results, 10-6 to 10-11, represents the likely range for the release of radioactivity from a Type B spent fuel package during a transatlantic crossing. It should be noted that the Charter Freighter is classified as an INF-2 vessel under the INF Code. The INF-2 vessel may have a single-hull structure. On the other hand, the fresh MOX fuel will be transported to Japan by a vessel with a double-hull structure in accordance with the special regulation in Japan as mentioned above. Therefore, the possibility of accidental damage to the package is far less and the probability of the severe ship accident leading to the release of radioactivity from a Type B package is far less than the range of 10-6 to 10-11/voyage. Watabe, et al. (1998) estimated that the probability of total-loss of an exclusive ship (double-hull structure) without accidental damage to a package shipping radioactive materials from Europe to Japan is 2.6 x10-5/voyage. The probability of submergence of ships, therefore, is considered to to be less than 2.6 x10-5/voyage.