Implications on cask criticality safety of a postulated impact accident

For many years, British Nuclear Fuels (BNFL) plc has been involved in the transportation of irradiated nuclear fuel from customers throughout the world to its reprocessing complex at Sellafield in Cumbria, United Kingdom. The nuclear criticality safety of this activity requires engineered safeguards that are effective under both normal and accident conditions. One possible accident is that the cask is dropped. This possibility is considered during the cask design process by ensuring that the cask design will safely withstand the 9-m drop test recommended in the International Atomic Energy Agency transport regulations. If fuel elements were to undergo massive fracture following an impact accident, releasing large quantities of UO{sub 2} fragments to the cask cavity, a criticality hazard may arise if fragments settle in a water-filled free volume. Calculations have shown that the reactivity effects are strongly determined by the amount of fuel assumed to fragment and also by the specific cask design that determines the degree of interaction between the intact fuel elements, the internal structure of the cask, and the assumed UO{sub 2}/water slurry. The sensitivity of some BNFL cask designs to the amount of UO{sub 2} assumed to be released means that arbitrary assumptions for UO{sub 2}more » release are not acceptable. Consequently, there is a clear need to evaluate an estimated upper limit to the possible UO{sub 2} fragment release for criticality safety studies under assumed accident conditions. Hence, a program of experimental work has been undertaken by BNFL to quantify the amount of UO{sub 2} that may be released.« less