Probabilistic Safety Evaluation of 48-inch Loaded Depleted and Natural UF6 Cylinders Involved in the TS-R-1 Regulatory Fire

In September 2000, the United States Enrichment Corporation (USEC) submitted an application to the U. S. Department of Transportation (DOT) for a certificate permitting the shipment of the series 48X and 48Y uranium hexafluoride (UF6) cylinders in their current configuration. Such approval is required under current IAEA regulations. The IAEA's 1996 Edition of Regulations for the Safe Transport of Radioactive Material: No. TS-R-1 (ST-1 Revised) contains several changes from the prior Safety Series No. 6. One such change, contained in Paragraphs 629 through 632, is new accident test requirements for UF6 cylinders containing 0.1 kg or more of material. Particularly affected are those cylinders containing natural or depleted UF6 since enriched material is shipped with an overpack that affords protection from the accident environment. The tests are pressure, mechanical loading, and thermal exposure. Of these three, the thermal test is the focus of our paper. The thermal accident test requirements are found in Paragraph 728 of TS-R-1. The acceptance criterion is that the loaded cylinder sustains the thermal test environment, i.e., 800°C plus convection, for 30 minutes without rupture. A significant amount of testing and analysis of bare, i.e., no specific thermal protection, cylinders has been performed by various entities but the results have been both marginally conclusive and at times contradictory. To evaluate the safety of continued shipment of the 48X and 48Y cylinders in their current configuration, USEC used a probabilistic \"risk-informed\" approach. This was done to determine the reasonableness of such thermal protection measures. We based the assessment on an examination of the probability of occurrence of the thermal environment during transport together with a consideration of several mitigating factors that reduce the risk of shipping bare 48-inch cylinders.