Spent nuclear fuel comprises a fraction of the hazardous materials packages shipped annually in the United States. In fact, at the present time, fewer than 100 packages of spent nuclear fuel are shipped annually. At the onset of spent fuel shipments to the proposed Yucca Mountain, Nevada, repository, the U.S. Department of Energy (DOE) expects to ship 400 - 500 spent fuel transport casks per year over the life of the facility.

Independent spent fuel storage installations (ISFSIs) are currently licensed for 20 years. However, delays in developing permanent spent fuel disposal capability require continued ISFSI storage beyond the 20-year term. This report provides a technical basis for demonstrating the feasibility of extended spent fuel storage in ISFSIs.

Until now, the Nuclear Regulatory Commission's (NRC) approval of criticality safety evaluations for spent fuel in transport and storage casks has been based on analyzing the fuel as though it were fresh and without burnable poisons. The well-known nuclide composition of fresh fuel has provided a straightforward and bounding approach for showing that spent fuel systems will remain subcritical under normal and accident conditions. Burnup credit refers to the approval of criticality safety evaluations that consider the decrease in fuel reactivity caused by. irradiation in the reactor.