US Light-Water Reactor spent fuel inventory-fissile distribution

Those conducting waste management studies to reduce the potential for a nuclear criticality accident in a future geological repository must examine the quantities and distribution of fissile isotopes that are present in discharged boiling-water reactor (BWR) and pressurized-water reactor (PWR) spent nuclear fuel (SNF) scheduled for disposition. The major fissile isotopes present in LWR fuels that impact criticality safety are the nuclides, 235U, 239Pu, and 24lPu. The sum of the quantities of these three nuclides, expressed as a percentage of the total amount of all U and Pu isotopes present in a batch of discharged fuel, determines the final enrichment of the fuel batch under consideration. The final enrichment provides an approximate measure of the nuclear criticality potential. As the final enrichment increases, the mass, geometry, or administrative controls that must be in place to prevent nuclear criticality become more stringent. Below an enrichment of about 0.7%, however, criticality is no longer a concern because the infinite multiplication factor for any heterogeneous or homogeneous mixture of fuel and water, even under conditions of optimum moderation, is less than unity. The current study examines the distribution of the final enrichment of the LWR SNF which was discharged through December 31, 1993, and which currently resides in the fuel storage pools of the various utilities or in one of several AFR facilities