Criticality safety issues associated with the burial of highly enriched nuclear fuel in a geologic repository

The purpose of this study is to provide some basic guidance regarding the criticality safety implications associated with long term burial of highly enriched nuclear fuel in a geologic repository. Generic calculations to determine the distance required to neutronically isolate neighboring fissile regions are reported. Two specific fuels are also addressed, Fort Saint Vrain and Shippingport PWR Core-2 Seed-2 (PWR). These fuels are addressed under both dry and flooded conditions. Fort Saint Vrain fuel contains a uranium-graphite fuel matrix, is graphite moderated and has a relatively low {sup 235}U loading ({approximately}1 kg per element). PWR contains a uranium-zirconium fuel matrix, is water moderated, zircaloy clad and has a {sup 235}U loading of {approximately}15 kg per cluster. These two fuels are representative of many of the fuels currently stored at the Idaho Chemical Processing Plant (ICPP). The spent fuel storage system under consideration involves filling cylindrical canisters with spent nuclear fuel. These canisters are then to be placed into a geologic repository. Salt and tuff are the two most likely materials in which the canisters will be buried. Canisters will require sufficient material between them to ensure that the overall reactivity stays within acceptable limits. Fuels will also have to be shown to be critically safe under a variety of conditions. These conditions include flooding and the loss of the physical integrity of the fuel