Mobility and Criticality of Plutonium in a Repository

In an underground repository for spent fuel, criticality is excluded initially by compliance with the disposal conditions. In the long term, critical accumulations of fissile material can be formed only by mobilization of uranium and plutonium from the waste forms and subsequent precipitation or sorption of these elements. This paper presents an overview of mechanisms relevant for mobilization and possible accumulation of U and Pu from disposed mixed-oxide fuel elements. Concentrations of fissile materials observed in laboratory corrosion experiments together with model approaches are applied to determine the degree of fissile material accumulation and the risk of a sustained nuclear chain reaction. A prerequisite of criticality in a repository is an accumulation of fissile materials. Since geometry, moderation, and neutron absorption properties cannot be forecast, the neutron multiplication factor k_inf is used (instead of k_eff) as a measure of the incidence of criticality. The factor k_inf is derived for several scenarios. Required critical masses and critical volumes are evaluated.

The accumulation of Pu onto solids is considered, and it is shown how selective enrichment of Pu and U may affect the risk of criticality. It is also shown that the criterion for criticality would be met only in the unrealistic case of selective sorption of ²³⁹Pu. Realistic sorption densities are too low to provide sufficient accumulation of fissile materials for criticality. This is particularly true if high Cl concentrations are present.