Abstract
One of the concerns regarding direct disposal of spent nuclear fuel (SNF) dual-purpose canisters (DPCs) currently in dry storage is criticality during disposal time frames (e.g., 10,000 years or more). Preconditioning DPCs with appropriate filler materials (e.g., cementitious materials, low-melting metals) before disposal could prevent post-closure criticality by means of moderator displacement. However, potential radiolysis of the filler material by ionizing radiation and release of reactive chemical species over repository time frames may negatively impact the mechanical and chemical properties of the filler materials. Evaluation of the energy deposited in candidate filler materials by SNF radiation is essential for the calculation of radiolytic species. This paper provides an evaluation of the total energy deposited by gamma and neutron radiation in a candidate filler material over 10,000 years for a representative pressurized water reactor SNF DPC. The canister is assumed to contain identical SNF assemblies with a burnup value of 55 GWd/MTU, which is the bounding burnup value for more than 95% of the SNF inventory discharged from 1968 to 2013. The evaluated energy deposited per unit volume of filler material between 10 years and 10,000 years after fuel discharge is approximately 9.3E+23 eV/cm3. Gas production from radiolysis was not evaluated and may be revisited in future work. Radiolysis may be the principal gas generation mechanism for a cementitious system.
