Effect of Impurities and Activated Structural Materials on Radionuclide Release from Disposed LWR-SMR Spent Nuclear Fuel

This study assessed the effect of activation products (such as C-14, Cl-36, Ni-59 and Mo-93) on small modular reactor (SMR) spent nuclear fuel (SNF) disposal by characterizing SNF from two SMRs and one large nuclear power plant and comparing them to a reference fuel from Finnish conventional nuclear power plants. This was followed by comparing the release rates of the SNFs from the near-field of a KBS-3 disposal canister to understand the significance of the impurities on the release rates.
For the characterization of SMR SNFs, a low-burnup and a high-burnup assembly configuration was defined based on the LDR lite and the Rolls-Royce SMRs respectively due to the large difference in reactor power densities and discharge burnups. In addition, SNF from a generic EPR-style large LWR was characterized. The characterization was carried out with the continuous-energy Monte Carlo code Serpent 2 in a 2D infinite lattice geometry with reflective boundary conditions. For both SMR reactors, characterization was performed both with and without the presence of impurities to understand the share of the inventory they constitute.
For the radionuclide transport modelling, a near-field model, consisting of a leaking disposal canister, 2D axisymmetric bentonite buffer, 2D discretised tunnel section and the interfaces to surrounding bedrock was created. It was assumed that four assemblies would be emplaced into one disposal canister for each characterized SNF respectively. The radionuclide release rates to a fracture intersecting the deposition hole were compared between the SMR SNFs and with the reference fuel. From the near-field, radionuclides transported through the geosphere in rock fractures simulated as single path flow channels surrounded by bedrock and fracture coating materials. The radionuclide release rates from geosphere were compared to those from near-field.
The results showed that including nitrogen impurities significantly increased the C-14 inventory respectively while inclusion of chloride impurities only accounted for a small increase of Cl-36 inventory. The increase in C-14 also significantly increased the release rates from the radionuclide transport model in the cases, where impurities were considered. Furthermore, the near-field and geosphere radionuclide release rates, were the highest for the high-burnup SMR and lowest for the low-burnup SMR. However, when comparing cumulative releases