Comparison of Post-Irradiation Experimental Data and Theoretical Calculations for Inventory Estimation of Long-Lived Fission Products in Spent Nuclear Fuel

The inventory estimation of long-lived fission products (LLFP) is essential for the long-term safety assessment of a geological disposal of high-level radioactive waste (HLW). ⁷⁹Se and ¹³⁵Cs are a main contributor to the total dose from the geological repository of HLW, owing to their solubility in the strata. In this study, the post-irradiation experimental data of LLFPs, such as ⁷⁹Se, ⁹⁹Tc, ¹²⁶Sn and ¹³⁵Cs, were compared with ORIGEN2 calculation using the data library of JENDL-3.3. A fragment of the UO₂ fuel pellet irradiated in a commercial Japanese PWR was dissolved with nitric acid in a hot cell. The resultant solution was filtered to remove insoluble residue. After Se, Tc, Sn, and Cs were chemically separated, the concentrations of ⁷⁹Se, ⁹⁹Tc, ¹²⁶Sn and ¹³⁵Cs were determined with an inductively coupled plasma quadrupole mass spectrometer (ICP-QMS). The concentration of ⁷⁹Se, ⁹⁹Tc, ¹²⁶Sn and ¹³⁵Cs in the sample solution were 0.78 ± 0.22, 101 ± 24, 3.2 ± 0.6, and 68 ± 6.0 ng/g of the sample solution (ng/g-sol.), respectively. The results for ⁷⁹Se and ¹³⁵Cs obtained in this study showed good agreement with those obtained through ORIGEN2 calculation. This indicates that ORIGEN2 calculation is applicable to the estimation of the amounts of ⁷⁹Se and ¹³⁵Cs generated during irradiation. In contrast, the experimentally determined concentration of ⁹⁹Tc and ¹²⁶Sn were equivalent to approximately 70% and 60%, respectively, of those obtained through ORIGEN2 calculation.