Numerical modelling of radionuclide inventory for RBMK irradiated nuclear fuel

After the final shutdown of Ignalina NPP Unit 1 in 2004 and Unit 2 in 2009, total amount of spent nuclear fuel (SNF) is approximately 22,000 of fuel assemblies. Radionuclide content in SNF and its characteristics are initial data for analysis of various subjects such as radiation shielding, thermal analysis and other processes that can affect the performance of SNF storage or disposal facilities’ components. For instance, ageing of the components is influenced by decay heat, neutron and gamma radiation, external environment conditions, mechanical stresses, etc. The first two factors can be defined only after SNF characteristics are investigated and irradiation levels are determined. Experimental investigation of radionuclide content and characteristics in SNF is expensive and complicated, so numerical methods are widely used. In this paper, results of numerical modelling of spent RBMK-1500 nuclear fuel characteristics and its variation with time are presented. Modelling was performed using SCALE computer code system TRITON module. Some calculation results are compared with available experimental data for RBMK-1000 spent nuclear fuel and a rather good agreement between numerical and experimental results is identified. Comparison of calculated gamma and neutron sources, activities and decay heats shows that results for RBMK-1500 fuel with various enrichments differ by 10–20%.