Assessment of CASMO-4 predictions of the isotopic inventory of high burn-up MOX fuel

The current paper presents an assessment of the depletion model in CASMO-4 based on highly-burnt MOX fuel data from the Programme ARIANE. Isotopic compositions calculated with CASMO-4 for a sample irradiated for 6 cycles in the Swiss PWR Beznau-1 have been compared with experimental data from the PSI and SCK·CEN radio-chemical laboratories. Both a 1-D pin-cell and a 2-D fuel-assembly model have been used, together with detailed and burn-up averaged irradiation histories. The results obtained (a) show that predictions for the most important actinides have errors below 10%, (b) highlight problems with Am and Eu isotopes, and (c) indicate experimental inaccuracies probably linked to the measurement techniques for ¹²⁹I and some metallic fission products. Additional studies, focused on refined pin-cell treatment, have found that such modeling effects can be significant for the prediction of some actinides, e.g. Pu, Cm and Am.