The quality of the isotopic composition of plutonium fuel degrades as the number of recycles increases in thermal reactors. There are in addition incentives to better utilise fuel by increasing the burnup. These two factors lead to an increase in the total loading necessary in mixed plutonium-uranium oxide fuel. Experimental validation of such systems will not be forthcoming for several years. In the meantime, benchmark exercises can offer a valuable indication of what level of development effort is required to address the possible shortcomings of the current physics methods.
This report describes two such benchmark studies concerning pressurized-water reactors. It deals with the calculation of physics parameters as they evolve with increasing irradiation for PWRs fuelled with MOX of two different isotopic qualities: one with a typical first-cycle MOX, and the second with a fifth-recycle plutonium having a poor isotopic quality. Methods required for obtaining good-quality results are also identified.