Validation of an MCNP4B whole-reactor model for LWR-PROTEUS using ENDF/B-V, ENDF/B-VI and JEF-2.2 cross-section libraries

A detailed three-dimensional, continuous-energy MCNP4B model of the LWR-PROTEUS critical facility has been developed for the analysis of whole-reactor characteristics using ENDF/B-V, ENDF/B-VI and JEF-2.2 cross-section sets. The model has been applied to the determination of the critical loading, as well as the evaluation of reactivity worths for safety/ shutdown rods, control rods, and individual driver-region fuel rods. The initially obtained results for the ®rst con®guration investigated (Core 1B) indicated that, for the same geome- trical and materials speci®cations, the ENDF/B-V data library yields the closest critical pre- diction (discrepancy of 640 40 pcm), followed by ENDF/B-VI (980 40 pcm) and JEF-2.2 (1340 40 pcm). The di€erences in results between the three data libraries were studied by considering the contributions of individual materials to the neutron balance. 235U and 238U cross-sections from JEF-2.2, for example, explain an e€ect of ~400 pcm. Re®nement of the materials speci®cations in the MCNP4B whole-reactor model, in particular the impurities assumed for the graphite driver of the driver and re¯ector regions, has been shown to reduce the ®nal discrepancy of the ENDF/B-V based ke€ff result to ~ 0.2%. The MCNP4B results for relative reactivity e€ects, such as control rod worths, are found to agree within experimental errors with the measured values.