Study of the PWR REA pulse width for realistic UO2 and MOX core designs using 3D kinetics methods

Abstract

For the development of new acceptance criteria for the analysis of Rod-Ejection-Accidents (REA) in Pressurized Water Reactor UO2 cores, full-width-at-half-maximum power pulse widths in the range 25–40 ms were employed in the analytical transient fuel behaviour studies. Due to the operation of MOX cores in Switzerland, a study using the CORETRAN 3D kinetics code was carried out at PSI to investigate and compare on the basis of real operated cycles MOX and UO2 cores, the pulse width magnitude as function of rod reactivity for postulated super-prompt critical REAs. This study is presented in a first part of this paper. Thereafter, to quantify the variation in pulse width when applying a different 3D kinetic solver, a benchmarking with the SIMULATE-3K code is carried out. This code-to-code comparison aims also to serve as an assessment of SIMULATE-3K for REA analyses of the Swiss cores since only CORETRAN had so far been benchmarked at PSI for this type of applications. Finally, to provide confidence in the results, particularly for pulse widths below 25 ms, sensitivity studies are performed to assess the effects of specific modelling options and assumptions applied in the REA 3D kinetic analysis and to compare the sensitivities between UO2 and MOX cores. On the basis of all these studies, it is found that the MOX core pulse width is usually around 5 to 10 ms lower than for UO2 cores. For rod worths close to 2.0$, corresponding to around 500 pcm above prompt criticality and which was obtained for many of the investigated MOX cycles particularly at end-of-cycle conditions, a representative MOX pulse width magnitude is found to be around 15 ms. To shortly address the applicability of these results, i.e. obtained for very specific MOX cores, additional calculations were performed for hypothetical cases with increased MOX core fractions and with higher MOX assembly Pu enrichments. And it was found that even for these types of cores, the pulse width would, although being slightly reduced, remain in the range 10–15 ms for rod reactivities up to 500 pcm above prompt criticality.