Reactivity effect of axial burnup shapes for a realistic PWR spent fuel transport cask

The reactivity effect of the asymmetry of axial burnup shape in burnup credit criticality safety is studied for a realistic PWR spent fuel transport cask based on the current OECD/NEA's Phase II-C benchmark problem. The axial burnup profiles varying from strong asymmetry to more or less no asymmetry based on in-core flux measurements are modeled in 21 zones. Continuous energy criticality calculations in 3-D model have been performed using the Monte Carlo code MCNP4B and the nuclear data library JENDL-3.2. Calculations show that keff value increases with increasing asymmetry degree of axial burnup shape. This leads to the fact that the end effect is enhanced by the possibly realistic asymmetry of burnup shape from 0 to 3.5%Δk/k and from 3.4 to 7.7%Δk/k respectively for average burnups of 32 MWd/kgU and 50 MWd/kgU. The end effect is more sensitive to the asymmetry of burnup shape for the higher burnup. For an axially distributed burnup, the axial fission density distribution becomes strongly asymmetric as is peak shifts toward the top end of the fuel zone where the local burnup is less than that of the bottom end. The peak of fission density distribution gets higher as either the asymmetry of burnup shape or the average burnup increases. (author)