Analysis of tungsten gray rods critical experiments using PARAGON with ultra-fine energy mesh methodology

New critical experiments using gray control rods with tungsten were recently performed at the Toshiba NCA critical facility. This paper presents analyses of these experiments using both stochastic and deterministic codes. We used the continuous energy Monte Carlo code MCNP and the Westinghouse lattice physics code PARAGON. The basic nuclear data source for the cross-sections is ENDF/B-VII.1. First, the tungsten data is validated against Monte Carlo calculations. The ultra-fine energy mesh with 6064 group cross-sections library was used in PARAGON to extend the validation of the methodology to the cold temperature conditions. Comparisons focused on reactivity and the measured fission rate distributions. The results show that the ENDF/B-VII.1 data adequately reproduces the measured tungsten gray rod reactivity worth. Also, the energy mesh and the methodology used in PARAGON are seen to be adequate in predicting the reactivity and fission rate distributions for these challenging and highly heterogeneous experiments. (author)