Numerical investigation of the random arrangement effect of coated fuel particles on the criticality of HTTR fuel compact using MCNP6

Abstract

This study investigated the random arrangement effect of coated fuel particles (CFPs) on the criticality of the fuel compact of the high-temperature engineering test reactor (HTTR). A utility program coupled with MCNP6, namely the realized random packing (RRP) model, was developed to explicitly model the random arrangement of CFPs in a specified fuel compact of the HTTR. The criticality and neutronic calculations for a pin cell model were performed using the Monte Carlo MCNP6 code with ENDF/B-VII.1 neutron library data. The reliability of the RRP model was confirmed by the insignificant variance of the infinite multiplication factor (kinf) among 10 different random arrangements of the CFPs. Next, the criticality of the RRP model was compared with those of a non-truncated uniform packing (NUP) model and an on-the-fly random packing (ORP) model, which is a stochastic geometry capability in MCNP6. The results indicated that there was no substantial difference between the NUP and ORP models. However, the RRP model presented a lower kinf (by about 0.32–0.52%Δk/k) than the NUP model. In addition, the difference in kinf was increased as uranium enrichment decreased. Investigation of the 4-factor formula showed that the difference in kinf was predominantly a result of the resonance escape probability, with the RRP model showing the smallest value.