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

MCNP6を用いた臨界時のHTTR燃料コンパクトにおける被覆燃料粒子の現実的ランダム配列モデルの検討

Ho, H. Q.   ; 本多 友貴; 後藤 実  ; 高田 昌二

Ho, H. Q.; Honda, Yuki; Goto, Minoru; Takada, Shoji

This study investigated the random arrangement effect of Coated Fuel Particle (CFP) on criticality of the fuel compact of High-Temperature engineering Test Reactor (HTTR). A utility program coupling with MCNP6, namely Realized Random Packing (RRP), was developed to model a random arrangement of the CFPs explicitly for the specified fuel compact of HTTR. The criticality and neutronic calculations for pin cell model were performed by using the Monte Carlo MCNP6 code with an ENDF/B-VII.1 neutron library data. First, the reliability of the RRP model was confirmed by an insignificant variance of the infinite multiplication factor (k) among 10 differently random arrangements of the CFPs. Next, the criticality of RRP model was compared with those of Non-truncated Uniform Packing (NUP) model and 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 k of about 0.32-0.52%k/k than the NUP model. In additions, the difference of k could be increased as the uranium enrichment decreases. The investigation of the 4-factor formula showed that the difference of k was predominantly given by the resonance escape probability, with the RRP model showing the smallest value.