Stochastic arranging of CFPs in HTTR and criticality benchmark considering different modeling of CFPs

In this paper besides proposing a new method for stochastic arranging of coated fuel particles (CFPs) in graphite matrix of HTTR, the effect of homogeneous and heterogeneous (including stochastic and regular arrangement) modeling of CFPs in criticality calculations of this reactor is investigated using MCNPX. In regular arrangement, each CFP was modeled as a fixed sphere in the center of a cubic lattice; subsequently, depending on fuel compact's packing factor, each fuel compact was filled with around 13,000 (approximate number of CFPs in fuel compact) of this cubic lattice. In stochastic arrangement, an algorithm was developed to distribute definite number of CFPs randomly in a cubic lattice; next each fuel compact was filled with the cubic lattice. Criticality calculations including effective multiplication factor and critical position of control rods in three core configurations (thin annular core, thick annular core and fully-loaded core) have been carried out. The effect of nitrogen impurity was further investigated in all calculations. Results were benchmarked with different simulation results presented in the IAEA-Technical Document No.1382. It was realized that heterogeneous modeling of fuel particles in both the regular and stochastic arrangements was more accurate and efficient approach than homogenous modeling .It was also observed that there is no significant difference between the results of regular and stochastic arrangements. In comparison with other simulations, presented in Tech-Doc1382, the simulated critical heights of control rods in the heterogeneous modeling were more consistent with the experimental data