Monte Carlo criticality calculations for a pebble bed reactor with MCNP

In this study, the criticality analysis for a pebble bed reactor, HTR-10, is performed with Monte Carlo simulations. The MCNP4B code package is utilized in the analysis with ENDF/B-VI continuous energy cross sections. The full core with the initial loading case is considered in simulations. The variation of the effective multiplication factor as a function of core loading height is also analyzed. Three different geometrical models are employed to see the effect of geometrical detail on the criticality calculations. Results are compared with diffusion calculations as well as the experimental data. Results show that the use of the homogenized fuel zone model does not yield acceptable results and underestimates the core criticality. However, the results obtained by using models with uniform and randomly distributed coated fuel particles in the fuel zone are in quite good agreement and there is not any systematic difference. Furthermore, criticality values do not change significantly with different random arrangements of coated fuel particles in fuel spheres. However, the random and irregular arrangements of pebbles may result in statistically different criticality values at least due to varying streaming effect.