A new strategy to simulate a random geometry in a pebble-bed core with the Monte Carlo code MCNP

MCNP™ is one of the most widely used codes and is suitable for the validation of a new computer code for pebble-bed core analysis which will be developed in the future. In a pebble-bed core not only the fuel and moderator pebbles are randomly packed, but also the fuel-to-moderator pebble ratio is irregular. It is quite difficult to directly apply these features in MCNP™ simulations. A new method, that can efficiently simulate the characteristics of the pebble-bed core by using the MCNP™ code, was developed in this study. The key idea of the method is that the material composing the unit cell expanded by repeated-geometry feature of the code is randomly determined and the continuity of material is conserved. This method was validated by the benchmark problems for the pebble-bed cores. If this method is implemented into the MCNP™ code, greater variety of analyses for the pebble-bed core can be possible.