A preliminary study of Monte Carlo code development for pebble bed reactor: A new approach on judgment of fission source convergence

The core of the pebble bed reactor has random and double heterogeneous structure. Due to the geometrical configuration, some specific and difficult techniques are essentially required for Monte Carlo simulation. A development project of a user friendly Monte Carlo code for pebble bed reactor has been being pursued by this research team. The purpose of the project is that the core geometry modeling, source option, and tally option are automatically determined and calculated by the code itself. As a part of the project, a decision method of the inactive cycle is proposed in this study. It is noted that if fission source positions in Monte Carlo calculation are not converged enough, the calculation bias is occurred. Therefore, the inactive cycle is used for the skip of the calculation until the fission source positions are converged. Some methods for the decision of the inactive cycle were known, however, the applicability of the methods into the Monte Carlo code is not sufficiently secured. In this study, a new approach for checking the fission source convergence is proposed for the enhancement of the applicability. For the verification, the inactive cycles of pebble bed core problems are estimated by using the proposed method and compared with the result of the Shannon entropy method