In this paper, the effects of internal reflector size on the neutron flux of PBMR reactors by means of MCNP code has been investigated as a part of establishing Monte Carlo computation system for PBMR core analysis. At first, high temperature library in this reactor is built by NJOY code then, cross sections in the library are used by MCNP code. Because of complex fuel structure, several simplifications are assumed in order to limit the need for any further approximations when defining code models. In neutronic simulation, at first, all the important neutronic parameters such as thermal and fast flux distribution and power density of the reactor core are compared with the results of other codes and the accuracy of the simulation has been validated. Then the best size of internal reflector has been chosen by comparing the neutron flux in some different sizes and obtaining the best form factor.
Keywords: PBMR, MCNP, NJOY, Monte Carlo, internal reflector, form factor, neutron flux