Development of a computer code system for the analysis of VHTR cores

KAERI is developing a new computer code system for an analysis of the very high temperature gas-cooled reactor (VHTR) cores based on the existing HELIOS/MASTER code system. Several methodologies were developed in order for the original light water reactor (LWR) code system to treat easily the unique VHTR characteristics such as a so-called double heterogeneity problem, the effects of a spectrum shift and a thermal up-scattering, a strong fuel/reflector interaction, etc. The method of a reactivity-equivalent physical transformation (RPT) and the equivalent cylindrical fuel (ECF) model are proposed to transform the double-heterogeneous fuel problem into a single-heterogeneous one in a cylindrical coordinate for both a prismatic fuel and a pebble-bed fuel. An eight energy group structure with appropriate group boundaries has been constructed in the MASTER diffusion nodal calculation, within which the issues of a spectrum shift and a thermal upscattering are resolved. The concern about a strong fuel/reflector interaction can be handled easily by applying the equivalence theory to a simple one-dimensional spectral geometry consisting of the fuel and reflector regions. By combining all the methodologies described above, a well-known two-step core analysis procedure has been established, where HELIOS is used for the transport lattice calculation and MASTER for the 3-D diffusion nodal core calculation. The applicability of our code system was tested against several core benchmark problems. The results of these benchmark tests show that our code system is very accurate and practical for an analysis of both the prismatic and pebble-bed reactor cores.