Development of a computer code system for the analysis of prism and pebble type VHTR cores Jae Man Noh, Kang-Seog Kim, Yonghee Kim, Hyun Chul Lee * Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong-gu, Daejeon 305-353, Republic of Korea article info Article history: Received 10 October 2007 Received in revised form 21 March 2008 Accepted 26 March 2008 Available online 13 May 2008 abstract Korea Atomic Energy Research Institute (KAERI) is developing a new computer code system for an anal- ysis of 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 the unique VHTR characteristics easily such as the 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-heter- ogeneous 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 up-scattering are resolved. The concern about a strong fuel/reflector interaction can be handled easily by applying the equivalence the- ory to a simple one-dimensional spectral geometry consisting of the fuel and reflector regions. By com- bining 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 revealed that our code system is very accurate and prac- tical for an analysis of both the prismatic and pebble-bed reactor cores. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction In the Nuclear Hydrogen Development and Demonstration (NHDD) project (Chang et al., 2007), KAERI is developing a very high temperature gas-cooled reactor (VHTR) for a hydrogen pro- duction. For the successful completion of this project, a new com- puter code system for the analysis of VHTR cores is under development based on the existing HELIOS(STUDSVIK, 2000)/MAS- TER(Cho et al., 1999) code system. This code system was originally developed for a light water reactor (LWR) core analysis and it adopts a well-known two-step core analysis procedure. The two- step procedure which has been widely used and well-proven for the analysis of LWR cores consists of, for the first step, a transport lattice calculation for simple spectral geometries to generate a few group cross-section table-sets and, for the second step, a diffusion nodal core calculation by using these table-sets. HELIOS is respon- sible for the first step and MASTER is responsible for the second step in our code system. In the VHTR reactor physics, there are several unique neutronic characteristics that cannot be handled easily by the conventional computer code system and the two-step analysis procedure for the analysis of LWR cores. Typical examples of such characteristics are the so-called double-heterogeneity problem due to the partic- ulate fuels randomly dispersed in a graphite matrix, the effects of a spectrum shift and a thermal up-scattering due to a graphite mod- erator, a strong fuel/reflector interaction, etc. In order to facilitate in an easy treatment of such characteristics, we developed some specific methodologies for our code system. The resonance self-shielding effect becomes significant if the fuel is lumped into small particles with multi-coating layers (so- called TRISO particles) and the particles are dispersed in a graphite matrix with a relatively low volume fraction. A simple volume- weighted homogenization (VWH) of a fuel zone with TRISO particles results in a significant reduction in the resonance self-shielding effect (Kim et al., 2004). This is a well-known dou- ble-heterogeneity problem which cannot be handled by most con- ventional lattice codes except for a few such as WIMS, APOLLO, and DRAGON (Halsall, 1996; Sanchez et al., 1987; Marleau et al., 2006). The method of a reactivity-equivalent physical transformation (RPT) has been proposed (Kim and Baek, 2005; Kim et al., 2006) to transform a double-heterogeneous fuel problem into a single- heterogeneous one, which renders the conventional lattice codes including HELIOS the capability to analyze the VHTR fuel elements. 0306-4549/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.anucene.2008.03.011 * Corresponding author. Tel.: +82 42 868 4807; fax: +82 42 868 8767. E-mail address: lhc@kaeri.re.kr (H.C. Lee). Annals of Nuclear Energy 35 (2008) 1919–1928 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene