PHYSOR 2004 - The Physics of Fuel Cycles and Advanced Nuclear Systems: Global Developments Chicago, Illinois, April 25-29, 2004, on CD-ROM, American Nuclear Society, Lagrange Park, IL.(2004) Monte Carlo Analysis of High Moderation 100% MOX BWR Cores using JEF2 and JENDL3 Nuclear Data Olivier Litaize *1 , Alain Santamarina 1 and Morgan Hervault 1 Stephane Cathalau 2 , Philippe Fougeras 2 , and Patrick Blaise 2 Toru Yamamoto 3 , Ryogi Kanda 3 , Masaru Sasagawa 3 and Tsukasa Kikuchi 3 1 CEA Cadarache DEN/DER/SPRC - Bldg. 230 - 13108 Saint Paul-lez-durance, France 2 CEA Cadarache DEN/DER/SPEx - Bldg. 238 - 13108 Saint Paul-lez-durance, France 3 Systems Safety Department, NUPEC Fujita-Kanko Toranomon Bldg., 5F,3-17-1, Toranomon,Minato-ku, Tokyo, 105-0001, Japan This paper presents a Monte Carlo analysis of the BASALA neutron physics pro- gramme implemented in the EOLE facility at CEA Cadarache. This programme comprised two types of cores simulating hot (BASALA-H) and cold (BASALA-C) conditions. Reference and `poisoned´ configurations were investigated in both cases. The BASALA-H core included a configuration with extra water rods while the BASALA-C core included configurations with cross-shaped control blades. Different kinds of physical parameters were measured and calculated as the effective mul- tiplication factor, reactivity worth of control blades, gadolinium pins and fission rate distributions. The experiment and calculation comparisons are satisfactory and demonstrate the ability of French and Japanese reference calculation tools to predict the main 100% MOX BWR neutronic parameters. KEYWORDS : monte carlo, tripoli-4, jef-2.2, jeff-3.0, mvp, jendl-3.2, MOX, BWR 1. Introduction This BASALA experimental programme was carried out from 2000 to 2002 in the framework of an international cooperation between NUPEC (Japan), CEA and COGEMA (France) as part of the study of high moderation 100% MOX BWR cores. Several BWR mock-ups loaded with 9 × 9 fuel assemblies (7% Pu content) were performed. The four central assemblies, in which MOX fuel pins were measured by gamma-scanning, are composed by different types of plutonium content (3%, 4.3%, 7% and 8.7%). Each fuel pin is cladded by zircaloy clads and aluminium over-clads. The main nuclear parameters, as the core reactivity, power distribution and reactivity worth of UO 2 -Gd 2 O 3 fuel rods and control blades, were measured and calculated by French and Japanese Monte Carlo codes. 2. Experimental Setting Each phase of the BASALA programme [1] started with a Reference configuration. Four addi- tional cores were performed with different kind of perturbations and criticality was achieved by adjusting the number of peripheral fuel pins [2]. * Corresponding author, Tel.(33)4 4225 7257, FAX (33)4 4225 7009, E-mail: olivier.litaize@cea.fr