Summary Record

The first workshop of the OECD/NRC Benchmark based on NUPEC BWR Full-size Fine-mesh Bundle Tests (BFBT) was held on 4 October 2004. The workshop was hosted by the Japan Nuclear Energy Safety (JNES) Organization. The BFBT Benchmark is sponsored by the US Nuclear Regulatory Commission (NRC), the OECD, and the Nuclear Engineering Program (NEP) of the Pennsylvania State University. The experimental data was produced during a measurement campaign by the NUPEC, Japan, and sponsored by the Japan Ministry of Economy, Trade and Industry (METI). This international benchmark, based on the NUPEC database, encourages advancement in this un-investigated field of two-phase flow theory with very important relevance to the nuclear reactor's safety margins evaluation. Considering the immaturity of the theoretical approach, the benchmark specification is being designed so that it systematically assesses and compares the participants' numerical models on the prediction of detailed void distributions and critical powers. Furthermore, the following points are kept in mind while the benchmark specification is being established. As concerns the numerical model of void distributions, no sound theoretical approach that can be applied to a wide range of geometrical and operating conditions has been developed. In the past decade, experimental and computational technologies have improved tremendously through the study of the two-phase flow structure. Over the next decade, it can be expected that mechanistic approaches will be more widely applied to the complicated two-phase fluid phenomena inside fuel bundles. The development of truly mechanistic models for critical power prediction is currently underway. These models must include elementary processes such as void distributions, droplet deposit, liquid film entrainment, etc. The BFBT benchmark consists of two parts (phases), each part consisting of different exercises: - Phase 1 Void Distribution Benchmark: Exercise 1: Steady-state sub-channel grade benchmark; Exercise 2: Steady-state microscopic grade benchmark; Exercise 3: Transient macroscopic grade benchmark. - Phase 2 Critical Power Benchmark: Exercise 1: Steady-state benchmark; Exercise 2: Transient benchmark. It should be recognized that the purpose of this benchmark is not only the comparison of currently available macroscopic approaches but above-all the encouragement to develop novel next-generation approaches that focus on more microscopic processes. Thus, the benchmark problem includes both macroscopic and microscopic measurement data. In this context, the sub-channel grade void fraction data are regarded as the macroscopic data and the digitized computer graphic images are the microscopic data