Summary Record

The fifth workshop for the OECD/NRC Benchmark based on NUPEC BWR Full-size Fine-mesh Bundle Tests (BFBT-5) was held on 31 March and 1 April 2008 in Garching, Germany. This international benchmark encourages advancement in the un-investigated fields of two-phase flow theory with very important relevance to the nuclear reactors' 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 were kept in mind while establishing the benchmark specification: 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 tremendously improved though 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 is made up of two parts (phases), each part consisting of different exercises: - Phase I - Void Distribution Benchmark: Exercise 1 (I-1) - Steady-state sub-channel grade benchmark; Exercise 2 (I-2) - Steady-state microscopic grade benchmark; Exercise 3 (I-3) - Transient macroscopic grade benchmark; Exercise 4 (I-4) - Uncertainty analysis of the steady state sub-channel benchmark. - Phase II - Critical Power Benchmark: Exercise 0 (II-0) - Pressure drop benchmark; Exercise 1 (II-1) - Steady-state benchmark; Exercise 2 (II-2) - Transient benchmark; Exercise 3 (II-3) - Uncertainty analysis of the steady state benchmark. It should be recognized that the purpose of this benchmark is not only to compare currently available macroscopic approaches but above-all to encourage the development of 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. The technical topics to be addressed at the workshop include: - Review of the benchmark activities after the 4. Workshop; - Presentation and discussion of summary of comparisons of final submitted results for Exercise 1 of Phase I (I-1); for Exercise 0 of Phase II (II-0); and for Exercise 1 of Phase II (II-1); - Presentation and discussion of comparison of final submitted results for Exercise 2 of Phase I (I-2); - Presentation and discussion of comparison of final submitted results for Exercise 3 of Phase I (I-3); - Presentation and discussion of comparison of final submitted results for Exercise 2 of Phase II (II-2); - Presentation and discussion of preliminary uncertainty results for Exercise 4 of Phase I (I-4); - Presentation and discussion of preliminary uncertainty results for Exercise 3 of Phase II (II-3); - Preparing a special issue in a journal with participants' BFBT papers; - Defining a work plan and schedule outlining actions to advance the two phases of the benchmark activities