Numerical simulation of the long-term radiolytic gas accumulation in diversified measurement pipeline for BWR using GASFLOW-MPI

Abstract

Unforeseen radiolytic gas combustions have occurred in many Boiling Water Reactors (BWRs) and caused severe damages in the past. This combustion is caused by the long-term accumulation of radiolytic combustible gas in a pipeline with a top dead-end due to the main steam continuous condensation. The main objective of this study is to conduct a numerical simulation for a diversified measurement pipeline for BWR with GASFLOW-MPI, in order to evaluate its the long-term radiolytic gas accumulation. Firstly, the model for the radiolytic gas absorption in the condensing film is developed in GASFLOW-MPI and is validated with the experiment of “Hamaoka pipe”. The calculated temperature profiles along the pipe centerline agree well with the experimental data. Then a 3D model for part of the diversified measurement pipeline of BWR is built and analyzed with GASFLOW-MPI to evaluate the long-term radiolytic gas accumulation. Moreover, two factors are discussed in this paper, one is the local condensation enhancement at the floating chambers which is, different from the rest part of the pipeline, not covered by insulator, and the other is the radiolytic gases absorption model. The result shows that there will not be strongly local accumulation of radiolytic gases at the “dead-end” of the pipeline, because the momentum of the reservoir can be balanced with the buoyancy caused by the density difference. Moreover, a mass balance can be established when the mass of radiolytic gases sucked from the reservoir is balanced with that absorbed in the film when considering the absorption model. As a result, the concentration of the radiolytic gases accumulated is far below the combustible limit for the pipeline.