PRELIMINARY ASSESSMENT OF THE LONG-TERM RADIOLYTIC GAS ACCUMULATION IN MODIFIED MEASUREMENT PIPELINE FOR BWR WITH GASFLOWMPI

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The non-condensable radiolytic gas could accumulate in a pipeline with dead end on the top induced by condensation due to imperfect insulation. This has already led to unforeseen combustion during the operation of BWR systems, and cost considerable damages. The chef strategy in Europe was to avoid radiolytic gas accumulations in branch lines from the very beginning. One possible solution is to modify the pipeline avoiding non-vented dead end at the top of pipeline systems and allowing the radiolytic gas to diffuse back to the main flow. The main objection of this study is to conduct a preliminary assessment for a new modified measurement pipeline for BWR with GASFLOW-MPI, evaluating the long-term radiolytic gas accumulation. The radiolytic gases can interact with the condensing film through absorption and degassing. The model for the non-condensable gas absorption in the condensing film is developed in GASFLOW-MPI to simulate this process. Then the experiment of the non-condensable enrichment in the pipeline of the Residual Heat Removal System (RHRS) for the Hamaoka Nuclear Power Plant and is modeled and simulated to validate the model. The calculated temperature profiles along the pipe centerline agree well with the experimental data indicating that GASFLOW-MPI can provide reasonable predictions for radiolytic gas accumulation in a complex pipeline. Then a 2D model for a modified measurement pipeline of BWR is built and analyzed with GASFLOW-MPI to evaluate the long-term radiolytic gas accumulation. Different from the cases in literatures, the modified pipeline has the reservoir (main flow) locating at the top and the dead end of locating at the bottom, meanwhile the horizontal parts is modified slightly ascending to allow the radiolytic gas to diffuse upwards. The result shows that the non-condensable gas will not be significantly enriched locally at the dead end of the pipe, because the gravity and the momentum work against with each other in this case. On one hand, the pipeline take suction from the main flow, which pushes condensed gas in the pipe to the bottom, meanwhile, the condensed gas mixture tends to move towards the reservoir because the gas enriched with non-condensable gases has lower density. In the end, a steady status can be reached under these two mechanisms, with the maximum concentration of non-condensable gases below the combustion limitation. Therefore, the long-term radiolytic gas accumulation can be mitigated through the modified design of the pipeline.

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