Mexico: Generation and Distribution of Hydrogen in a BWR Reactor in a Severe Accident Scenario

Ever since the TMI-2 accident, the nuclear industry has aimed to have a better understanding of severe accidents and its consequences. The computer codes, like MAAP and MELCOR, developed to get this broader knowledge struggle to produce a consolidated result of what the sequence of events of the quantitative results are in a severe accident scenario. The lessons learned from the different severe accident occurred in the nuclear industry history have helped to develop de the Severe Accident Guidance (SAGs), which when followed reduce the hydrogen generation and the overall accident consequences. This paper studies a severe accident scenario in a BWR Mark II where no SAGs are followed in MELCO 2.1 and MELCOR 1.8, these simulations reinforce the findings that, although the same code is used, the different amount of hydrogen is generated from one version of the code to another. Also, a short and long term SBO in a Mark II BWR are analysed with MAAP4; in the first simulation the generation of ex-vessel hydrogen decreases from 400 kg to 100 kg while , in-vessel hydrogen generation decreases from 380 kg to 160 kg when severe accident guidelines are followed. The second case ,LTSBO, shows how when severe accident water addition (SAWA) starts by injecting 400 gpm through the RPV combined with a wetwell vent containment, hydrogen reduces from 200 to 80 kg ; a second vent reduces accumulated hydrogen from 140 to 60 kg until the hydrogen generation stabilizes at about 9 hours into the event. This work demonstrates how the use of the SAMGs provides a clear strategy to mitigate the consequences combining the lessons learned from the severe accidents experiences by the nuclear industry and the experimental data provided by experiments and modelled by computer codes like MAAP and MELCOR. (author)