Abstract
In this study, uncertainty and sensitivity analyses were performed with MELCOR 2.2.18 to study the hydrogen generation (figure-of-merit (FoM)) during the in-vessel phase of a severe accident in a light water reactor. The focus of this work was laid on a large generation-III pressurized water reactor (PWR) and a double-ended hot leg (HL) large break loss of coolant accident (LB-LOCA) without a safety injection (SI). The FPT-1 Phebus integral experiment emulating LOCA was studied, where the experiment outcomes were applied for the plant scale modelling. The best estimate calculations were supplemented with an uncertainty analysis (UA) based on 400 input-decks and Latin hypercube sampling (LHS). Additionally, the sensitivity analysis (SA) utilizing the linear regression and linear and rank correlation coefficients was performed. The study was prepared with a new open-source MELCOR sensitivity and uncertainty tool (MelSUA), which was supplemented with this work. The FPT-1 best-estimate model results were within the 10% experimental uncertainty band for the final FoM. It was shown that the hydrogen generation uncertainties in PWR were similar to the FPT-1, with the 95% percentile being covered inside a ~50% band and the 50% percentile inside a ~25% band around the FoM median. Two different power profiles for PWR were compared, indicating its impact on the uncertainty but also on the sensitivity results. Despite a similar setup, different uncertainty parameters impacted FoM, showing the difference between scales but also a significant impact of boundary conditions on the sensitivity analysis.
Highlights
This study focused on nuclear power plants (NPP) with pressurized water reactors (PWR) reactors, and the scope was limited to a severe accident phase before the reactor pressure vessel (RPV) failure
The best estimate (BE) results for the FPT-1 were compared with two experimental datasets ([18,29,30,58]) and selected best-estimate results found in the literature—see
Similar behavior was present for the best estimate MELCOR model, but the oxidation peak ended at ~80 g
Summary
In the course of a severe accident (SA) in a light water reactor (LWR), large quantities of hydrogen gas can be generated. In nuclear power plants (NPP) with PWR reactors, the hydrogen is a threat for safety barriers and especially containment buildings It creates a risk of global hydrogen combustion (detonation or deflagration), which can compromise the containment integrity. A single severe accident integral code run provides deterministic results, which has to be treated with a large margin of confidence. It allows the estimation of different phenomena but cannot be trusted in the same way as typical. We have to remember that even relatively small initial disturbances can propagate with accident progression to substantial differences It is one reason why severe accident studies with integral codes should be supplemented with uncertainty studies and sensitivity when possible. Nowadays, growing and easy access to cheap computational resources makes it possible
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