Study of hydrogen risk in a PWR-W containment during a SBO scenario; Tau parameter definition and application on venting strategy analysis

Abstract Hydrogen management is still one of the main nuclear safety topics because of its violent reaction with oxygen. During a severe accident, hydrogen can be generated and it can be released into the containment atmosphere. To deal with this threat, the severe accident management guidelines must be used. These guidelines include several actions to coup with the hydrogen challenge, including the venting strategy. However, these guidelines do not normally help the operators in deciding when the optimal moment to vent is. In this study, a PWR-W GOTHIC 3D containment model is used to simulate a station blackout scenario. The venting and spraying strategy and their impact on hydrogen risk are evaluated in a sensitivity analysis. For this goal, more than 60 sequences with different timing of venting and spraying have been performed and analyzed. To compare all simulations between each other, a quantitative approach to hydrogen risk was needed. Therefore, hydrogen risk is estimated and quantified using a new parameter named Tau. This parameter proposal takes into account three variables for each cell that account for likelihood and consequences of a hydrogen burn: the time a hydrogen cloud spends within the flammability limits, the mass of hydrogen that is within the flammability limits, and the combustion regime reached. With this parameter, a single value can be obtained to quantify the hydrogen risk in a full transient. Finally, reviewing the sensitivity analysis, it is observed that hydrogen risk is highly dependent on the venting strategy used, increasing it if performed prior to the hydrogen release, or decreasing it if performed during or after the hydrogen release peak. To conclude, optimal time windows for venting are proposed in order to reduce the hydrogen risk during the transient.

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