Accident Localisation System (ALS) of Ignalina NPP is a “pressure suppression” type confinement, which protects the population, employees and environment from the radiation hazards. According to the Safety Analysis Report for Ignalina NPP ∼110 m3 of hydrogen is released to ALS compartments during the Maximum Design Basis Accident. However in case of beyond design basis accident, when the oxidation of zirconium starts, the amount of generated hydrogen could be significantly higher. If the volume concentration of hydrogen in the compartment reaches 4%, there is a possibility for a combustible mixture to appear. To prevent the possible hydrogen accumulation in the ALS of the Ignalina NPP during an accident the H2 control system is installed. The results of the performed analysis derived the places of the possible H2 accumulation in the ALS compartments during the transient processes and assessed the mixture combustibility in these places for a beyond design basis accident scenario. Such analysis of H2 distribution in the ALS of Ignalina NPP in case of BDBA was not performed before.

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