Bruce NGS A/B Assessment of Reactor Vault Fans on Air Mixing Patterns
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Abstract
The development and results of numerical simulations of air mixing patterns in the CANDU Bruce Nuclear Generating Station reactor vault, as a function of vault cooling fan combinations, are presented. The results of this analysis will serve as a basis for selecting appropriate fan combination scenarios to consider in the upcoming post- LOCA hydrogen-air-steam mixing analysis. Following a severe reactor accident in which fuel cooling is impaired, a significant amount of hydrogen may be produced from the steam/Zircaloy reaction and subsequently released into containment. The hydrogen ignition system mitigates the consequences of hydrogen burns to within acceptable safety limits. Igniters deliberately initiate a bum of the hydrogen-air-steam mixture as it reaches its flammability limits. However without adequate mixing, the igniters may become blinded by a region of non-flammable hydrogen mixture while an unfavourable hydrogen mixture forms elsewhere. The vault cooling fans play an important role in promoting mixing in the vault atmosphere. To help assess the effects of vault cooling fans on air mixing, an analysis was carried out to identify the air mixing patterns as a function of different fan availability combinations. The three- dimensional containment code, GOTHIC, was used to model the Bruce containment with modelling emphasis on the reactor vault geometry and the vault cooling system fans. Twenty-five fan combination air mixing simulations and eight tracer gas fan dispersion simulations were performed. The results showed that air mixing patterns created by individual fans can be superimposed to determine the effects of various fan combinations, there was symmetry of flow patterns between the west and east vault halves, and there was a general absence of significant stagnant regions in the reactor vault.
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