Moderator Heat Sink Behaviour Under Severe Accident Conditions as Modelled by MAAP-CANDU

Abstract

In apostulated severe accident in a CANDU reactor, which assumesthe moderator cooling system is unavailable, the moderator would heat up and pressurize until the calandria-vessel rupture disks burst. The subsequent moderator boiling and expulsion would uncover some fuel channels that would heat up and disassemble. The magnitude of moderator expulsion affectsthe accident progression, in-core hydrogen production, and fission product releases.The integrated severe-accident simulation code MAAP-CANDU uses simple two-phase moderator models. In this scoping paper three parameters, that affect the moderator expulsion, were explored: the void concentration factor, churn turbulent factor, and number of rupture disks that simultaneously burst. For a CANDU 6 station blackout scenario, changing these parameters within reasonable limits increased the moderator expulsion from 32% (standard parameter values) to 58% of the initial inventory, reducing the time to calandria vessel dryout by ~2.7 hours. Thus these three parameters significantly change the accident progression and consequences, affecting Severe Accident Management (SAM) actions like calandria vessel refill. Additional analytical and experimental work is required to derive best-estimate inputs for modelingmoderator expulsion.