Consequences of Severe Accident Mitigating Strategies in CANDU Reactors

Abstract of the technical presentation presented at:
International Severe Accident Management Conference (ISAMC-2018)
October 15–18, 2018

Prepared by:
Mohamed E. Shawkat
Canadian Nuclear Safety Commission

Abstract:

Management of severe accidents requires the implementation of different strategies that aim to: delay and/or prevent the accident from leading to severe consequences; maintain the integrity of the radiation barriers; eliminate or minimize the release of the fission products to the environment; and achieve long-term stable and controlled reactor conditions. The use of multiple mitigating strategies provides the operator with several options to prevent accident progression and mitigate the radiological consequences, which increases the likelihood of achieving the severe accident management goals.

However, excessive or unmonitored implementation of the mitigating strategies may have adverse consequences that result in challenging the integrity of the radiation barriers and limit the expected benefits of these strategies. With the existing multiple strategies, careful attention should be given to their adverse impacts, in order to avoid amplifying them when several strategies are applied.

This paper summarizes the strategies proposed by the severe accident management guidelines (SAMG) for CANDU reactors and outlines their advantages and adverse consequences. A network-wise structure was developed to illustrate the relationships between the proposed strategies, their positive and negative outcomes, and the SAMG goals.

The SAMG actions in CANDU reactors can generally be categorized into four groups:

• deployment of water from available reservoirs within the plant to the steam generators’ secondary side to maintain them as heat sinks for the reactor core; this is done using equipment installed in the plant or via or gravity-driven flow that requires minimal operator intervention
• deployment of water from available reservoirs within or outside the plant to the reactor core, moderator, vault/shield tank and containment, via installed or portable equipment
• addition/utilization of passive mitigating equipment to avoid failure of the plant infrastructure; examples of such equipment include overpressure protection devices, passive autocatalytic recombiners (PARs) and containment-filtered venting system (CFVS)
• use of portable generators to restore the functionality of containment safety systems such as air cooling units, hydrogen igniters and vacuum pumps (if they become unavailable)

Nuclear power plant operators are required to consider both positive and negative effects of the proposed strategies in their SAMG by identifying the envelope or the limits where implementing each proposed strategy is considered advantageous with minimum or no adverse consequences. 

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