Canadian Regulator’s Perspective on Severe Accident Management of Candu Reactors: In-Vessel Retention

Abstract of the technical presentation presented at:
International Seminar “In-vessel retention: outcomes of IVMR project” (IVMR 2020) and Forum on Future International IVR activities.
January 21-23, 2020

Prepared by: J. Sun, S. Gyepi-Garbrah and A. Persaud
Canadian Nuclear Safety Commission


The Canadian Nuclear Safety Commission (CNSC) is assessing strategies to support severe accident management of CANDU reactors in Canada. One of these strategies, in-vessel retention (IVR), has the likelihood of being successful by virtue of the CANDU design’s unique geometry and potential large heat sink availability. The CNSC is supporting research and development on investigating the possibility that external cooling of the Calandria vessel (which contains a large amount of water), could help to slow down the progression of a postulated severe core damage accident, prevent corium overheating and form a crust at water-cooled surfaces.

If a severe accident were to occur, the unique geometry and potential large heat sink availability of the CANDU reactor may increase the chance of IVR success by providing and maintaining the cooling of core reactor materials. The IVR strategy may also prevent progression of the severe accident to the ex-vessel phenomena, which could lead to the generation of a large amount of non-condensable and combustible gases from molten core concrete interaction (MCCI) and could result in containment integrity challenges.

This presentation will consists of three parts:

  • Highlight the regulatory roles and approach in the evaluation of IVR.
  • Provide an update on the status and knowledge of IVR, including completed experimental and analytical projects to support the achievement of IVR.
  • Highlight Canadian regulator’s IVR research and development activities in upcoming and future studies.

IVR is a key strategy for severe accident management for CANDU reactors in Canada. Robust and easily deployable emergency mitigating equipment that is implemented as a post-Fukushima defence-in-depth measure will provide water makeup to various strategic areas of the reactor and will therefore increase IVR success. Past and present experimental studies, as well as analytical investigations and future work, support a better understanding of key degradation mechanisms challenging IVR strategy. One of the IVR challenges and associated research activities deals with the effect of horizontal and vertical penetrations on the calandria vessel. The results of these investigations will enable CNSC staff to improve their understanding of IVR-related phenomena and have a better approach during the evaluation of the licensees’ submissions on IVR strategy for severe accident management.

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