CANDU heat sinks improvements as a follow up to Fukushima Daiichi Accident "The regulator perspective"

An abstract of the technical presentation presented at:
ENC 2014, The European Forum to discuss Nuclear Technology Issues, Opportunities & Challenges
Marseille, France
May 11–15, 2014

Prepared by:
N. Mesmous and C. Harwood
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

Heat sinks are combination of systems or portions of systems that contribute to conveying heat to the ultimate heat sink (UHS); the ultimate goal of heat sink systems is to provide heat removal from the heat source (reactor core, irradiated fuel, pump heat) to the UHS. The UHS is a medium to which the generated heat can always be transferred. This is an inexhaustible natural body of water or the atmosphere.

The purpose of this paper is to provide a summary of the Canadian Nuclear Safety Commission (CNSC) recommendations related to improving the heat sink strategy as a follow up to the Fukushima Daiichi Accident (FDA).

As a follow up to FDA, CNSC staff tasked the Nuclear Power Plant (NPP) licensees to review the lessons learned from the FDA and re-examine the NPP safety cases. The reviews have examined the CANDU defence-in-depth strategy and considered events more severe than those that have historically been regarded as credible, and evaluated their impact on the NPPs safety. Availability of emergency equipment was shown to be crucial during the FDA and its availability could have allowed terminating the severe accidents early enough to prevent any radioactive releases to the environment. Licensees presented appropriate evaluations of the means to provide coolant make-up to the primary Heat Transport System (HTS), boilers, moderator, calandria vault, and irradiated fuel pools.

The CANDU defence-in-depth strategy assures effectiveness of physical barriers against the release of radioactive materials. It ascertains that the three basic safety functions (controlling the power, cooling the fuel and containing the radioactive material) are preserved, and that radioactive materials do not reach people or the environment. Prevention of beyond design basis accidents and mitigation of severe accidents can be further improved by providing additional make-up systems capable of delaying core melt for an extended period. These provisions would reduce the likelihood and consequences of severe core damage and large release of radioactive materials.

In general, CANDU reactors have a large inventory of water available for passive cooling in the secondary cooling system, the primary HTS, the moderator and the calandria vault. This inventory could be augmented with appropriate operator actions. As with other designs, the CANDU reactors have the capability of passively removing decay heat provided that water can be injected to the boilers and that there is no significant leakage of primary coolant.

CNSC staff is satisfied that Canadian NPPs are safe and have a robust design that relies on multiple layers of defence. The design ensures that there will be no impact on the public from credible events. The emergency mitigating equipment offers additional protection against more severe events that are much less likely to occur.