ICONE23-1053 OPPORTUNITIES AND NEED TO FURTHER UPGRADE CANDU REACTORS FOR IMPROVED SEVERE ACCIDENT MITIGATION AND REDUCTION OF RISK

Abstract

Independent deterministic analyses of system response and severe accident progression after a station blackout scenario for CANDU single and multi-unit reactors have unveiled a number of design vulnerabilities that cause uncontrolled pressure boundary ruptures; premature expulsion of coolant from main loops and from the moderator heat sink; direct exposure of core debris and fission product releases to the containment; thermo-mechanical failure of the thin shell Calandria vessel welds; accelerated production of hydrogen with containment boundary failures by steaming as well as by sparsely populated PARS units potentially exposed to high concentration deuterium/hydrogen they are unable to adequately mitigate. A number of design enhancements can however be undertaken to minimize risk from a severe accident by eliminating or avoiding some of the undesirable system responses. Engineered design measures to also avoid some the undesirable accident progression paths have been summarized in the paper. The Canadian regulatory body CNSC prepared a number of design enhancement requests as part of post Fukushima reviews but then accepted measures and submissions that do not meet the public expectations for risk reduction after Fukushima on all counts. Without thoughtful and timely design changes, consequences of a severe core damage in a CANDU reactor can pose risks that are still unacceptable especially after the hype that surrounded the utility 'Stress Tests' and regulatory 'Action Items' that followed Fukushima disaster in 2011. It is stressed that operating CANDU and other similar PHWRs in India are unique; have an enviable safety record; have maintained very high operational loads and the observations in this paper relate to requirements for mitigating a range of potential severe accidents whose probability has been demonstrated to be relatively high given that less than 15000 reactor years of operation have resulted in 3 severe accidents that have caused more off-site damage than ever anticipated. That is something the founding fathers did not consider within the design basis and the current regulating bodies have a difficult time wrapping their head around. Utilities that operate the plants have shown no visible leadership in self assessments; have rejected any attempts to enlighten them and the risk posed by continued operation of these plants can only be minimized by an open discussion of the totality of issues as attempted in this paper.