The most painful lessons that engineers will learn from reviews of the Fukushima disaster relates to the unacknowledged failure of regulators, designers and utilities in better retrofitting existing reactors in a timely manner to better withstand and mitigate known severe accident related challenges to reactor and containment integrity. Commercial nuclear power reactors have operated for over 50 years, and the first severe accident progression studies and understanding of related phenomena began to mature over 30 years ago. However, some very basic accident prevention, mitigation and management measures have not been required by the regulators and hence not initiated by the utilities.

In public reviews that inevitably followed, severe accidents in power reactors at Three Mile Island, Chernobyl and Fukushima have been often presented as site specific aberrations in design, operations, safety culture and acts of God beyond mortal imagination. If a sincere soul searching, regulatory overhaul and actual, effective, timely and far reaching measures are not taken for operating reactors, the world is bound to witness recurring severe core damage scenarios followed by series of studies that will predictably conclude that such severe accidents can happen only in other jurisdictions and in other designs and that ‘our’ reactors are ‘safe’ and only minor enhancements are required to placate the anxious public or inquiring regulators.

The paper examines the role played by regulators and other Canadian stakeholders in failure to ensure that the so-called residual risk from operating nuclear reactors is minimized in a timely manner. It presents specific examples of known deficiencies in PHWR designs that may degrade a sustained loss of power from a recoverable outcome into significantly more severe consequences. It discusses ideas for potential design retrofits and reiterates the need for more open, concerted and cooperative efforts internationally in accident progression and consequence analyses and supporting experiments.

Taking station blackout as example of an instigator of a severe accident, the paper discusses certain elements of the current PHWR designs that accelerate the onset and progression of core damage and present substantially degraded opportunities for mitigation and control. It demonstrates how in many cases, a sustained loss of power in a PHWR may cause a containment bypass with early and unacceptable off-site consequences. Many issues discussed in this paper are common to other reactor designs and regulatory jurisdictions.

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