Principles of operation of CANDU multi-unit containment systems

Analysis has shown that the 'negative pressure' containment (NPC) concept is flexible and efficient in meeting the various challenges presented by a broad spectrum of initiating failures ranging from operational upsets to failures of the largest piping in the heat transport system. Containment envelope ventilation is isolated promptly and the integrated overpressure period is minimal. A substantial holdup period is provided to remove the bulk of radiologically important fission products from the containment atmosphere. If and when controlled venting becomes necessary to avoid long term leakage, radiological consequences are minimized by treating the effluent stream and by providing the flexibility to interrupt the release during unfavourable weather conditions. The NPC concept incorporates both the features identified as 'compartment venting' and 'filtered atmospheric venting'. These were recognized separately as the most effective means of reducing major accident consequences following LWR meltdown sequences. CANDU does not have a credible meltdown sequence; however, pressure and effluent control are important risk-reducing function in many accident sequences. This is particularly important when it is recognized that containment envelopes are not absolute containers, and so might be impaired at the time of an accident. The dual-failure requirements of the AECB Siting Guide properly recognize this reality. The NPC containment concept is well equipped to respond to the full range of accident sequences important to public safety. The actual consequences of major accidents are expected to be much lower than those calculated for the use in reactor licensing. The eventual objective is to bring nuclear safety equipment expenditures down, closer to a reasonable balance relative to other societal risk-mitigation expenditures