On-power containment integrity monitoring in CANDU multi-unit stations

Based on a general review of current on-power test methods and experience in CANDU multi-unit containments, it is concluded that such tests make a significant contribution to plant safety. In particular continuous monitoring at low pressure differentials merits further development and more widespread application. Current on-power tests include individual component testing, quarterly reduced pressure tests (typically at -15 kPa (g)), and continuous pressure trend monitoring at normal operational pressure of -3 kPa (g). A continuous monitoring concept is outlined which consists of a periodically updated mass balance. Instrument error uncertainty for this technique was estimated to be on the order of a 1 cm hole. However systematic fluctuations (often attributable to physical causes) dominate the error analysis in on-power tests. With precautions on sampling interval, a moving regression may be used to generate a leakage rate time series such that the fluctuations can be bounded or eliminated. Experience to-date has indicated that most containment boundary impairments are detectable by component tests or continuous monitoring. On-power tests methods are capable of addressing a significant portion of the containment failure mode spectrum. Station risk assessment and regulatory testing requirements are identified as means by which these methods can be credited in demonstrating containment integrity. (orig.)