Consequences of a Single Feeder Break Accident in CANDU Power Reactors

Abstract

An accident scenario in the Canadian Pressurized Heavy Water Reactor (CANDU) in which the flow in one channel is reduced due to an inlet feeder break is considered. Different break sizes are studied and radioactivity released to the atmosphere (131I and Xe + Kr) in each case is estimated using AECL Computer Codes FIREBIRD, PRESCON, HOTSPOT and CURIES. (The channel with the maximum power of Pickering - B generating station was used in the analysis as an upper limit best estimate).Six break sizes were used in the analysis of discharge rates. These were 6.45,10.97,12.26,12.9,14.2 and 25.8 cm2. Only 10.97 and 12.9 cm2 sizes were used for further analysis.The activity released from breaks of smaller size than stagnation were found to cause more activity released because the discharge rate of steam from these breaks is less and therefore the building overpressure period is longer. That is before the dampers open to relief the pressure to the containment, the fuel would have spent longer time at high temperature, and thus more than the free activity of the channel is released to containment.Both cases of intact (single failure) and impaired (dual failure) containment are analysed. The activity released to the atmosphere in the event of single failure was estimated to be below the permissible regulatory limit. For dual failure (feeder break plus containment impairment), it exceeds this limit. This indicates that unless established means other than the high reactor building pressure signal are employed to open the pressure relief panels and relief the reactor building pressure to containment, the radioactivity released to the atmosphere in the case of dual failure would be more than the permissible limit if such an accident should take place.