Comparison of Channel Behavior for Initial and Aged CANDU Core

The timing of the fuel channel failure is important for this accident because it determines the amount and temperature of molten material in the channel, which can be discharged into the moderator. Channel failure results in increased coolant flow through the channel, and therefore terminates the channel heat-up. This limits the fission product releases from the fuel as well as the mass of the molten material. Therefore, it is conservative to select assumptions that tend to delay a channel failure. In this study, the channel failure timing and amount of molten material after the feeder stagnation break were evaluated for an initial core and an 11 EFPY (Effective Full Power Year) aged core of CANDU, respectively. Here, the aged core indicates that the core was assumed to be operated for 11 EFPY after the refurbishment and was considered the aging effect of the main heat transport system such as the pressure tube creep, magnetite deposition, and feeder orifice degradation. This was carried out to verify the aging effect on the safety analysis, specifically in the case of the amount of molten material following the feeder stagnation break. Because conservative assumptions are the basic characteristics of a deterministic safety assessment, the conservativeness of the aged core assumption can be derived from the evaluation results. Since the molten material in the channel can be discharged into the moderator after the channel failure, the more molten inventory for the aged core can have a bad influence on safety. Therefore, the aged core model was found to be more conservative, and it is reasonable to consider the aging effect in the safety assessment