Assessment of the impact of fueling machine failure on the safety of CANDU-PHWR

A survey of possible LOCA (Loss-of-Coolant Accident) initiating events that might take place for CANDU-PHWRs (Canadian Deuterium Uranium-Pressurized Heavy Water Reactors) has been conducted covering both direct and indirect initiators. Among the 22 initiating events that were surveyed in this study, four direct initiators have been selected and analyzed briefly. Those selected were a pump suction piping break, an isolation valve piping break, a bleed valve failure, and a fueling machine interface failure. These were selected as examples of failures that could take place in the inlet side, outlet side, or PHTS (Primary Heat Transport System) interfaces. The Pickering NGS (Unit-A) was used for this case study;A postulated mechanical failure of the fueling machine during the fueling process was studied in detail, considering the effect of the location of the pressure tube under fueling. At certain levels of a small-size LOCA that may be caused by failure of the fueling machine, the ECCS (Emergency Core Cooling System) injection may be needed to cool down the core. The availability of the ECCS as a long term safety backup system following the LOCA occurrence, was analyzed;Fault Tree Analysis methodology was used to assess the occurrence probability of the fueling machine failure, and also the unavailability of the ECCS. The PREP and KITT computer codes were used to estimate the failure probabilities. From these computations, the failure probability of the fueling machine interface is estimated as 1.08 x 10(\u27-5), and the unavailability of the ECCS is 3.63 x 10(\u27-3);The entire system interaction following the fueling machine failure was examined. The accident sequences resulting from partial failure of the safety systems were predicted using the event tree technique;Double failure (failure of the protective devices to operate when the process equipment fault occurs) and a triple failure (failure of the protective devices and the ECCS as well as the process equipment) were found to be highly improbable