ABSTRACT
One of the important safety features of PHWR is the ability of the moderator to act as a backup heat sink during emergencies. If moderator subcooling is insufficient in a PHWR, the external surface of the fuel channel may experience considerable dryout following occurrence of a Large Loss of Coolant Accident (LLOCA), irrespective of successful deployment of the Emergency Core Cooling System (ECCS). The current analysis models raise the concern that insufficient moderator subcooling may cause failure of multiple fuel channels, and ECCS may become ineffective to cool some un-failed channels due to degradation in the coolable geometry of the core. The number of fuel channels at risk of failure depends on the specific conditions and analysis assumptions.
Application of the CNSC Risk-Informed Decision Making (RIDM) process was requested with the objective of providing information to decision makers on the risks associated with this issue, and recommendations for risk control measures.
The paper presents the activities of the risk assessment team followed the steps of the CNSC RIDM process, and application of the RIDM specific tools and guidance for identification and assessment of the risks, identification of potential risk control measures and for consideration of time-at-risk, as well as the main results of this exercise. A brief overview of the CNSC RIDM process and tools is also discussed.
