A regulatory perspective on the establishment of fuel safety criteria for the large loss of coolant accident

Abstract of the technical paper presented at:
Technical Meeting on Fuel Integrity during Normal Operating and Accident Conditions in Pressurised Heavy Water Reactors (PHWRs)
Bucharest, Romania
September 24 to 27, 2012

Prepared by:
Ali El-Jaby
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

The analysis of the large loss of coolant accident (LLOCA) scenario for CANDU pressurised heavy water reactors (PHWRs) in Canada has been affected by periodic discoveries that have impacted the predicted consequences of the event to the extent that the safety margins have been significantly eroded.

Canadian Nuclear Safety Commission (CNSC) staff is currently actively monitoring an extensive initiative by the Canadian nuclear industry to develop a new analytical framework, known as the composite analytical approach (CAA), which is aimed at demonstrating that the safety margins for the LLOCA scenario are much larger than those currently predicted using the more conservative limit of operating envelope (LOE) analysis methodology.

Part of the industry effort to demonstrate that larger safety margins exist consists of a re-evaluation of the fuel safety criteria being used in the LLOCA safety analysis. This includes a systematic process to identify the physical barriers relevant to the accident, their various failure mechanisms and their associated failure limits. The principal output from this process is the establishment of fuel safety criteria (also know as derived acceptance criteria) which are defined with a certain margin to the failure limits. This process also includes a review of the existing experimental database, which could be used as the technical basis for the proposed fuel safety criteria, the identification of additional experiments needed to address gaps in knowledge, and a review of the analytical capability of the current code toolset to assess adherence to the LLOCA fuel safety criteria.

Pending CNSC concurrence towards the use of the CAA, including its subsequent implementation by all licensees, the CNSC has instituted a set interim fuel safety criteria for sheath temperature, centreline melting, and maximum enthalpy deposition. In addition, the CNSC has established a regulatory process to address any adverse findings which may impact the LLOCA safety margins under the current LOE analysis framework.

This paper will provide an overview of the CAA for the LLOCA analysis and, in particular, a Canadian regulatory perspective on the process that should be followed when defining fuel safety criteria, including a discussion on the relationship the criteria have with the analysis methodology(ies) that licensees intend to use to demonstrate compliance. This will be followed by a detailed discussion on the current status of LLOCA safety margins, including the rationale behind the CNSC interim fuel safety criteria and the associated process to address adverse findings.