Evaluation of severe accident mitigation actions through simulation

An abstract of the technical paper presented at:
PHWR Safety 2014 / CANSAS-2014
Ottawa, Ontario
June 23–26, 2014

Prepared by: Q. Lei, C. French, P. Devitt and A. Viktorov
Canadian Nuclear Safety Commission
Ottawa, Ontario, Canada

Abstract

Implementing countermeasures or actions to mitigate the consequences of a severe accident, should it occur in a nuclear power plant (NPP), is an essential part of severe accident management to reduce radiological risks to the public and environment. These mitigation actions, together with the technical basis (a set of severe accident management strategies and enabling instructions), are developed and documented in the station-specific severe accident management guidelines (SAMGs). Accelerating the implementation of the existing SAMG program for each Canadian NPP is one of the action items raised by the CNSC in light of the lessons learned from the Fukushima Daiichi nuclear accident.

This paper describes an analytical approach to evaluating the SAMG-specified actions, as one of the means to validate SAMG. Simulations are performed with and without crediting operators’ actions with an aim at enhancing our understanding of the SAMG-specified actions and providing necessary detailed information used to assess the feasibility and efficiency of those actions under severe accident conditions.

Specifically, the intent of this analytical approach is to:

• gain insight into SAMG-specified actions
• verify their merit and advantages
• ascertain their potential negative impacts
• characterize the environmental conditions of the reactor prior to, during, and following a mitigating action
• provide feedback to SAMG refinement if necessary, and
• document SAMG action simulations to facilitate selection of recommended mitigating actions during an accident by the technical support centre experts

The severe accident management strategies applicable for Canadian NPPs are implemented using a set of severe accident guidelines: SAG-1 inject into the heat transport system, SAG-2 control moderator conditions, SAG-3 control calandria vault conditions, SAG-4 reduce fission product releases, SAG-5 reduce containment hydrogen, SAG-6 control containment conditions, and SAG-7 inject into containment. Simulations of selected actions as specified in SAG-2 and SAG-3 using the severe accident computer code MAAP4-CANDU are documented elsewhere, e.g., Reference 1 for SAG-2. The focus of this paper is on the description of the general methodology used to simulate the progression of a severe accident while some selected mitigation actions as specified in SAG-4 and SAG-5 are evaluated.

Reference:

[1] Q. Lei, C. French, P. Devitt, and A. Viktorov, ’Understanding and Evaluation of Severe Accident Mitigation Actions‘, to be published in the 19^th Pacific Basin Nuclear Conference (PBNC 2014), Vancouver, Canada, August 24-28, 2014.