FEATURE CASE STUDY:
Severe Accident Management Guidelines (SAMG)

International Safety Research, in association with RABA (Toronto) and COMEX (US) developed the proposed methodology for designing Severe Accident Management Guidelines (SAMG) for CANDU reactors (Canada Deuterium Uranium). SAMGs allow operators to deal with accidents that go beyond the design basis or the scope of the emergency operating procedures normally in use by nuclear power plants.

The Challenges

SAMGs previously developed focused on Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) technologies. Critical safety functions and accident dynamics are different and call for a different approach in the case of CANDUs. Furthermore, there has been no harmonization of Emergency Operating Procedures (EOP) in the various CANDU plans; some are event-based while others are symptom-based. There are few CANDU symptom-based procedures capable of dealing with significant core damage. Finally, there are significant differences between the design of single and multi-unit CANDU plans.

The Solution

Our team recommended the adoption of a Westinghouse Owners Group approach, modified for CANDU. The first recommended step was the creation of an EPRI-like Technical Basis Document, which defines the main strategies for dealing with severe accidents (CHLAs) and focuses on effects that could be expected if one or more of the CHLAs are implemented during an accident.

The proposed model for SAMGs would contain Severe Accident Guides (SAGs) and Severe Challenge Guides (SCGs), accessed through the use of a Diagnostic Flow Chart (DFC) when the critical safety parameter exceed a predetermined limit. The entry point into SAMGs would be the failure of the existing EOPs to gain control of accident progression and return the critical safety parameters to acceptable values. The proposed approach requires the creation of two Severe Accident Control Room Guidelines (SACRGs) for use prior and after, respectively, establishment of the TSC. The first of these can be developed from the existing symptom-based EOPs. Similarly, Severe Accident Exit Guidelines (SAEGs) must be developed.

The approach also critically examines the respective roles of the TSC and the Shift Supervisors in the application of SAMGs and proposes training requirements for all senior personnel involved in SAMGs.

Severe Accident Management Assessment for Gentilly-2

In response to regulatory requests, ISR conducted an analysis of the total loss of pump house for the Gentilly-2 nuclear power station and applied the basic SAMG methodology to recommend physical and procedural strategies and modifications to improve the ability of the plant to manage this event.