CNSC Review of Industry's Capability to Model a Multi-Unit Severe Accident

Abstract of the technical paper/presentation presented at:
7th International Workshop on CANDU Safety Association for Sustainability (CANSAS 2018)
October 15‒18, 2018

Prepared by:
Q. Lei and K. Leung
Canadian Nuclear Safety Commission

Abstract:

The two Fukushima action items (FAIs) pertinent to multi-unit severe accident modelling were generated

• FAI 3.2.1 ‒ evaluation of modelling of multi-unit plants in severe accidents
• FAI 3.2.2 ‒ plan for improved modelling of severe accidents in multi-unit plants

In response to these two FAIs above, the Canadian CANDU industry conducted an initial evaluation of the severe accident modelling capability for multi-unit stations. As a result, the industry proposed an improvement and confirmation method by running multiple instances of MAAP4-CANDU in parallel. This so-called “severe accident software simulator (SASS) solution” approach provided a practical means to simulate multi-unit events and benchmarking to verify the currently used containment scaling approach.

The objective of the CNSC staff review of the SASS project results is to confirm industry’s capability to model a multi-unit severe accident in a multi-unit shared containment station.

The regulatory review led to the conclusion that the industry has evaluated the capability of the MAAP4-CANDU computer code to simulate severe accidents in a multi-unit station. The SASS solution project successfully demonstrated that the industry has the capability to model multi-unit severe accidents by using the SASS approach, forcing function full-containment model, and the scaled-containment model. Each of the approaches has its own strengths and limitations, which will be explained in the presentation.

The regulatory review also revealed that the scaled-containment model does not appear to provide a realistic prediction of containment failure due to pressure spikes in containment such as those resulting from core collapses, shield tank overpressure failures, or steam explosions in a multi-unit severe accident. The presentation will provide a discussion on this aspect.

Keywords: severe accident management, SAM evaluation, SAMG, SAMG verification and validation, severe accident analysis, MAAP-CANDU simulation, multi-unit severe accident

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