CNSC review of industry's capability to model a multi-unit severe accident

The two Fukushima Action Items (FAIs) pertinent to multi-unit severe accident modeling were generated: FAI 3.2.1 Evaluation of modeling of multi-unit plants in severe accident and FAI 3.2.2 Plan for improved modeling of severe accidents in multi-unit plants In response to these two FAIs above, the Canadian CANDU industry had conducted an initial evaluation of the severe accident modeling capability for multi-unit stations. As a result, the industry had proposed an improvement and confirmation method, namely, 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 a 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 conclude that the industry has evaluated the capability of the MAAP4-CANDU computer code to simulate severe accidents in a multi-unit station. The Severe Accident Software Simulator (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 realistic prediction of containment failure due to pressure spikes in containment such as those resulting from core collapses, shield tank overpressure failures, steam explosions, etc. in a multi-unit severe accident. The presentation will give a discussion on this aspect. (author)