Approach to Conservative Deterministic Safety Analysis for Design Basis Accidents Based on Regulatory Requirements
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
E4-1 NPP Transients and Accidents Analysis

Noreddine Mesmous (CNSC)
Janusz Kowalski (CNSC)


Canadian reactor licensing practice has evolved along the Limit of Operating Envelope (LOE) approach, in which the plant operating and safety systems parameters are set at their limiting values. In this type of analysis, it is assumed that the conservatisms inherent in setting input plant parameters at their limits will cover the impact of uncertainties associated with the code models. Although LOE methodology provides some conservatism in the analysis results it is not certain if it off-sets the uncertainties associated with modeling parameters relevant to the analyzed event.

A new Conservative Analysis Method (CAM) has been developed that takes into account uncertainties in input modeling and plant parameters when performing conservative safety analysis for design basis accidents. TheCAMapproach is consistent with the Canadian Nuclear Safety Commission (CNSC) and the International Atomic Energy Agency (IAEA) regulatory requirements. This methodology requires knowledge of key input parameters and the important output parameter(s). Only parameters important for highly ranked phenomena are selected as key input parameters. The selection is performed using the PIRT/PKPIRT processes and confirmed by sensitivity analysis.  For each key input parameter, the analysis value for the modeling parameters, the analysis set points for the plant parameters and the actuation of the mitigating system(s) are defined and their impact on the safety analysis results assessed. 

An example is presented to show the application of this methodology. The example involves a comparison of the LOE, Best Estimate (BE) andCAMresults simulating a small break loss of coolant accident in a generic CANDU 6 station. The calculations are performed with the CATHENA computer code coupled with the LEPCON program.

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