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The Analytic Hierarchy Process in Global Assessments

Abstract of the technical paper/presentation presented at:
FISA 2019, 9th European Commission Conference on EURATOM Research and Training in Safety of Reactor Systems
Pitesti, Romania, June 4-7 2019

Prepared by:
Sanja Simic
Technical Specialist, Reactor Thermalhydraulics Division
Canadian Nuclear Safety Commission


In Canada, Class I Nuclear Facilities Regulations were amended in 2017 to require nuclear power plant (NPP) licensees to carry out a Periodic Safety Review (PSR) at an interval specified in their operating licence. Canadian nuclear power plant licensees conduct their PSRs in accordance with the CNSC’s regulatory document REGDOC-2.3.3, Periodic Safety Reviews, which is consistent with international guidance provided in the IAEA’s Safety Guide SSG-25.

A PSR is an established method for identifying safety improvements for aging NPPs, and has also been shown in Canada to be an effective means for determining scope of their refurbishments. A Global Assessment and an Integrated Implementation Plan (IIP) represent the results of a PSR, with the overall objective of presenting an integrated evaluation of an NPP’s overall safety against modern codes and standards. This is achieved by taking into account a balanced assessment of all findings identified in Safety Factor Reports (SFRs). Global Assessments identify the Global Improvement Opportunities (GIOs) and take into account all the strengths and gaps from SFRs and practicable corrective actions and/or safety improvements proposed in IIP to provide an overall assessment of safety and the acceptability of long-term operation. GIO ranking is an important step of a PSR, owing to its impact on the extent and timeliness of the benefits to be achieved by the resolution each opportunity for improvement identified.

Analytic Hierarchy Process (AHP) is one of the methodologies used to rank GIOs. The application of AHP begins with a problem being decomposed into a hierarchy of criteria (Value Trees) so as to be more easily analyzed and compared in an independent manner. In Global Assessments, a Value Tree begins with a cardinal objective such as "Enhanced confidence in the continued NPP safety and reliability of electricity production" and a set of fundamental objectives as main branches, such as "Enhanced confidence in the design basis" or "Enhanced confidence in the safety analysis". Each fundamental objective (Tier 1) is then expanded and supported with more specific objectives (Tier 2 and Tier 3). After Value Trees are constructed licensees make systematic pair-wise comparisons of objectives within the same Tier, in order to assign them varying weights depending on importance to the cardinal objective. Thereby, AHP transforms the pair-wise comparisons, which are most often empirical, into numerical values that are further processed and compared. This capability of converting empirical data into numerical values is the main distinctive contribution of the AHP technique when contrasted with other comparing techniques.

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