PD IEC/TR 61838:2009 - Page 2

Nuclear power plants. Instrumentation and control important to safety. Use of probabilistic safety assessment for the classification of functions. | Status: Confirmed, Current

1 Scope This Technical Report provides a survey of some of the methods by which probabilistic risk assessment results can be used to establish "risk-based" classification criteria, so as to allow FSEs to be placed within the four categories established within IEC 61226. The application of risk-based classification (categorisation) techniques, in conjunction with the role-based deterministic approach to classification given in IEC 61226 Edition 3, will continue to be decided by the utility and/or regulator within the National Regulatory frameworks. However, these approaches would be expected to take due account of internationally agreed approaches such as expressed in IAEA standards and guides. However, those are essentially high level and for instrumentation and control systems IAEA have left it to IEC TC45 SC 45A to determine the detailed approaches available and to express them in standards. There is an increasing level of consensus on the topic of classification; however there is some way to go yet. Edition 1 of this technical report published in 2001 assisted in the revision of IEC 61226 published in 2005. The scope of this revision to IEC 61838 is to stimulate debate on this subject and encourage the convergence of views so that further revision to IEC 61226 can be agreed to bring it into line with the latest IAEA guidance, i.e. to explicitly include consideration of aspects such as risk and time lines of response. The safety principles and the usefulness of a risk-based approach to classification are discussed and a description of four different approaches is presented. Two of these approaches are applied to a practical example and the results compared as a means to evaluate the robustness and generality of the risk-based approach. In other respects, references are given in this report to IEC and IAEA documents, which relate directly to the topic. This report also discusses the limitations associated with the use of either a risk-based approach or a role-based approach on its own, either of which would be inconsistent with the guidance soon to be released in IAEA NS-G-1.14.