Insights from Pilot Applications of Risk Informed Methodology to Inservice Inspection of Nuclear Power Plant Piping
F. Ammirato, EPRI NDE Centre, Charlotte, USA
ABSTRACT
Nuclear power plants world-wide rely on inservice inspection programs for integrity management. In the United States and many other countries, Section XI of the ASME Code is used to define the locations to be inspected, the frequency of inspection, and inspection methods. Field experience accumulated over nearly 3 decades has confirmed that the location selection rules of Section XI do not effectively identify locations most susceptible to degradation or those systems most important to nuclear safety. Thus, an improved approach is warranted that will focus inspection resources on those systems and locations most important to integrity management. Risk informed methods have been developed and applied to inservice inspection that bring significant improvements in inspection effectiveness. Plant owners and regulators have recognised the value of risk informed approaches to develop inspection strategies as an alternative to current prescriptive rule-based approaches. Risk informed methods have been applied to define an effective inspection strategy for several US nuclear plants in a pilot study aimed at demonstrating the method, identifying the risk reduction associated with inspection strategies, and obtaining actual results needed for regulatory review and acceptance. This development effort and the pilot studies have resulted in development and demonstration of useful models for evaluating the risk reduction that can be achieved through application of alternative inspection strategies. The inspection effectiveness defined as the effectiveness of selecting the most risk significant location and applying appropriate inspection methods, is a key element of the model. Actual field experience is used in the model to estimate pipe leak and rupture frequencies for the various damage mechanisms. The method focuses on damage mechanisms that may be operative in piping systems and provides criteria for determining where these mechanisms may be active. Recent results have confirmed that risk informed methods can be applied to piping system inspection. Benefits realised form this approach includes improved safety, reduced cost, and reduced radiation exposure. The method also meets regulatory initiatives for implementation of risk informed and performance based approaches. This paper will review EPRI's risk informed inspection methodology and present results from the pilot studies. In particular, the changes in overall risk between the new method and Section XI inspection programs will be discussed, including the role of inspection effectiveness. The issue of repeatedly inspecting the same locations versus random inspection in subsequent inspections will also be addressed in terms of risk reduction benefits of each approach. These results provide important insights for designing effective inspection techniques and qualification programs for ISI systems.
Publication Source: First International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components , 20 - 22 October 1998, Amsterdam, Netherlands. Held by the Joint Research Centre of the European Commission. Publisher:Woodhead Publishing Limited