Nuclear equipment parts classification: a functional modeling approach

Nuclear power plants comprise a large number of components designated as safety-related. These safety-related components are subject to a special quality assurance program to assure their availability when needed, and as a result, substantiation of the product quality and suitability of these items incurs considerable costs to utilities. Furthermore, the number of vendors capable of or willing to meet nuclear-unique requirements has dwindled, forcing utilities to procure commercial grade items and dedicate them for safety-related use. Functional classification of parts, sometimes called part classification, seeks to identify the particular subset of parts associated with a safety-related component that is not relied upon for the parent component to accomplish its safety-related functions. This paper presents a functional modeling approach to part classification which is based on Goal Tree Success Tree and Master Logic Diagram (GTST-MLD). A case study for a pressure relief valve installed in an emergency diesel generator shows that the GTST-MLD provides a functional decomposition framework within which the various functions of the component can be modeled in a transparent, systematic and logical fashion in connection with a hierarchical topology of the structural elements.