DAMAGE MECHANISM-SPECIFIC NDE METHODS SUPPORT RISK-INFORMED INSPECTIONS

摘要

Risk-informed inservice inspection (RISI) programs are currently being designed to effectively concentrate the limited and costly examination resources on systems and locations most relevant to plant safety and operation. Current RISI programs are concentrating on applying RISI principles to plant piping systems as plant owners will reap the greatest benefits from RISI on their piping systems, as compared to other components within the plant. The thought process used in the selection of nondestructive evaluation (NDE) methods and procedures in a RISI program is changing toward integrating NDE into an integrity management program, with a concentration on understanding failure mechanisms and their relationships to specific NDE methods. Identifying which damage mechanisms may be operative in specific locations and applying the appropriate NDE methods to detect the presence of these damage mechanisms is fundamental to application of an effective RISI plan. Considerable information is already available on inspection for damage mechanisms such as intergranular stress corrosion cracking (IGSCC), creep cracking, thermal and mechanical fatigue, and flow accelerated corrosion (or erosion-corrosion). Similar procedures are under development for other damage mechanisms that may occur individually or in combination. Guidance is provided on qualification and application of NDE procedures in a RISI framework to facilitate implementation by plant staff or contractors. With the RISI process, examiners focus on the specific examination location and, with the knowledge of which damage mechanism could (and is expected to) occur there, perform the appropriate NDE such that that damage mechanism may be detected and identified. Performing NDE techniques inappropriate for the anticipated damage mechanism does not add to the detection probability and is discouraged due to the unnecessary expense associated with the examination. As such, examiners should be made aware of the characteristics of each damage mechanism and its relationship with the NDE methods being implemented in order to perform an effective examination.