By: Christopher Lohse
There have been several industry initiatives to support optimization of examination requirements for various items/components (both Class 1 and Class 2 components) in lieu of the requirements in the ASME Code, Section XI. The ultimate objective of these initiatives is to optimize the examination requirements (through examination frequency reduction, examination scope reduction, or both) while maintaining safe and reliable plant operation. There are various examples of examination optimization for both boiling water reactors (BWRs) and pressurized water reactors (PWRs). Each of these technical bases for examination optimization relies on a combination of items. The prior technical bases have relied on: (1) operating experience and prior examination results as well as (2) some form of deterministic and/or probabilistic fracture mechanics. For BWRs, the two main technical bases that are used are BWRVIP-05 and BWRVIP-108. These technical bases provide the justification for scope reduction for RPV circumferential welds, nozzle-to-shell welds, and nozzle inner radius sections. For PWRs, the main technical basis for RPV welds is WCAP-16168. These technical bases are for the RPV welds of BWRs and PWRs which represent just a small subset of the examinations required by the ASME Code, Section XI. Therefore, the industry is evaluating whether technical bases can be optimized for other components requiring examinations.