Abstract
Pressurized water reactor (PWR) steam generator (SG) Class 1 and Class 2 components are required to be inspected every ten years by ASME Code, Section XI. A comprehensive survey of over forty years of examination results showed that no known degradation has occurred in these components. A comprehensive study involving stress analyses and probabilistic and deterministic fracture mechanics analyses indicated that the components are flaw tolerant and determined that the inspection of these components can be deferred beyond the current 10-year requirement without compromising plant safety.
A proposed Code Case has been introduced in ASME Code, Section XI that would allow the inspection of SG components to be deferred to 30 years from the date of the last inspection. Specifically, the Code Case addresses the following ASME Code, Section XI Examination Categories and Item Numbers:
• Class 1, Category B-B, pressure-retaining welds in vessels other than reactor vessels (Item Nos. B2.31, B2.32 and B2.40).
• Class 1, Category B-D, full penetration welded nozzles in vessels (Item No. B3.130).
• Class 2, Category C-A, pressure-retaining welds in pressure vessels (Item Nos. C1.10, C1.20 and C1.30).
• Class 2, Category C-B, pressure-retaining nozzle welds in pressure vessels (Item Nos. C2.21 and C2.22).
The technical bases for the proposed Code Case are provided in three EPRI reports (3002014590, 3002015906 and 3002023713). The first two reports were used by lead plants to request relief from the NRC for a 30-year deferral of the examination of the subject components. After many interactions with the NRC, relief was granted to the lead plants; however, the NRC also identified key parameters that should be considered in future requests. Lessons learned from interactions with the NRC were incorporated into the third report, which specifically addressed the 30-year deferral period with consideration of the key parameters identified by the NRC. This third report was only recently published and has not yet been used by a plant to seek relief for the 30-year deferral.
This paper provides the technical basis for the 30-year deferral of the examination of SG components in support of the proposed Code Case using the results of the evaluations in the three EPRI technical basis documents plus additional evaluations specifically addressing the 30-year deferral and the lessons learned through the Request for Alternative process by the lead plants.
