A Simplified Stress Intensity Factor Relation for Assessment of External Axial Flawed Safety-Related Nuclear Power Plant HDPE Piping

Abstract

PE piping has been used successfully for over 50 years in the natural gas distribution, water systems, petrochemical industry piping, and specialty applications for corrosive fluids. The application of PE pipes to safety-related service-water nuclear power plant (NPP) piping, demands superior material and fracture toughness performance at elevated stresses and thermal loads/gradients. The ASME Boiler and Pressure Vessel Code Case N-755 was developed to identify requirements that PE piping must meet and material qualification based on allowable stress levels in the presence of cracks/flaws and material resistance to crack growth. The PE and NPP industry intend to use PE pipes in safety-related, service-water applications and have conducted material evaluation tests that are similar in nature to those used earlier for qualifying PE materials for other applications listed above. Coupon and PENT tests that include controlled notches that simulate the stress-intensity factor (KI) of a typical axial flaw in PE pipe form the basis of current assessment procedures for PE material resistance to slow-crack growth (SCG). The applicability of these assessment methods, and experience from other applications, to specifying allowable flaw depths is of interest to the industry and is the subject of much discussion. In this study, a detailed investigation was conducted to assess KI for PE pipes with various outside diameters (OD) and a range of OD axial surface crack/flaw depths, different SCG test specimens with prescribed initial notch, notched-pipe tests (NPT) prescribed by international standards. A simplified equation to represent the SIF variation with crack/flaw depths for a large group of PE pipes (with same DR value) and same axial semi-elliptical surface flaws was developed.