Structural Integrity of HDPE Piping and Joints in Nuclear Safety-Related Applications

Abstract

The ASME Boiler and Pressure Vessel Codes Committee (BPVC) has recently passed Revision 1 of Code Case N-755 that describes the requirements for the use of High-Density Polyethylene (HDPE) pipe for the construction of Section III, Division 1, Class 3 buried piping systems for service water applications in nuclear power plants. The Code Case N-755 was developed by Special Working Group – Polyethylene Pipe (SWG-PP) within Section III (Design) of the BPVC. The United States Nuclear Regulatory Commission (USNRC) has not as yet approved this Code Case N-755 for generic use in regulatory decisions. However, two Relief Requests for installation of polyethylene (PE) Piping in safety-related applications at US Nuclear Power Plants (NPP) have been approved by USNRC.Since 2007, confirmatory research has been conducted on the issues for the USNRC involving structural integrity and service life of both HDPE pipe and fusion joints in safety-related service water applications.As is well established, the limiting failure mode for the long term performance of HDPE pressure piping is slow crack growth (SCG), which is governed by the sustained stress levels (pressure), and increases exponentially with elevated temperature. Hence a comprehensive approach is being adopted to study SCG in both parent and butt-fusion HDPE materials involving a combination of coupon tests, full-scale stress rupture tests on razor-notched pipe specimens, advanced finite element analysis, and analytical approaches to service life prediction of HDPE piping.This paper outlines several findings from the confirmatory research, while setting the stage for a fracture mechanics based methodology to analyze stress rupture data from both notched pipe tests (NPT) and coupon tests including the standard Pennsylvania Notch Test (PENT) and other Single Edge Notched Tension (SENT) tests. In addition to conventional approaches such as Rate Process Method (RPM), both linear and nonlinear elastic fracture mechanics based parameters such as Stress Intensity Factor (KI), and J-integral are recommended to analyze the data and predict the critical allowable flaw size for given service loading conditions (pressure and temperature) and expected service life. The work focuses solely on PE 4710 bimodal HDPE resins with cell classification of 445574C per American Society for Testing and Materials (ASTM) D3350, which are the only materials allowable for nuclear safety-related applications.Copyright © 2012 by ASME