Evaluating Properties of Chemically-Aged High Density Polyethylene Piping Material Used in Nuclear Power Plants

Abstract High Density Polyethylene (HDPE) pipes are increasingly used in safety-related components, such as Essential Service Water (ESW) systems in nuclear power plants (NPPs), including buried sections. However, there are safety concerns with HDPE pipes in safety-related components. Even though there is a general belief that HDPE pipes have service lives of 50 years or more, there is limited evidence supporting this assumed service lifetime and associated performance. The available methods for testing HDPE pipe failures and service lifetime have limitations, as they do not account for both chemical degradation and mechanical stresses. The testing methods are solely based on the mechanical strength of the HDPE materials. Further, the methods show two types of pipe failures: (i) ductile pipe rupture occurring with ballooning of the pipe specimen and yielding of the HDPE material and (ii) nonductile slit and pinhole failures. In the methods, the allowable service life (i.e., for 50 years or more) is dependent on the level of stress applied at the pipe wall. HDPE pipes could undergo chemical degradation in the form of oxidative degradation due to the chemical environment in contact with the external and internal surfaces. For the HDPE pipes in NPPs, the internal environment is service water, which contains oxygen and radical generating disinfectants, such as chlorine or chlorine dioxide (CIO2). The presence of oxidizing species in the service water leads to oxidative degradation. Oxidative-degradation resistance of HDPE is increased by adding antioxidants, such as stabilizers and carbon black; however, when these antioxidants deplete from HDPE, the dissolved oxygen and other chemical species degrade the polymer at the pipe inner surface. This degradation leads to reduced molecular weight and mechanical strength. When degradation is severe enough, the embrittled surface layer develops cracks, which tend to propagate through the pipe wall, driven by the internal pressure. Laboratory experiments were conducted to determine the depletion rate of antioxidants from HDPE blocks, and to determine the strength of the material as the antioxidants are depleted. The experiments were conducted using ClO2 solution as an oxidizer. A model was developed to predict the antioxidant level using the depletion rate computed from the experiments. The model was used to predict the time-dependent concentration of antioxidant in an HDPE component.