In United States HDPE pipes are increasingly used in safety-related components such as Essential Service Water (ESW) systems including buried sections at the nuclear power plants (NPPs). Because carbon steel piping in the ESW system corrodes with age carbon steel piping requires costly maintenance and has become a safety concern. To mitigate corrosion risks NPP operators have begun to replace carbon steel ESW piping with HDPE. However here are regulatory and safety concerns regarding the use of HDPE pipes in safety-related components. Even though there is a general belief that HDPE pipes have service lives of 50 years or more with minimal degradation and thus are safer compared to carbon steel pipes there is limited evidence supporting this assumed service lifetime and associated performance. The U.S. Nuclear Regulatory Commission (NRC) identified several issues related to the allowable service life conditions (i.e. temperature and stress) pipe fusion and inspection that need resolution before NRC allows general use of HDPE pipes by NPP operators. Therefore NRC has not approved the ASME Code that governs the design and installation of the HDPE pipes and is allowing HDPE usage by approving relief notices.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. Available 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 in the failure area and (ii) nonductile slit and pinhole failures. In the available test 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 materials undergo chemical degradation in the form of oxidative degradation due to the chemical environment in contact with the external and internal surfaces of the HDPE pipe. 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 (ClO2). The external environments are generally soil or air. 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 significantly 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 diminished mechanical properties of HDPE. When degradation of the inner surface material is severe enough the embrittled surface layer develops cracks which tend to propagate through the pipe wall driven by internal pressure. The principal objective of this work is to evaluate material properties of chemically-aged high density polyethylene (HDPE) piping used in nuclear power plants (NPPs). This paper will present data on HDPE degradation from various experimental studies and methodology to estimate the remaining service lifetime of the HDPE piping from the experimental data.