Evaluating Properties of Chemically-Aged High-Density Polyethylene Piping Material Used in Nuclear Power Plants, 20–R8432
Inclusive Dates: 12/13/13 – Current
Background — The principal objective of this project is to evaluate the material properties of chemically aged high density of polyethylene (HDPE) piping used in nuclear power plants (NPPs). In nuclear power plants located in the United States, HDPE pipes are increasingly being used in safety-related components, such as essential service water (ESW) systems including buried and above-ground sections. 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, nuclear power plant operators have begun to replace carbon steel ESW piping with HDPE. There are regulatory and safety concerns regarding the use of HDPE pipes in safety-related components. Even though there is a generalized 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. Therefore, this project work includes evaluating properties of the material as it ages and estimating its remaining service life.
The available methods for testing HDPE pipe failures and service lifetime have limitations, as they do not account for both chemical and mechanical degradation. The available testing methods are solely based on the mechanical strength of HDPE materials. These 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 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, 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 the oxidative degradation. The oxidative-degradation resistance of HDPE is increased by adding antioxidants, such as stabilizers and carbon black. When these antioxidants are significantly depleted 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 strength of HDPE. When degradation of the inner surface material is severe enough, an embrittled surface layer develops cracks, which tend to propagate through the pipe wall, driven by internal pressure.
Approach — The overall technical approach consists of the following four elements: mechanical testing, chemical treatment under oxidative conditions, developing a model for estimating antioxidant concentration and depletion with time, and designing a method for in-situ measurement of the antioxidant level and extent of oxidative degradation in the in-service HDPE pipes. In the overall approach, mechanical testing is used to correlate the antioxidant level and oxidative degradation to mechanical properties. The mechanical testing data will be used as a master curve for predicting the service lifetime of HDPE pipes. For the third objective, which involves predicting the evolution of the antioxidant concentration in the HDPE pipes, a model is being developed based on diffusion theory. The sample data will be used to estimate model parameters. It is important to develop a model because antioxidant concentration in HDPE pipe will vary in the radial pipe direction, and this variation is difficult to measure. Finally, a tool to measure the antioxidant concentration as a function of depth from the external surface of the pipe will be developed. The measured antioxidant level will help determine the antioxidant concentration profile in the pipe and overall antioxidant level to quantify the extent of oxidative degradation.
Accomplishments — The project accomplishments include insight gained on the mechanical and chemical properties of the HDPE piping materials as it ages in oxidizing solution. Two types of samples were prepared for the aging tests. The first type consisted of "dog bones" of a HDPE (TUB121) prepared by injection molding and aged at 40 °C in either a baseline solution (pH = 2) or a solution containing an oxidant, chlorine dioxide (ClO2) (average concentration of 82 ppm for duration of exposure experiment) at a pH of 2. The second type consisted of four-inch thick blocks of HDPE exposed at 40 °C from one side with a solution 90 to 140 ppm of ClO2 at pH of 2. For the HDPE blocks, samples were later machined into dog bones or discs over a series of depths from the surface so as to ascertain a correlation between the depth and material properties. After exposure tests, samples were analyzed by tensile, oxidative induction time (OIT), Fourier Transform Infrared spectroscopy, creep, and dynamic mechanical analysis for molecular weight.
Figure 1 compares the tensile results and OIT data for the first type of samples (dog bones made of TUB121). It is found that mechanical and chemical properties of the exposed samples decreased rapidly with exposure to the ClO2 solution. For example, the ultimate tensile strength decreases by 50 percent after two weeks of exposure to ClO2 solution. This correlated with the loss of antioxidant in the HDPE, as exhibited by the dramatic reduction of the oxidative induction time after a week of exposure. These data indicate that both mechanical and chemical properties of HDPE significantly change for the samples after exposure to the ClO2 solution.
Initial analysis of the second type samples (four-inch thick blocks) also shows a rapid decrease in OIT values; however, as shown in Figure 2, these phenomena are limited to the exposed surface. As seen in Figure 2, the OIT values for samples prepared from the bulk of the HDPE remained more or less unaffected by the exposure. This surface limiting aspect is also noticed in the mechanical properties. For example, the ultimate tensile strength of the samples directly from the exposed surface shows significant reduction in mechanical properties compared to the bulk and unexposed material. However, the ultimate tensile strength of the samples at a small distance from the exposed surface is the same as the bulk and unexposed material, and remains unchanged.
The experimental data suggest that mechanical and chemical properties of the HDPE piping material are correlated. The data also suggest that the degradation in the HDPE material properties due to exposure is expected to be limited to certain depth from the exposed surface. Additional work is underway to further verify the later observation.