Evaluating Properties of Chemically-Aged High-Density Polyethylene Piping Material Used in Nuclear Power Plants, 20-R8432
Inclusive Dates: 12/13/13 – 08/12/15
Background — The principal objective of this project is to evaluate material properties of chemically aged high-density polyethylene (HDPE) piping used in nuclear power plants (NPPs). In the United States NPPs, HDPE pipes are increasingly being used in safety-related components, such as essential service water (ESW) systems, including buried and aboveground sections. Because carbon steel piping in the ESW system corrodes with age, it 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. There are concerns, however, 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 was aimed at 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, because 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: ductile pipe rupture occurring with ballooning of the pipe specimen and yielding of the HDPE material in the failure area, and 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 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. 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 consisted of four elements: (i) mechanical testing, (ii) chemical treatment under oxidative conditions, and (iii) developing a model for estimating antioxidant concentration and depletion rate with time. In the overall approach, the mechanical testing was used to correlate the antioxidant level and oxidative degradation to mechanical properties. The mechanical testing data was used as a master curve for predicting the service lifetime of HDPE pipes. In Element iii, involving predicting the evolution of the antioxidant concentration in the HDPE pipes, a model was developed based on diffusion theory. The sample data from Element ii was used to estimate model parameters. It was important to develop a model because antioxidant concentration in a field HDPE pipe varies with the radial thickness due to gradual depletion of the antioxidant. Further, in the laboratory scale experiments with limited time exposure, this variation is difficult to replicate due to very low depletion rates of the antioxidant. The model was used as an extrapolation tool to estimate long-term depletion of antioxidant for a given thickness of the pipe. The model results were used to estimate service lifetime of the HDPE with a given wall thickness.
Accomplishments — The project accomplishments included characterizing the mechanical and chemical properties of the HDPE piping materials as they age in oxidizing solution. Two types of samples were prepared for the aging tests. The first 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, ClO2 (average concentration of 82 ppm for duration of exposure experiment) at a pH of 2. The second type consisted of 4-in. 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 to develop 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 samples (dog bones made of TUB121). It was 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 after exposure to the ClO2 solution.
Analysis of the second type samples (4-in thick blocks) also shows a rapid decrease in OIT values. As shown in Figure 2, for the relatively short exposure times used in the tests, these phenomena are limited to a short distance from 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 the mechanical properties compared to the bulk and unexposed material. However, 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 over the experimental exposure times.
The OIT values in Figure 2 were used to estimate the antioxidant concentration with depth in the 4-in block sample. The concentration values with time were used to estimate the depletion rate of antioxidant from the HDPE matrix. This was input in the diffusion-based model, and antioxidant levels were predicted in a 4-in thick wall HDPE pipe. The model computed values of the normalized antioxidant levels are presented in Figure 3.
The following conclusions are drawn from this work:
- Oxidative induction time is a good indicator of loss of antioxidants from HDPE. It was further observed that the strength of the material decreases by 50 percent or more when the antioxidants are depleted from the bulk of HDPE. Considering this, loss of antioxidants at 50 percent with respect to initial concentration of HDPE is a reasonable reference or threshold to estimate the service life of HDPE components in NPPs.
- When the material is exposed to an oxidizing environment, the loss of antioxidants from HDPE is controlled by the diffusion of the antioxidants from the material. The diffusion of the antioxidants through the bulk material is a slow process, as observed in the block experiments. Therefore, long-term exposure experiments are needed to accurately determine the depletion rate of the antioxidants from the material.
- There are no readily available methods to measure in-situ antioxidant levels of an in‒service HDPE component, such as pipes at nuclear power plants. The most feasible method is to use coupons to measure the depletion rate of the antioxidants from HDPE.
- Depletion rate data from coupons or laboratory experiments can be used to obtain key model parameters. These parameters can be input in the diffusion-based dynamic mass‒balance model to extrapolate the cumulative levels of the antioxidants in the in‒service HDPE component. A threshold value of 50 percent can be used to estimate the service life of HDPE components.