Antioxidant Depletion in HDPE Geomembranes Exposed to Low-Level Radioactive Waste Leachate

Abstract

Low-level radioactive waste (LLW) and mixed waste (MW) disposal facilities in the United States are required to have a service life in excess of 1000 yr. Understanding the rate of degradation of high-density polyethylene (HDPE) geomembranes used in LLW and MW facilities is necessary to assess their service life. HDPE geomembranes (GM) undergo degradation in three stages: antioxidant depletion (Stage I), induction time to the onset of polymer degradation (Stage II), and polymer property degradation (Stage III). This paper focuses on antioxidant depletion in LLW leachate. HDPE geomembrane coupons (2-mm thick) were immersed in radioactive synthetic leachate (RSL) with chemistry representative of leachate in LLW disposal facilities operated by the U.S. Department of Energy's environmental restoration programs. Depletion was assessed at four temperatures (25, 50, 70, and 90 °C). To identify the effect of radionuclides on antioxidant depletion, comparative tests were conducted with nonradioactive synthetic leachate (NSL) having the same chemistry as the RSL leachate, except radionuclides were excluded. Control tests were conducted with deionized water. Specimens were removed periodically and tested to determine their mechanical and chemical properties. Antioxidant depletion was measured by both standard and high-pressure oxidative induction time (OIT) tests. The antioxidant depletion rate increased with exposure temperature and was higher for GMs immersed in RSL. Arrhenius modeling was used to extrapolate the laboratory data at elevated temperatures to a typical LLW liner at 15°C. The predicted time to antioxidant depletion for a GM in a composite liner is 624 yr.