Presentation on theme: "Long-Term Performance & Life Expectancy of Engineered Barriers: Applied Research by CRESP’s Landfill Partnership Craig H. Benson, PhD, PE, DGE Wisconsin."— Presentation transcript:
Long-Term Performance & Life Expectancy of Engineered Barriers: Applied Research by CRESP’s Landfill Partnership Craig H. Benson, PhD, PE, DGE Wisconsin Distinguished Professor CRESP Landfill Partnership University of Wisconsin-Madison 28 November
On-Site Disposal Facility (OSDF): aka LLW or MW Landfill Final Cover System Liner System MONITORING WELLS Figure courtesy M. Othman, Geosyntec Consultants
Challenges – Predicting the Future Engineering Property Time (years) As-Built ACAP Exhumations ? ? ? Analogs
Landfill Partnership Priorities 1.Develop confidence in the long-term (1000 yr) performance of OSDF designs. 1.Develop an understanding of degradation mechanisms affecting containment systems in OSDFs. 1.Develop confidence in models used for PAs of OSDFs and characterize uncertainty in model predictions. 1.Create and evaluate monitoring strategies that build confidence in the performance of OSDFs. 5 Understanding engineering properties, as well as how and why properties change, is essential to create confidence in outcomes of PAs.
Current Research Initiatives 1.Long-term physical and chemical changes in bentonite barriers (e.g., geosynthetic clay liners). 1.Chemical properties of LLW leachates. 2.Life expectancy of geosynthetic materials used for barriers systems (e.g., geomembranes). 3.Sorptive and diffusive characteristics of barrier materials for transport analyses in PAs. 1.Coupling of erosion and hydrology in cover systems. 2.Mercury sequestration materials for MW disposal. 6
Degradation of Geomembranes
Geomembrane Degradation 8 Day-long discussion at Paducah in March ‘11. Literature suggests lifespan may be more than 1000 yr. No data for conditions in LLW or MW facilities. No credible scientific data for LLW or MW to draw inference, but methods from solid waste literature
Durability Tests in Synthetic LLW Leachate 9 Data from Oak Ridge, Fernald, Hanford, Idaho, & CNSC to define realistic OSDF LLW/MW leachates. Accelerated degradation tests using elevated temperature in immersion cells.. No credible scientific data for LLW or MW to draw inference, but well-defined methods in solid waste literature
Testing Program 10 2 mm HDPE smooth geomembrane from GSE 20, 50, 90 o C 3, 6, 9, 12, 18, 24, 30, 36 mos Leachates: - DI water - Synthetic LLW leachate w/o radionuclides - LLW leachate with radionuclides Stress crack resistance, OIT, MFI, mechanical properties
Leachate Analysis: Tc 3 of 4 sites are similar – one is much higher 3 of 4 are temporally invariant, one is diminishing.
Leachate Analysis: U 3 of 4 sites are similar – one is much higher Two temporally invariant, one increasing, one diminishing.
Leachate Analysis: Sr Strong temporal trends. Appear to level out at 3 yr both sites. Similar concentrations both sites.
Leachate Analysis: 3 H - Tritium 3 of 4 sites are similar – one is much higher MSW leachate has higher tritium than 3 of 4 OSDFs No distinct temporal trends.
Leachate Analysis – pH Neutral to slightly alkaline Cementitous grouts contribute to alkalinity Temporally invariant
Major Cations - ERDF & OSDF Leachates 16 Solid bars correspond to MSW leachate. Na and K less abundant in LLW/MW leachate compared to MSW leachate
Trace Elements 17
Erosion of Rip-Rap and Gravel Admixture Covers Rip-rap surface layer
Erosion of Rip-Rap and Gravel Admixture Covers Gravel Admixture Topsoil Surface Layer
Percolation Rate 20 Flow out of Tailings Flow into Tailings Gravel admixture and rip rap surface layers comparable in terms of erosion protection. Rip rap surface layer collects water. Gravel admixture releases water.
Transport Parameters: Diffusion & Sorption in Earthen Barriers 21 Synthesis of literature on radionuclide transport parameters for barrier materials. Wealth of information on bentonite and natural soils; non for more traditional clay barriers. None for geomembranes.
Transport Parameters: Diffusion and Sorption in Geomembranes 22 Diffusion and sorption tests for geomembranes. Evaluating new geomembrane with exceptionally low radon diffusion coefficient.
Bentonites - Geosynthetic Clay Liners For low hydraulic conductivity, Na bentonite granules must swell to from a gel (paste). Gel must be maintained to retain low hydraulic (~ m/s) conductivity. If granules do not swell and form gel, higher hydraulic conductivity (>10 -7 m/s). Typical Cross-Section LowerGeotextile UpperGeotextile GranularBentonite
Importance of Bound Cation Valence Na-Bentonite in DI Water (monovalent) – crystalline + osmotic hydration Na-Bentonite in Calcium (Ca 2+ ) Rich Water (divalent) – crystalline hydration only.
25 Understand conditions that promote rapid hydration. Characterize relative rates of hydration and cation exchange. Conditions needed for long service life: rapid hydration & protection from wet-dry cycling.
Summary 1.Research program focused on long service life of LLW disposal facilities – developing the science to provide confidence in PAs and facilities. 2.Current focus is on barrier systems – liners, covers, reactive layers. 3.Focusing on understanding mechanisms, engineering behavior and properties, and practical application. 4.Use experiment and theory (models) in tandem to couple observation with predictive capability. 5.Employ field data when possible to ground research in observed behavior. 26