Polyethylene encapsulatin of nitrate salt wastes: Waste form stability, process scale-up, and economics

Abstract

A polyethylene encapsulation system for treatment of low-level radioactive, hazardous, and mixed wastes has been developed at Brookhaven National Laboratory. Polyethylene has several advantages compared with conventional solidification/stabilization materials such as hydraulic cements. Waste can be encapsulated with greater efficiency (i.e., more waste encapsulated per drum) and with better waste form performance than is possible with hydraulic cement. The properties of polyethylene relevant to its long-term durability in storage and disposal environments are reviewed. Response to specific potential failure mechanisms including biodegradation, radiation, chemical attack, flammability, environmental stress cracking, and photodegradation are examined. These data are supported by results from extensive waste form performance testing including compressive yield strength, water immersion, thermal cycling, teachability of radioactive and hazardous species, irradiation, biodegradation, and flammability. The bench-scale process has been successfully tested for application with a number of specific "problem" waste streams including nitrate salts, sludges, incinerator ash, reactor-generated evaporator concentrates (sodium sulfate and boric acid), and ion exchange resins. A production-scale technology demonstration of polyethylene encapsulation of nitrate salt waste, sponsored by the Department of Energy (DOE), Office of Technology Development (OTD), Division of Demonstration, Testing and Evaluation (DT&E) is planned in conjunction with Rocky Flats Plant. In support of the technology demonstration, a scale-up feasibility test was conducted by processing simulated nitrate salt waste with a production-scale extrusion system. Output rates in excess of 454 kg/hr (1000 lb/hr) were achieved, ana process data were in close agreement with bench-scale parameters. Quality assurance and performance testing of the resulting waste form confirmed scale-up feasibility. Use of this system at Rocky Flats Plant can result in over 70% fewer drums processed and shipped for disposal, compared with optimal cement formulations. Based on the current Rocky Flats production of about 1 x 10 6 kg (2.2 x 10 6 lbs) of nitrate salt per year, polyethylene encapsulation can yield an estimated annual savings between $1.5 million and $2.7 million, compared with conventional hydraulic cement systems.

Long-Term Durability of Polyethylene for Encapsulation of Low-Level Radioactive, Hazardous, and Mixed Wastes

“…It is insoluble in virtually ali organic solvents (at ambient temperature) and is resistant to many acids and alkaline solutions [13]. A summary of polyethylene suitability for use in specific chemical environments is given in Reference [2].…”

Review of encapsulation technologies

“…Therefore, this discussion is limited to polyethylene. "Polyethylene is an organic polymer material of crystalline-amorphous structure, formed through the polymerization of ethylene gas" (Kalb et aL, 1991b). By controlling the design of polyethylene's molecular structure, hundreds of compound variations with differing properties may be produced.…”

“…By controlling the design of polyethylene's molecular structure, hundreds of compound variations with differing properties may be produced. These variations are produced by manipulating process parameters and selecting particular additives, and are characterized by the degree of crystallinity (Kalb et aL, 1991b). The degree of crystallinity determines the density of a particular polyethylene, which affects a number of material properties, as listed in Table 2.…”

“…Medium-density polyethylenes (0.926 -0.940 g/cm3) can be produced either by adjusting process pressures or mixing LDPE and HDPE materials. Although Table 2 indicates that higher-density polyethylenes have greater mechanical strengths and are more resistant to harsh chemicals, LDPE is more favorable because its increased branching results in a greater resistance to biodegradation (Kalb et al, 1991b). BNL uses LDPE for its encapsulation process because it does not require high temperatures and pressures and it is therefore more cost effective than HDPE (Kalb et al, 1991b).…”

“…Typically, this means that the encapsulating material must be resistant to leaching and degradation by the contaminants which will be in contact with the encapsulant (40 CFR 268.45). The DOE laboratories that have been testing this technology have not only tested encapsulants for leachability and chemical degradation, they have also tested some encapsulants for: biodegradation; tolerance to irradiation; environmental stress cracking; photodegradation; flammability; compressive strength; and ability to tolerate thermal cycling (Kalb and Coiombo, 1984;Kalb et al, 1991b;Kalb etal., 1993). These tests have also shown that encapsulation is an appropriate treatment technology for mixed waste streams including: evaporator concentrate salts, sludges, incinerator ash, ion exchange resins, blowdown solutions, and molten salt oxidation residuals.…”