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DRUM-SCALE TESTS OF IN-SITU THERMAL DESORPTION TREATMENT OF INEEL BURIED WASTES

The U.S. Department of Energy (DOE) is investigating in-situ thermal desorption (ISTD) as a treatment for TRUcontaminated buried waste at the Idaho National Environmental and Engineering Laboratory (INEEL) sub-surface disposal area (SDA). The SDA contains Rocky Flats Plant (RFP) wastes, including drums of potassium nitrate/sodium nitrate salts, drums of volatile organic chlorides in mineral oil adsorbed on calcium silicate-based adsorbents, and drums of combustible debris. ISTD treatment envisioned for these wastes will place relatively low power rod heaters, rated at about 300 watts per foot of rod length, in a grid pattern throughout the SDA treatment area. The rod heaters will be enclosed in a perforated six-inch diameter steel casing. Each of these heater assemblies will be sealed at the treatment area surface, so that a slight vacuum applied at the head of individual heater assemblies will draw desorbed or thermally decomposed waste constituents, and soil air and moisture, through the heater assembly into an offgas treatment system. Temperatures within the perforated casing are expected to be approximately 1100 o F (593 o C), with temperatures in the surrounding waste/soil ranging downward from 750 o F (399 o C). In practice, heat will be applied until all the surrounding waste/soil is 220 o F (104 o C) or greater, with temperatures at six inches from the heater assembly expected to reach approximately 842 o F (450 o C). This approach to SDA waste treatment is being tested at the drum scale at the MSE Technology Applications, Inc. (MSE-TA) test facility in Butte, Montana. The work is being conducted under the direction of Bechtel BWXT Idaho, L.L.C. (BBWI). The objective of these tests is to assess the effectiveness, at drum scale, of ISTD treatment of INEEL buried waste, particularly with regard to questions of waste reactivity, offgas composition, contaminant destruction and removal effectiveness (DRE), and actinide fixation. In calendar year (CY) 2003, MSE-TA completed one drum-scale shakedown test and three drum-scale waste treatment tests. The waste treatment tests used surrogates for organic sludge, nitrate salts, and a mixture or organic sludge and combustible debris. The organic-sludge surrogate contained a mixture of cutting oil, halogenated solvents, and inorganic adsorbents. The nitrate-salt surrogate consisted primarily of sodium and potassium nitrate. The combustible-debris surrogate was a mixture of cotton rags, paper, and plastic beads. A mixture of water and soil, formulated to simulate the heat of vaporization of the organic-sludge halogenated solvents, was used for the shakedown test. Heat-up rates for the organic, nitrate, and combustible debris surrogates were compared to the heatup rate for the non-combustible shakedown feed. Faster heating rates were noted for the organic sludge and combustible debris surrogates, and were assumed to be due to combustion of the surrogate waste material. Air flow and electrical power to the heater assembly were found to limit the rate of combustion. For the organic-sludge surrogate, the constituent halogenated solvents were not detectable in the offgas, as long as electrical power was applied to the heater assembly. When the heater electrical power was turned off, constituent halogenated solvents and products of incomplete combustion were found in the offgas, even though the core of the drum continued to self-heat from combustion. The organic-sludge/combustible-debris mixture self-heated at a greater rate than did the organic sludge alone. Data from the offgas analyses for this test were not available for inclusion in this paper. Heat up for the nitrate salt surrogate was slower than for the other tests, including the water/soil shakedown test. The nitrate-salt surrogate partially melted and foamed, nearly plugging the drum outlet line, after ~65 hours of heating.