(51e) High Level Waste Vitrification Progress at the Savannah River Site

The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies High Level Waste (HLW) for repository internment. DWPF has been processing radioactive waste since March 1996 and has chemically treated 2.7 million gallons of sludge to produce over 2,500 canisters (>10 million pounds) filled with HLW glass. Progress towards processing all of SRS's HLW sludge, consisting of four major steps: sludge washing, pretreatment, vitrification, and canister decontamination/sealing, and processing improvements will be discussed. Salt waste is pretreated in separate processes prior to being fed to DWPF.

Sludge waste consists of insoluble metal hydroxides (primarily iron, aluminum, calcium, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate). The particle size of the solids is typically 1-20 microns and the resulting slurry behaves as a Bingham Plastic. Sludge washing is performed to reduce soluble salts to ~1M (sodium basis) by allowing the solids to settle, decanting as much supernatant as possible, and then diluting the remaining supernate with water. This process is repeated until the targeted supernatant concentration is reached.

The pretreatment process in the Chemical Processing Cell (CPC) adds nitric and formic acids to the sludge to lower pH, destroy nitrite and carbonate, and reduce mercury and manganese in the Sludge Receipt and Adjustment Tank (SRAT). Acidification of the sludge also allows higher solids concentrations to be processed by lowering the yield stress of the sludge. Glass formers are added and the batch is concentrated to the final solids target (~45 wt% total solids) in the Slurry Mix Evaporator (SME). During these processes, hydrogen can be produced by noble metal catalyzed decomposition of excess formic acid. The ratio of nitric to formic acid is adjusted to target a glass pool with 20% of the iron having a valence of 2. Pretreated salt waste is sent to DWPF in dilute slurries and aqueous streams. The water added by these streams is boiled off during the SRAT cycle.

Vitrication of the waste is performed using a joule-heated, slurry-fed melter. The SME product slurry is pumped at ~ 0.6 gallons per minute into the melter, forming a cold cap on the molten glass surface. Drying and calcination of the slurry feed occurs in the cold cap. Thermal reduction of feed components (particularly manganese) can occur below the cold cap as the calcined feed converts to glass, releasing gas that can cause an insulating layer of foam between the cold cap and the melt pool. Controlling the reduction/oxidation potential of the feed so that at least 10% of the iron in the final glass is reduced to valence 2 has been shown to minimize foaming. The glass is poured into a stainless canister by pulling a vacuum on the canister. Glass properties are controlled by careful selection of the glass former to ensure acceptable glass product is produced.

The canister of glass is sealed with a temporary plug immediately after pouring. The canister is decontaminated by sandblasting the exterior with frit, then a permanent plug is welded in place to seal the canister. The canister is then placed into a temporary storage facility awaiting transfer to a permanent repository.