The Hanford Tanks: Environmental Impacts and Policy Choices (1996)

The following is a brief description and analysis of the alternatives based on information from the DEIS. It was prepared and is included here as a convenience to the reader of this report.

This alternative calls for management of the tank farms consistent with current waste management programs, with monitoring for only 100 years to provide a consistent basis for assessing health and environmental impacts. Spare double-shell tank space is to be maintained in the event of a tank leak. The alternative does not comply with either federal or state requirements for storing hazardous or high-level waste, now or in the future. Discussion of it in the DEIS is treated as a pro forma requirement.

The tank farms would be managed consistent with current waste management programs, with monitoring for only 100 years to provide a consistent basis for assessing health and environmental impacts. The double-shell tanks would be replaced with 26 new ones as needed, presumably at the end of the existing tank design life of approximately 50 years. Approximately 1 percent of the waste would be left in the abandoned double-shell tanks, around which a permanent marker would be emplaced. This alternative does not comply with either federal or state requirements for storing hazardous or high-level waste, now or in the future. After 100 years, the tank wastes would be in the same unacceptable long-term disposal condition they are in today.

All pumpable liquids would be evaporated from double-shell tanks, with the concentrate being returned to the tanks. The condensates would be routed to the Hanford Site 200 Area Effluent Treatment Facility, where all effluents would meet discharge limits. The single-shell and double-shell tanks would be filled with gravel and covered by caps (Hanford Barriers) over each tank farm, marked with surface and subsurface markers. Institutional control of the buried tanks would terminate after 100 years. Each tank would need to be characterized to determine that the concentrated residues are safe, i.e., not explosive or otherwise likely to present a hazard at some future time. The alternative, representing a low-cost approach to managing the tank waste, does not comply with either federal or state requirements for storing hazardous or high-level wastes, now or in the future.

All pumpable liquids from double-shell tanks would be evaporated, and the concentrates would be returned to the tanks. The condensate would be routed to the Hanford Site 200 Area Effluent Treatment Facility, where effluents would be treated to meet currently applicable discharge limits. The tank dome space would be filled with sand, and the waste plus the sand would be melted by joule heating with graphite electrodes to 1,450 to 1,600°C to produce a vitreous mass (each in situ vitrification melter would require approximately 160 MW). Thermally unstable solids in the waste (nitrates/nitrates, organic compounds, ferrocyanides, etc.) would decompose during the melting, producing off-gases, and volatile materials would vaporize. Four in situ vitrification units would be on site, with at least two operating at all times. A thermal oxidizer would be provided to complete organic compound destruction. A tank farm confinement facility would be constructed over an entire tank farm for containment during treatment operations. Movable-wall buffer areas would provide a safe operating area. Shielding for personnel would be provided. An off-gas system consisting of water

scrubbing, high-efficiency particulate air (HEPA) filtration, charcoal bed sorption, and electrostatic precipitation is to be provided.

Caps (Hanford Barriers) would be constructed over the tank farms. The variability of waste composition among tank farms (and among tanks) dictates the need for specific knowledge of tank waste composition and of the safety implications of that information. Fluxing additives may be required to achieve proper melting. Inspection and sampling of the final waste form may be difficult. If treatment of hazardous wastes can be demonstrated to be adequate, the vitrified wastes might meet RCRA land disposal requirements. However, the near-surface disposal would not meet DOE Order 5820.2A, requiring disposal of readily retrievable high-level waste in a geologic repository.

The ex situ/in situ alternative is intended to bound the impacts from a combination of a wide range of alternatives, including treatment of some tanks by the in situ and capping alternative and some by the ex situ alternatives. It presents a concept of recognizing that different tanks should be treated differently, depending upon their specific attributes. In the version evaluated in the DEIS, approximately one-half of the tanks would be treated ex situ, based on an evaluation of treatment alternative on a tank-by-tank basis. The retrieved wastes would be treated according to the ex situ intermediate treatment alternative, while the tanks with wastes not retrieved would be treated according to the in situ fill and cap alternative. Selection of tanks for treatment would require extensive characterization. The in situ tanks would not meet RCRA land disposal requirements for hazardous waste or the DOE policy of readily retrievable high-level waste in a geologic repository.

This alternative is similar to the ex situ/in situ combination alternative, differing in that it allows for a judicious selection of the tanks by focusing treatment on the biggest contributors to long-term risk (⁹⁹Tc, ¹⁴C, ¹²⁹I, and ²³⁸U), while limiting the volume of waste to be processed.

Approximately 23 tanks would be processed instead of 70 tanks, as in the ex situ/in situ combination alternative. Two treatment facilities would be constructed for ex situ treatment; one would be for combined separation and treatment of low-activity waste, and the other would be a high-level waste treatment facility. Selection of tanks to treat would depend on results of future characterization of the tanks. Waste contained in the tanks left for in situ treatment would follow the in situ fill and cap alternative. The benefit foreseen for this alternative is that up to 85 percent of the greatest contributors to long-term risk would be disposed of ex situ, while only approximately 26 percent of the waste would need to be retrieved and treated. The implementation aspects are the same as those for the phased implementation alternative and the in situ fill and cap alternative. This approach deals with many of the limitations of availability of funding and, at the same time, addresses the real risks associated with the tanks. (However, it may be found that many of the tanks that must be emptied to get 85 percent of the long-term risk isotopes out would be among the most difficult to empty.)

All wastes under this alternative would be handled as high-level waste, and there would be no separations. Otherwise, this alternative is similar to the ex situ intermediate separations alternative. Wastes would either be vitrified or calcined. The primary matrix of calcination would be sodium carbonate, resulting in a finely divided powder that must be compacted to produce dense pellets or briquets. Off-gas treatment would be the same as for any vitrification alternative. This alternative produces a large volume of high-level waste and meets all applicable regulations for disposal of radioactive, hazardous, and mixed wastes, assuming they are all contained in the final waste form. However, the final waste forms may not meet geologic disposal waste acceptance criteria.

As much of the waste as practicable would be removed from the tanks and separated into high-level and low-activity (low-level) waste

fractions. Slurry pumping would be used to extract wastes from the double-shell tanks. Hydraulic sluicing plus hydraulic arm retrieval would be used to remove the single-shell tank wastes, crush chunks as necessary, and transfer slurries to interim storage in double-shell tanks or directly to a pretreatment facility. Sludge washing, enhanced sludge washing, solid/liquid separation, and ion exchange would be used to produce high-level and low-level waste streams. Solutions of salts from washing would be sent to ion exchange to remove cesium and then to a low-activity waste vitrification facility for concentration, mixing with glass formers, and vitrification. Additional liquid processing may be necessary to remove certain radionuclides (e.g., technetium and strontium) as well as organic compounds from the low-activity waste to meet on-site disposal requirements. The sludge remaining after washing, along with the separated cesium, would be sent to the high-level waste vitrifier, where it would be mixed with glass formers and vitrified. The vitrified off-gas systems would consist of water scrubbing, HEPA filtration, cupric oxide (CuO) bed sorption for oxides of sulfur (SOx), and catalytic reduction of oxides of nitrogen (NOx). The vitrified low-activity waste in the form of cullets would be mixed with a matrix material and put into large containers for near-surface, retrievable disposal on the Hanford Site. A cap (Hanford Barrier) would be put over the low-activity waste, and markers would be installed; controls would be terminated after 100 years. Vitrified high-level waste would be put in temporary storage in an aboveground interim facility on the Hanford Site. The low-activity waste form requirements have not been defined, and selection of vitrifiers has not been made. In addition, many mechanical features of this alternative have not been demonstrated.

This alternative is similar to the ex situ intermediate separations alternative, but with additional, extensive separations to remove components of the high-level sludge from recovered tank wastes. The goals are to minimize the number of high-level waste canisters and produce low-activity waste that meets USNRC low-level waste Class A standards or as low as reasonably achievable (ALARA), whichever is lower. Processing operations to separate elements such as uranium, plutonium, neptunium,

thorium, americium, lanthanide elements, cesium, strontium, and technetium would be used. The separations include sludge washing, caustic and acid leaching, solvent extraction, and ion exchange. Destruction of organic compounds and ferrocyanides would be carried out by wet air oxidation by holding the liquid at 325°C and 2,000 psi in the presence of oxygen for 1 hour. Plutonium/uranium extraction (PUREX) followed by transuranic extraction (TRUEX) would be used to remove residual americium, trivalent lanthanides, and bismuth. Bismuth would be stripped with sodium and ethylene diamine tetracetic acid (EDTA), and the lanthanides and americium with dilute nitric acid. Americium would then be separated from the lanthanide elements by cation exchange. Raffinate from this step would be processed by displacement ion exchange. The TRUEX raffinate containing cesium, strontium, and technetium would be processed by crown ether extraction to remove strontium. The cesium in the raffinate would be isolated by adsorption on an ammonium phosphomolybdate column and dissolved in caustic. Final concentration of cesium would be by ion exchange on a resorcinol-formaldehyde column. The subsequently eluted cesium would be sent to high-level waste processing and treatment. Any cesium, or plutonium carried over into the cesium stream, would be sorbed on silicotitanate. Technetium in the raffinate from the crown ether extraction would go to a strong base ion exchanger for removal as the pertechnetate ion. Technetium, strontium, plutonium, and cesium would all go to the high-level waste. Bulk chemicals such as water, nitric acid and sodium hydroxide would be recovered and recycled. Excess caustic would go with the low-activity waste. Chromium would be processed in a step to reduce it to trivalent chromium, which precipitates as the hydroxide and is removed by centrifugation, and sent to a separate waste processing step as a mixed waste. Concentrated sodium nitrate and aluminum nitrate solutions may be purified by crystallization. Subsequent operations would parallel those for the ex situ intermediate alternative operations. Processing equipment would be decontaminated for on-site disposal in a low-activity waste burial ground. Processing facilities are decontaminated and entombed in place.

The phased implementation alternative assumes tank waste remediation in two steps, or phases. The first phase (Phase 1 in the DEIS) entails operation for up to 10 years (1997 to 2007) of two low-level waste separation and vitrification facilities, one of which would also include high-level waste vitrification. Approximately 20 million gallons (76 million liters) of tank waste would be processed. Wastes would be stored pending availability of both an on-site storage facility and a geologic repository. The second phase (Phase 2 in the DEIS) upgrades the facilities in Phase 1 and uses them for another 10 years. In addition, a full-scale low-level waste separation and immobilization facility and a high-level vitrification facility would be built. All wastes (99 percent) would be removed from both single-shell and double-shell tanks. Sludge washing, caustic leaching, ion exchange, and other separations “as required” would be used to separate the tank wastes into high-level and low-activity wastes. High-level waste would go to a geologic repository; low-activity waste would go to near-surface storage on site.