portion from the high-level portion of the remaining sludge were to be undertaken at a later date as more advanced separations technology became available. The objective of this phased approach was to start disposing of tank waste as soon as possible while minimizing the volume of HLW to be vitrified and sent to a geologic repository.

The advanced separations technologies were to be developed in the laboratory and subsequently tested on a pilot scale in B Plant (one of the three original chemical separations facilities in the Hanford Site 200 Areas, built in 1944). Upon successful demonstration on a pilot-scale basis, full-scale production capability would be installed in the B Plant canyon for the processing of the remaining double-shell tank waste.

Design of the Hanford low-level grout facility and the Hanford Waste Vitrification Plant was initiated in the late 1980s. Construction of the grout facility was completed in 1987 and 500,000 gallons (2,000 m³) of low-level supernatant were processed into grout in 1988.

In 1986, DOE agreed that all mixed waste (i.e., those wastes containing both radioactive and hazardous chemicals) at its sites were subject to the provisions of the Resource Conservation and Recovery Act of 1976 (RCRA). In 1987, officials of Region 10 of the U.S. Environmental Protection Agency delegated regulatory authority for all mixed waste at the Hanford Site to the Washington State Department of Ecology. The Hanford tank wastes are deemed to contain hazardous substances under RCRA provisions and, as a consequence of the new regulations, the tank wastes also become subject to State of Washington regulation.

In February 1988 negotiations were initiated among the DOE, the U.S. Environmental Protection Agency, and the Washington State Department of Ecology on a compliance agreement for cleanup of the Hanford Site. In May 1989, the three parties signed the Hanford Federal Facility Agreement and Consent Order (also called the Tri-Party Agreement, or TPA) (Washington State Department of Ecology, U.S. Environmental Protection Agency, and U.S. Department of Energy, 1996 - current amendment) which established milestones enforceable by law for specific cleanup actions. The strategy for cleanup of tank waste incorporated in the TPA was that implemented for the 1988 Record of Decision (U.S. Department of Energy, 1988a).

Several safety issues concerning day-to-day operations were identified in early 1990. These safety issues were deemed to pose unacceptable risks for continued operations without corrective actions. Originally, 54 tanks (48 single-shell tanks and 6 double-shell tanks) were identified as having potential for release of highly radioactive material in the event of an uncontrolled temperature or pressure excursion. These 54 tanks were designated as Watch List Tanks in response to Public Law 101-510, Section 3137, ''Safety Measures for Waste Tanks at Hanford Nuclear Reservation," of the National Defense Authorization Act for Fiscal 1991. Sixteen tanks have subsequently been removed from the Watch List.

The immediate concern for resolution of the safety issues resulted in major changes in programmatic needs that preempted the previously established disposal program strategy promulgated by the 1988 Record of Decision and the 1989 TPA. The redirection of technical and financial resources to resolve or mitigate the safety issues and the imposition of operating restrictions caused slippage in the schedules of activities supporting the disposal program outlined in the TPA. Additionally, tank safety issue resolution was expected to require double-shell tank space that was dedicated to supporting the disposal program. In some cases, the Hanford baseline schedules could not be recovered, and, consequently, the 1989 TPA provisions regarding tank waste could not be met.

Following the issuance of the 1988 Record of Decision and the 1989 TPA, knowledge of conditions of the single-shell tank waste evolved considerably, resulting in some changes in thinking that previously had favored in situ disposal. Also, RCRA and the Nuclear Waste Policy Act of 1982, as amended, requirements ran counter to the in situ disposal approach. As a result,