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The U.S. Department of Energy's (DOE's), Office of River Protection (ORP) located at Hanford Washington has established a contract (1) to design, construct, and commission a new Waste Treatment and Immobilization Plant (WTP) that will treat and immobilize the Hanford tank wastes for ultimate disposal. The WTP is comprised of four major elements, pretreatment, LAW immobilization, HLW immobilization, and balance of plant facilities. This paper describes the technologies selected for pretreatment of the LAW and HLW tank wastes, how these technologies were selected, and identifies the major technology testing activities being conducted to finalize the design of the WTP.
Chemical pretreatment of nuclear wastes refers to the sequence of separations processes used to partition such wastes into a small volume of high-level waste for deep geologic disposal and a larger volume of low-level waste for disposal in a near-surface facility. Pretreatment of nuclear wastes now stored at several U. S. Department of Energy sites ranges from simple solid-liquid separations to more complex chemical steps, such as dissolution of sludges and removal of selected radionuclides, e. g. , 90Sr, 99Tc, 137CS, and TRU (transuranium) elements. The driving force for development of chemical pretreatment processes for nuclear wastes is the economic advantage of waste minimization as reflected in lower costs for near-surface disposal compared to the high cost of disposing of wastes in a deep geologic repository. This latter theme is expertly and authoritatively discussed in the introductory paper by J. and L. Bell. Seven papers in this volume describe several separations processes developed or being developed to pretreat the large volume of nuclear wastes stored at the US DOE Hanford and Savannah River sites. These papers include descriptions of the type and amount of important nuclear wastes stored at the Hanford and Savannah River sites as well as presently envisioned strategies for their treatment and final disposal. A paper by Strachan et al. discusses chemical and radiolytic mechanisms for the formation and release of potentially explosive hydrogen gas in Tank 241-SY-101 at the Hanford site.
Radioactive wastes resulting from over 40 years of production of nuclear weapons in the U. S. are currently stored in 273 underground tanks at the U. S. Department of Energy Hanford site, Idaho National Engineering and Environmental Laboratory, Oak Ridge Reservation, and Savannah River site. Combined, tanks at these sjtes contain approximately 94,000,000 gallons of waste in a variety of forms including liquid, concrete-like salt cake, and various sludges. More than 730,000,000 curies of several radioactive isotopes are present in the underground tanks. Certainly, one of the greatest challenges facing the U. S. Department of Energy is how to characterize, retrieve, treat, and immobilize the great variety of tank wastes in a safe, timely, and cost-effective manner. For several years now, the U. S. Department of Energy has initiated and sponsored scientific and engineering studies, tests, and demonstrations to develop the myriad of technologies required to dispose of the radioactive tank wastes. In recent times, much of the Department of Energy R&D activities concerning tank wastes have been closely coordinated and organized through the Tanks Focus Area (IF A); responsibility for technical operations of the TF A has been assigned to the Pacific Northwest National Laboratory.
Between 1943 and 1986, 149 single-shell tanks (SSTs) and 28 double-shell tanks (DSTs) were built and used to store radioactive wastes generated during reprocessing of irradiated uranium metal fuel elements at the U.S. Department of Energy (DOE) Hanford Site in Southeastern Washington state. The 149 SSTs, located in 12 separate areas (tank farms) in the 200 East and 200 West areas, currently contain about 1.4 x 105 m3 of solid and liquid wastes. Wastes in the SSTs contain about 5.7 x 1018 Bq (170 MCi) of various radionuclides including 9°Sr, 99Tc, 137Cs, and transuranium (TRU) elements. The 28 DSTs also located in the 200 East and West areas contain about 9 x 104 m3 of liquid (mainly) and solid wastes; approximately 4 x 1018Bq (90 MCi) of radionuclides are stored in the DSTs. Important characteristics and features of the various types of SST and DST wastes are described in this paper. However, the principal focus of this paper is on the evolving strategy for final disposal of both the SST and DST wastes. Also provided is a chronology which lists key events and dates in the development of strategies for disposal of Hanford Site tank wastes. One of these strategies involves pretreatment of retrieved tank wastes to separate them into a small volume of high-level radioactive waste requiring, after vitrification, disposal in a deep geologic repository and a large volume of low-level radioactive waste which can be safely disposed of in near-surface facilities at the Hanford Site. The last section of this paper lists and describes some of the pretreatment procedures and processes being considered for removal of important radionuclides from retrieved tank wastes.
This report establishes the need and technical feasibility for using in-tank pretreatment processes for destruction of organic complexants and removal of 9°Sr, transuranic (TRU) elements, and 99Tc from double-shell tank (DST) liquid wastes. Neither 9°Sr nor 99{Tc} have to be removed from any DST solution to obtain vitrified product containing less than the Nuclear Regulatory Commission (NRC) criteria for Class C commercial low-level waste (LLW). To meet the NRC criterion for Class C LLW, TRU elements must be removed from liquid wastes in three (possibly five) DSTs. No 9°Sr will have to be removed from any solution for the total vitrified waste from both DSTs and single-shell tanks to meet a goal of
This report describes the bench-scale pretreatment processing of actual tank waste materials through the entire baseline WTP pretreatment flowsheet in an effort to demonstrate the efficacy of the defined leaching processes on actual Hanford tank waste sludge and the potential impacts on downstream pretreatment processing. The test material was a combination of reduction oxidation (REDOX) tank waste composited materials containing aluminum primarily in the form of boehmite and dissolved S saltcake containing Cr(III)-rich entrained solids. The pretreatment processing steps tested included caustic leaching for Al removal solids crossflow filtration through the cell unit filter (CUF) stepwise solids washing using decreasing concentrations of sodium hydroxide with filtration through the CUF oxidative leaching using sodium permanganate for removing Cr solids filtration with the CUF follow-on solids washing and filtration through the CUF ion exchange processing for Cs removal evaporation processing of waste stream recycle for volume reduction combination of the evaporated product with dissolved saltcake. The effectiveness of each process step was evaluated by following the mass balance of key components (such as Al, B, Cd, Cr, Pu, Ni, Mn, and Fe), demonstrating component (Al, Cr, Cs) removal, demonstrating filterability by evaluating filter flux rates under various processing conditions (transmembrane pressure, crossflow velocities, wt% undissolved solids, and PSD) and filter fouling, and identifying potential issues for WTP. The filterability was reported separately (Shimskey et al. 2008) and is not repeated herein.