INTRODUCTION The mission of the River Protection Project (RPP) is to store, retrieve, treat, and dispose of the highly radioactive waste in Hanford Site tanks in an environmentally sound, safe, and costeffective manner. The waste is stored in 28 active double-shell tanks and 149 single shell tanks. Although new waste additions stopped in 1980, the single-shell tanks (SSTs) continue to store over 30 million gallons of radioactive waste left over from decades of plutonium production for defense purposes. In 2004, the last pumpable liquid was removed from the SSTs except for those tanks being retrieved.
BACKGROUND Delays in the construction and completion of the Hanford Waste Treatment and Immobilization Plant have resulted in the realization that waste will continued to be stored in these tanks for several more decades, resulting in a service lifetime of nearly 100 years. As result of these delays and to improve the understanding of SSTs integrity, the Department of Energy and Washington River Protection Solutions, LLC (WRPS), the USDOE Hanford Site tank contractor, developed an enhanced SST Integrity Project (SSTIP) in 2009. An expert panel on SST integrity, consisting of various subject matters experts in industry and academia, was created to provide recommendations supporting the development of the project. Working with the Washington State Department of Ecology, key recommendations were used to develop the basis for the M-45-10-1 Change Package for the Hanford Federal Agreement and Compliance Order, which is also known as the Tri-Party Agreement.
DESCRIPTION OF THE SINGLE-SHELL TANK SYSTEM The Hanford radioactive waste is contained in 149 single-shell tanks (SSTs) and 28 double-shell tanks (DSTs). The SST tank farms were constructed over a 20 year period as needed to support the reprocessing of fuel. Four farms were started in late 1943; two were completed in 1944, and two were completed in 1945. The rest of the SST farms were started and finished at various times between 1946 and 1964, see Figure 1 for typical construction photo. The first four farms consisted of four 55,000 gallon tanks and twelve 530,000 gallon tanks. The other farms were built with three different capacities: 530,000, 750,000, and 1,000,000 gallons. In total, 149 SSTs, in 12 farms, were built for the storage of radioactive wastes at the Hanford Site.
As previously stated, four different tank types were constructed (see Figure 2). The first, Type I, have a 20 foot diameter, 38 foot height, and hold 55,000 gallons. The second, Type II, have a 75 foot diameter, 32 foot height, and hold 530,000 gallons. The third, Type III, also have a 75 foot diameter, but had a 39 foot height, and hold 750,000 gallons. The fourth, Type IV, was broken down into three sub-types. All three Type IV tanks - Types IVA, IVB, and IVC - had a 75 foot diameter and hold 1,000,000 gallons, with heights ranging from 46 feet to 48.75 feet.
In addition to the increasing volume of the tanks, other design features changed over the years. The Type I have 15-inch thick flats lids and all other tank types have 15-inch thick concrete domes. The Type I and Type II tanks both have 12-inch thick reinforced concrete walls, and dished bottoms. The Type III tanks also have dished bottoms, but the walls were increased to 15 inches The lower portion of the tank wall on Type IV tanks was increased to 24 inches to accommodate the increased wall height. The Type IV tanks went to flatter bottom designs: pan (or with a slight depression in the center) for the Type IVA tanks and flat for the other Type IV tanks. The bottom and the wall were welded with a fillet weld for the Type IVA and IVB tanks, but the Type IVC design has a 4-inch radius knuckle. For the increased heat loaded in the Type IV tanks, they were equipped with Air Lift Circulators up to four in the Type IVA tanks, four in the Type IVB tanks, and 22 in the Type IVC tanks.
Early failures of some single shell tanks, some potentially from stress corrosion cracking (SCC) of the SSTs carbon-steel liners, resulted in leakage of waste from the SSTs to the surrounding soil. This leakage led to a decision by the U.S. Atomic Energy Commission (predecessor to the U.S. Energy Research and Development Administration and subsequently the DOE) in the 1960s to initiate construction of DSTs with improved design, materials, and construction. The construction of the DSTs began in 1968 with the sixth farm being completed in 1986. All of the DSTs have a nominal million-gallon waste capacity. The free liquids from SSTs have been transferred to DSTs as part of the SST interim stabilization program, which was completed in fiscal year (FY) 2005. Eventually, the remaining solids (i.e., sludge and salt cake) and interstitial liquid in the SSTs will also be retrieved and transferred to DSTs for subsequent processing and disposal; after that, the disposition of the SSTs will take place per the applicable requirements.
At this point, the structural integrity program for SSTs is limited to ensuring that structural adequacy is maintained throughout SST waste retrieval and closure. However, since negotiations under the Tri-Party Agreement related to the schedule for waste treatment and vitrification have extended the use of the SSTs, the DOE established an extensive program for SST integrity.
Single Shell Tank Operational History The SSTs received alkaline waste from multiple nuclear fuel reprocessing operations, starting in 1944. The initial radioactive wastes were principally derived from three different chemical processing operations, each of which produced several different types of waste; the bismuth phosphate process, Reduction Oxidation (Redox) process, and Plutonium Uranium Extraction (PUREX) process. The bismuth phosphate process only recovered plutonium from irradiated reactor fuels. The Redox and PUREX processes recovered both plutonium and uranium from the fuel.
The bismuth phosphate wastes discharged to the tanks were later processed to recover uranium from the wastes by using the tributyl phosphate (TBP) process. Potassium ferrocyanide was used to scavenge cesium ion from this waste. The oldest tanks (241-B, 241- BX, 241-BY, 241-C, 241-T, 241-TX, 241-TY, and 241-U farms) were constructed to receive waste from bismuth phosphate plants and received other wastes (e.g., low heat wastes from the Redox and PUREX plants and waste from uranium metal recovery). The Redox high heat wastes were stored in the 241-S and 241-SX farms. The PUREX high heat wastes were stored in 241-A, and 241-AX farms. The 241-SX, 241-A, and 241-AX designs allowed the storage of boiling wastes so water could be removed from the tanks to conserve space for the retention of radioactive materials. Tanks in the 241-A, -AX, and -SX Farms experienced high temperatures ranging from 2000 F to 5940 F. Other operations including the in-tank solidification (ITS) and tank farm evaporators were used to remove water and concentrate the wastes.
Waste additions to the SSTs ceased in 1980 and pumpable liquids have been transferred from the SSTs to the double-shell tanks (DSTs). SST wastes are slated for retrieval and treatment in a Waste Treatment Plant and Immobilization (WTP) that is currently under construction. Technical issues have delayed the schedule for initiating operations of the WTP. The delays to the WTP will necessitate extended storage in the SSTs, most of which are beyond their design life. The most recently built, 241-AX farm, tanks had a design life of 25 years which expired in 1990. Design life is based on steel liner corrosion rather than concrete degradation.
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