Enhanced Chemical Cleaning in Congested Steel Tanks - 13022

Savannah River Site in South Carolina, USA is a former nuclear weapons materials production facility dating from the 1950's. The mission for the site has evolved from nuclear weapons material production to a new mission called Enterprise SRS that supports clean energy, nuclear materials stewardship, and environmental remediation. Savannah River Site has 51 underground waste storage tanks, all of which were placed into operation between 1954 and 1986. Fig 1 shows the type and status of the tanks. There are four types of waste tanks - Types I through IV. Type III tanks are the newest tanks, placed into operation between 1969 and 1986. There are 27 Type III tanks. Type I tanks are the oldest tanks, constructed in 1952 through 1953. Type II waste tanks were constructed in 1955 through 1956. There are eight Type IV tanks, constructed in 1958 through 1962. Four of these Type IV tanks, Tanks 17 through 20 in F-Tank Farm, have been isolated, removed from service, and grouted. Some urgency exists to clean up and close the tanks. Twelve tanks without full secondary containment have a history of leakage. Sufficient waste has been removed from these tanks such that there are currently no active leak sites. The first tank, Tank 1F, lacking full secondary containment, began receiving waste in 1954. This tank is still in service. Closure of the remaining tanks is planned by 2028. Approximately 38 Mgal of radioactive waste, containing 318 million curies (MCi) of radioactivity, are currently stored in 47 active waste storage tanks located in two separate locations, H-Tank Farm (29 tanks) and F-Tank Farm (18 tanks). This waste is a complex mixture of insoluble metal hydroxide solids, commonly referred to as sludge, and soluble salt supernate. The supernate volume is reduced by evaporation, which also concentrates the soluble salts to their solubility limit. The resultant solution crystallizes as salts. The resulting crystalline solids are commonly referred to as salt cake. The salt cake and supernate combined are referred to as salt waste. The sludge component of the radioactive waste represents approximately 2.9 Mgal (8% of total) of waste but contains approximately 155 MCi (49% of total). The salt waste makes up the remaining 35.1 Mgal (92% of total) of waste and contains approximately 163 MCi (51% of total). Of that salt waste, the supernate accounts for 19.3 Mgal and 151 MCi and salt cake accounts for the remaining 15.8 Mgal and 12 MCi25. The sludge contains the majority of the long-lived (half-life > 30 years) radionuclides (e.g., actinides) and strontium. The sludge is currently being stabilized through a vitrification process that immobilizes the waste in a borosilicate glass matrix. The SRS is unique in the United States in that it has an integrated capability to process tank waste into final forms suitable for environmentally safe ultimate disposal. This capability is illustrated by the process flowsheet given in Fig. 3. The SRS is a leader in environmental remediation and the development of technology in support of this mission. Sludge at SRS can be grouped into two general types: F-Area sludge, and H-Area sludge. Each type is made up of both an aqueous fraction and a solids fraction. Assuming a volume of about 19,000 liters, representative mass quantities of the major constituents (i.e., accounting for about 90 to 95% of the mass) are shown in Table I. After the sludge is removed from a tank, it is prepared to become feed for the Defense Waste Processing Facility (DWPF), where it will be vitrified into a borosilicate glass matrix for eventual long-term disposal. As part of preparing the feed, the glass qualifications have a limited tolerance for salt, and therefore require that the sludge must be washed. Normally, during washing the sodium concentration must be reduced from greater than 6 molar to less than 1 molar. The remaining solids are disposed of through DWPF, while the spent wash water, containing the aqueous and solubilized salts are eventually processed through Salt Processing and ultimately disposed of through Saltstone. In Saltstone, the low curie liquid is combined with grout and solidified for long term disposal. Before the tanks can be closed, SRS prepares the HLW tanks for closure in three phases: bulk waste removal, heel removal, and chemical cleaning. The SRS HLW tanks must be very clean in order to support closure due to the high specific activity of the residual waste and the close proximity to the water table. For closure, an average allowed remaining residual volume has been estimated to be about 190 to 1,900 liters per tank, or if assumed to be spread-out evenly on a tank bottom, about 0.025 to 0.4 cm deep. Considering the size of the tanks and the internal cooling coils, the corrosion products alone would likely exceed the allowed residual. Most of the tanks contain up to 7 kilometers of cooling pipes that make cleaning the residual materials from the tanks very challenging. Areva, working with Savannah River Remediation (SRR), has tested and proven an enhanced chemical cleaning process using a dilute concentration of oxalic acid to dissolve and remove the residual material from the tanks. An advantage of the enhanced chemical cleaning (ECC) process is greatly reduced effluents compared to other available treatment processes. This paper will review the testing and performance of the oxalic acid cleaning process using simulated waste and actual waste from the tanks. Preliminary design of the equipment to perform the chemical cleaning of the tanks was completed. Fabrication of the equipment is expected to begin as soon as funds are available from the Department of Energy. Areva's approach to the ECC process is based on years of providing turn-key services to its customers. Areva has also performed detailed evaluations of the dissolution chemistry of both the sludge and salt simulants demonstrating the applicability of the Areva ECC process.