SRS fills first mega-volume disposal unit with grout

Saltstone Disposal Unit (SDU) 6, the first of seven mega-volume disposal units built at the Savannah River Site near Aiken, S.C., has been filled with grouted legacy tank waste, according to the Department of Energy.

Last week's announcement from the DOE marked a milestone in efforts to clean up nuclear waste that for decades had been stored underground in 51 carbon-steel tanks at the site. SDU 6, which accepted its first shipment of waste material in 2018, is the first of the mega-volume units approved in 2012 to fill completely.

Another mega-volume unit, SDU 7, was listed as operational on the DOE's website as of May 2026.

Processes at SRS: The waste sent to Savannah River's SDUs is low-level waste material that has undergone several stages of processing.

Historically, nuclear waste stored in carbon-steel tanks was treated with sodium hydroxide. This neutralized the nitric acid used to dissolve fuel rod material in the waste, eventually resulting in two waste products: sludge (a highly radioactive substance that would settle at the bottom of the tank, later disposed of through vitrification, or immobilization in glass) and salt waste.

Since 2020 at SRS, salt waste has gone to the site's Salt Waste Processing Facility (SWPF) so that radionuclides can be removed and sent for vitrification, while the remaining low-level waste material, called decontaminated salt solution, or DSS, has been transported to SRS's Saltstone Processing Facility. There, the DSS is combined with fly ash and slag to form a saltstone grout, which is then sent to an SDU to harden. The grout serves as a form of permanent disposal that, according to prior research, offers stability and immobilization of radionuclides among its benefits.

Although a significant portion of the waste sent to SDU 6 had been processed at the SWPF, the SWPF came on line only in 2020. Prior to the start of the SWPF's operations, SDU 6 received LLW from a facility that operated temporarily at SRS, the Actinide Removal Process/Modular Caustic Side Solvent Extraction Unit, as well as from SRS's Tank Closure Cesium Removal demonstration project.

Additionally, while SDU 6 was the first of the mega-sized units to be constructed and to begin operations, smaller SDUs also have been in use at SRS. SDUs 1 and 4 both are rectangular units built in the mid-1980s that no longer receive waste. Construction of SDUs 2, 3, and 5 finished between September 2012 and September 2013; each of those three cylindrical units holds about three million gallons of saltstone grout.

Comparatively, mega-disposal units 6 through 12 each will hold an estimated 33 million gallons of grout when filled. According to a DOE fact sheet, "The larger SDU design will result in substantial cost savings over the life of the project because of economies of scale, requiring less infrastructure and materials to design and build."

Progress continues with the remaining mega-volume SDUs. Along with SDU 7, which is currently operational, SDUs 8, 9, and 10 all have passed leak tightness tests and have been deemed ready to operate. Concrete construction of SDU 11 was completed in July 2026, and initial construction work on SDU 12, including a multilayer foundation, recently was completed, according to the Savannah Business Journal.