The two tanks – called 5 and 6 – held about 730,000 gallons each of Cold War radioactive waste, which has been removed, according to a U.S. Department of Energy news release. Then, the tanks were filled with a custom-made grout that permanently seals them in place.
The site has 49 underground storage tanks, some of which are leaking. Tanks 18 and 19 were closed last year, marking the first DOE tanks closed nationwide since 2007.
An agreement among the South Carolina DHEC, the U.S. Environmental Protection Agency and DOE required the operational closure of tanks 5 and 6 by December 2015.