Where does nuclear waste go when nobody wants it anymore?
That's the perennial dilemma facing the folks at Savannah River Site who are spending decades - and billions of your dollars - to dispose of radioactive material generated by a half century of Cold War nuclear weapons production.
One of the newest options involves mammoth structures known as "saltstone disposal units," or SDUs. Each rectangular module holds two round containers with a joint capacity of about 5 million gallons.
The first such SDU was completed just this year, and will start accepting waste in November 2012. It covers 2.5 acres and cost $38.4 million.
It is only the first. The long-range plans call for 39 more, all within SRS.
The material of concern is the site's liquid waste - now stored in 49 underground storage tanks, 12 of which are leaking.
The high-level waste in the aging tanks includes thick liquids, sludge with a consistency similar to peanut butter and a caustic material that turns to salt.
The low-level salt material is blended with cement and slag, then mixed to form a liquid grout that is pumped into the SDU, where it hardens to a permanent state.
The salt waste material accounts for much of the volume in the decaying Cold War waste tanks, but the peanut-butter-like sludge is the most radioactive and much more dangerous. That high-level material heads in a different direction, to the Defense Waste Processing Facility at SRS, where it is "vitrified" in glass and sealed inside steel canisters.
About 3,000 canisters processed during the past decade remain in storage at SRS. Some of the material was earmarked for permanent burial in Nevada's Yucca Mountain repository - a project that has since been scrapped by the Energy Department.
Now the waste has nowhere to go, and a permanent solution - other than leaving it in South Carolina - might be a long way off.