Hanford workers are investigating the possibility of a recent leak at one of the nuclear reservation's underground tanks of high-level waste after an unexplained drop in the contents of Tank SX-104.

"We want to protect human health and the environment and in that process treat it as a potential leaker until we can prove otherwise," said Steve Pfaff, the Department of Energy federal project director for tank retrieval.

DOE contractor CH2M Hill Hanford Group is taking a conservative approach, knowing that leaks can occur because of weld failures in the steel liners of the concrete shells of the oldest tanks, said Herb Berman, a CH2M Hill vice president and chief engineer.

However, there are other possible explanations, including changes in the waste from the installation of a pipe in the tank or gas escaping from the solids of the tank. Tank SX-104 was built in the early 1950s and is suspected of having leaked about 6,000 gallons of waste before 1988.

It's one of 149 single-shell tanks built to hold high-level radioactive waste left from processing plutonium for the nation's nuclear weapons program during World War II and the Cold War.

DOE and CH2M Hill have removed pumpable liquid from the underground tanks and are working to remove the remaining waste and store it in 28 double-shell tanks until it can be treated for disposal.

A May check of the level of the waste in the enclosed tank found it had dropped 3.384 inches since October. It holds about 310,000 gallons of salt cake and 136,000 gallons of sludge. In addition, it has about 47,700 gallons of liquid trapped in the pores of the salt cake and sludge.

Checks of the tank level have been increased from four times a year to weekly, with last week's measurement showing the level had risen a twentieth of an inch.

Checks also are being made in the seven monitoring wells that ring the underground tank for an increase in radiation levels that might be caused by a leak.

"We haven't seen any increase," Berman said. But he also noted that the dry wells have not detected possible previous leaks. The tank waste would be likely to gel once it cooled outside the tank rather than seeping through the soil to the monitoring wells.

Levels in the tank also were observed to fluctuate in 1997 and 1998. That variation was traced to response to barometric pressure, Berman said.

A team assessing the current fluctuation also is looking at whether the tank may have "burped" some gas trapped in its solids into the upper part of the tank. In that case the drop would be sudden and then the waste level might slowly rise.

That's a possible scenario given the slight increase in the tank level last week, Berman said.

The team also is considering whether water added to the tank in December 2006 may finally have permeated some of the solids, resulting in a drop in the waste level. A water lance was used to break up the waste enough then to insert a new pipe for monitoring into the tank's waste.

Contingency plans are being made in case the assessment team does conclude a leak occurred. Because the leak would be underground, the threat would be to the environment rather than an immediate risk to Hanford workers, Pfaff said.

DOE might respond by retrieving the waste from Tank SX-104 sooner than planned or making changes to how the waste is retrieved to reduce the possibility of leaks.