It's highly likely that radioactive liquid collected by a system to detect leaks in a Hanford double-shell tank did not come from inside the tank, according to results of a recently completed investigation.
The finding eases concerns that Tank AY-102, Hanford's oldest double-shell tank, may be leaking radioactive waste into the soil beneath it.
A comprehensive study done for the Department of Energy by contractor Washington River Protection Solutions shows "with a high level of confidence" that contaminated liquid collected in a leak detection pit for Hanford's oldest double-shell tank did not come from within the tank, the investigation concluded.
Instead, the pit likely caught some rain water that dripped through the soil, picking up contamination in the soil left during more than 40 years of operating Tank AY-102, and then infiltrated the leak detection pit, according to information in the report. The pit collects water between the bottom of the tank's outer shell and the underground concrete platform beneath the tank.
The inner shell of Hanford's Tank AY-102 slowly is leaking waste into the space between its inner and outer shells. But that waste was not believed to have reached the environment until contamination was found June 20 in the leak contamination pit, raising concerns that the outer shell of the tank also was leaking.
In July, DOE said further tests then to determine if the outer shell was leaking had been inconclusive and had not shown that the contamination came from the tanks.
Although results of the completed investigation are encouraging, DOE will continue its look at the leak detection pit to validate the investigation's conclusion.
It plans to send robotic technology down the leak detection pit, possibly later this year.
Equipment would be lowered from the ground about 65 feet down the pit and then it would go laterally into the collection pipes of the detection system. Technology for the test has not been selected, and sensitive equipment will have to be able to withstand a high-radiation environment.
DOE also will continue to monitor the area between the tank's two shells and the leak detection pit, said Erik Olds, of the DOE Office of River Protection.
The radioactive liquid pumped from the leak detection pit June 20 was contaminated with cesium, which Hanford workers check for as a radioactive marker to determine tank waste contamination. But the level of cesium was a million times more dilute in the pumped liquid than in liquid samples drawn from the tank, according to the completed investigation report.
The liquid pumped from the pit also had sodium and nitrate contamination, but it was several thousand times more dilute than the liquid in the tank, the investigation report said.
One theory had been that the pump lowered into the pit to remove the liquid had been contaminated previously. The pump was taken apart at Hanford's 222-S Laboratory and contamination was found within it, but it was not consistent with tank waste, according to the report.
The Washington State Department of Ecology, a Hanford regulator, has been sent the report and is reviewing it.
"We stand solid in our belief that the tank needs to be pumped," said Jane Hedges, manager of Ecology's nuclear waste program, in a statement Thursday.
DOE has taken actions need to prepare to immediately begin pumping liquid from the tank, if warranted, Olds said.
However, about 22 months would be needed to prepare to also remove sludge from the tank.
Some liquid would need to be left in the tank unless all waste is pumped because the waste generates heat.
In addition to the inner leak at Tank AY-102, six single-shell tanks at Hanford also may be leaking radioactive waste and Hanford officials are continuing an investigation of those tanks.
Hanford has 56 million gallons of radioactive and hazardous chemical waste stored in underground tanks until it can be treated for disposal.
Waste, left from past production of weapons plutonium, is being pumped from 149 single-shell tanks to 28 newer double-shell tanks.
-- Annette Cary: 582-1533; email@example.com; Twitter: @HanfordNews