The last eight double-shell waste tanks built at Hanford had only minor construction issues, according to the final construction review released by the Department of Energy on Tuesday.
The review concluded that double-shell tanks in the group called the AP Tank Farm were in better condition overall following construction than Tank AY-102, which is leaking radioactive waste between its shells. It was the first tank to be built.
The interior leak is suspected to be due in part to difficulties with the tank's construction, prompting a look at the construction history of the other double-shell tanks. The tanks hold radioactive waste left from the past production of plutonium for the nation's nuclear weapons program.
The AP Tank Farm was the sixth group of tanks and was built from 1982-86. About 5 to 12 percent of welds in those tanks required rework during construction, compared to 34 percent in the bottom of the inner shell of Tank AY-102, according to the review done by DOE contractor Washington River Protection Solutions.
The construction reviews became an issue when Sen. Ron Wyden, D-Ore., obtained the reviews that had been finished by early March and said they contradicted DOE's statements that construction difficulties on Tank AY-102 were an isolated problem.
He concluded that six tanks in addition to Tank AY-102 have construction problems that could increase the risk of leaks and 13 more tanks may have construction problems that will shorten their life span.
He has asked for DOE to provide a plan to address tank construction issues, including considering building more waste storage tanks.
DOE said the interior leak in Tank AY-102 likely was the result of construction difficulties and a combination of high heat waste that is sitting above waste without an added corrosion inhibitor at the bottom of the tank. None of the other double-shell tanks have that combination of waste, according to DOE.
The construction review on Tank AY-102 concluded that "construction difficulties and trial-and-error repairs left the primary tank bottom with residual stresses that could not be foreseen by the designers. These provided a fertile incubator for sustained corrosion to take place."
The Washington State Department of Ecology has ordered DOE to start removing waste from the tank by Sept. 1.
Construction reviews concluded that enhanced inspection was needed on the next three tanks built after Tank AY-102, which included the second tank in the AY Tank Farm and the AZ Tank Farm.
Some problems also were found that were similar to those in Tank AY-102 in the next three tanks, those in the SY Tank Farm built from 1974-77. The 13 tanks in the AW and AN Tank Farms, built from 1976-80, also had some issues that leave room for uncertainty in long-term tank integrity, the reviews said.
-- Annette Cary: 582-1533; email@example.com; Twitter: @HanfordNews