An inspection of Hanford's second-oldest double-shell tank holding radioactive waste has found no evidence of a leak.
The result of the inspection was good news for taxpayers as the Department of Energy works to assess the extent of deterioration in massive underground tanks that are expected to be needed to hold radioactive waste for decades to come.
"Confirming the integrity of AY-101 is a positive step," Tom Fletcher, DOE assistant manager of the Hanford tank farms project, said in a statement. "We are continuing to accelerate surveillances of other tanks in an effort to ensure the protection of the workers, public and environment."
In October a leak of radioactive waste from the inner shell of Hanford's oldest double-shell tank, Tank AY-102, was confirmed. It was the first leak detected within one of Hanford's double-shell tanks and raised concerns about whether other double-shell tanks might be deteriorating.
Hanford has 56 million gallons of radioactive waste left from the past production of weapons plutonium being held in underground tanks. Waste from 149 leak-prone, single-shell tanks is being pumped into newer double-shell tanks, some of which may need to hold waste for 40 more years until all of it can be treated for disposal.
But already the first of the double-shell tanks have been in use for four decades, the amount of time they were designed to be used.
Since the initial leak within Tank AY-102 was discovered, Hanford workers have been using video cameras to check for leaks in six of the oldest double-shell tanks, starting with the second oldest, Tank AY-101.
"We finished up in AY-101 without seeing anything unusual in the annulus that would indicate a leak," Del Scott, project manager for contractor Washington River Protection Solutions, said in a statement Monday. The annulus is the 30-inch-wide area between the inner and outer tank walls.
Construction was being done in 1969 on both of the first two tanks, but some of the construction problems with Tank AY-102 were corrected on the second tank, AY-101.
Construction problems started on the first tank when the base of the outer shell was built with thin steel plates and bulges were created when they were welded. An eight-inch layer of hard insulating material was poured as a refractory over the steel bottom of the outer shell, but it cracked as the bulges moved.
Welding the inner shell on top of the refractory proved difficult and 36 percent of the tank's welds were rejected after an inspection. Some welds were redone as many as four times before they passed a radiography examination, according to recent report on the history of Tank AY-102.
Workers used three-eighths inch steel plates for the base of the outer shell of the second tank rather than the one-quarter inch steel used on the first. The percentage of rejected welds dropped from 36 percent to 10 percent on Tank AY-101, according to the report.
"It just did not have the number of construction difficulties," said John Britton, spokesman for Washington River Protection Solutions. And the construction problems it did have were less severe, he said.
"Ecology is pleased to hear that nothing significant was discovered in the inspection of AY-101," said Dieter Bohrmann, spokesman for the Washington State Department of Ecology, the regulator for the Hanford tank farms. "The state obviously hopes that is also the case when inspections are done on other double-shell tanks."
On Monday, work to inspect double-shell tanks moved to Tank AZ-101, where video cameras will be lowered down risers that give access from the ground to the space between the tank's inner and outer walls.
The inner shell of the first double-shell tank continues to leak waste at a slow rate. An estimated 190 to 520 gallons of waste have leaked from the inner tank, but a significant portion of the liquid has evaporated, leaving an estimated 20 to 50 gallons of drying waste between the shells.
"Everything is on the table" as DOE and Washington River Protection Solutions join with the state Departments of Health and Ecology to decide what to do about the leak within Tank AY-102, Britton said.
There is no evidence that waste has leaked from the outer shell of the tank into the soil beneath the tank.
But an existing pump has been reconnected to the electrical system that could be used to pump liquid waste from Tank AY-102, Britton said. The tank holds about 850,000 gallons of radioactive waste -- all but about 150,000 gallons of it liquid waste.
Methods that could be used to empty the entire tank also are being considered, according to DOE.
In addition another pump as been staged to pump liquid waste from the annulus, if needed.
However, most of the limited double-shell tank space is needed to hold waste being emptied from single-shell tanks, some of which have held waste since World War II. DOE has a consent decree deadline approved in federal court to empty all 16 single-shell tanks in the grouping called C Tank Farm by fall 2014.
To make more space in the double-shell tanks, the 242-A Evaporator could be run more frequently to reduce the volume of liquid waste in double-shell tanks.
In addition engineering studies are being conducted to see if some of the double-shell tanks could safely hold more waste.
The recommendation of the Hanford Advisory Board to build more waste storage tanks also is being considered, Britton said.
DOE has said building one double-shell tank could cost about $100 million and building a group of six tanks could take five to seven years.
-- Annette Cary: 582-1533; email@example.com; @HanfordNews