A sample taken from between the inner and outer walls of a Hanford double-shell tank confirms the material is consistent with the radioactive waste held in the tank, according to preliminary test results.
It's more evidence that at least one of Hanford's double-shell tanks, which are needed to hold waste for decades to come, may be deteriorating. Finding waste outside the inner shell of Tank AY-102 is a first for a Hanford double-shell tank.
The Department of Energy expects to know enough by the end of next week to declare whether or not Tank AY-102 is leaking from its inner shell.
Hanford has 56 million gallons of radioactive and hazardous chemical waste held in underground tanks until they can be treated for disposal. The waste is left from the past production of plutonium for the nation's nuclear weapons program.
The waste is being pumped from leak-prone single-shell tanks, some of them built as early as World War II, into 28 newer double-shell tanks. The double-shell tanks might have to hold waste for as long as another 40 years until all the waste can be treated.
It is important to remember that the material within the walls of Tank AY-102 is stable, said Tom Fletcher, DOE assistant manager of the tank farms, in a message to DOE employees Thursday.
No radioactive material has been found outside the outer wall of the tank, and there is no indication of radioactive contamination in the leak-detection pit outside the tank, he said.
The issue was discovered in August during video monitoring of the area between the inner and outer walls that was designed as an overflow space if the inner steel liner were to leak. The outer shell is made of steel and covered with steel-reinforced concrete.
A video camera, inserted down a tank riser that had not previously been used for visual examinations, Riser 90, showed two side-by-side areas of contamination. One was a dry mound about 24 by 36 by 8 inches.
A small sample collected in connection with the video inspections showed the material was radioactive but provided little other information.
Workers next sent video equipment down the remaining nine risers that provide access into the area between the tank's shells, finding nothing unusual near eight of the risers.
But more unusual material was found near one riser, Riser 83, which was on the opposite side of the tank from where contamination had initially been spotted near Riser 90.
It was particularly concerning because a a photo of the same area between the two shells of the tank near Riser 83 in 2006 showed it was clean then.
The sample collected in that area after the video inspection "is largely consistent with the type of material found in that tank," according to Fletcher.
But before reaching a conclusion on whether the inner shell of the tank has leaked, more samples will be collected from the material spotted in August near Riser 90. Results will be analyzed by a technical panel of Washington River Protection Solutions, the DOE contractor for the Hanford tank farms.
Tank AY-102 is among the oldest of Hanford's double-shell tanks, going into service in 1971, and is just past its design life of 40 years. It has a capacity of about 1 million gallons and holds about 850,000 gallons of waste.
Hanford workers have increased monitoring to make sure there is no change in conditions since finding the waste between the shells of Tank AY-102. Longer term, work will be done to determine if other double-shell tanks might have similar issues.
-- Annette Cary: 582-1533; email@example.com