DOE looks at flammable gas issue in tanks

By Annette Cary, Tri-City HeraldNovember 12, 2013 

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Workers are shown here at Hanford's C Tank Farm, where radioactive waste is being emptied from leak-prone underground tanks.

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The Department of Energy could know this spring whether a potential flammable gas issue will delay emptying the last two tanks required to meet a court-enforced consent decree deadline.

DOE is required to have all 16 underground, single-shell tanks in the group called C Tank Farm emptied of radioactive waste before October 2014.

However, in June, DOE notified the state of Washington that emptying the final two of the 16 tanks by that deadline was at serious risk because of the potential flammable gas issue. Six of the tanks still hold some radioactive waste from the past production of weapons plutonium at Hanford.

The two tanks at issue have more waste to be removed than other remaining C Farm Tanks. Plans call for pumping it into two sturdier double-shell tanks where the waste will be stored until it can be treated for disposal.

Pumping to remove 316,000 gallons of waste from Tank C-102 had been expected to begin about a year ago. But a Washington River Protection Solutions engineer raised the question of whether there might be a flammable gas issue that had not been previously considered, based on an incident in Denmark, said Tom Fletcher, DOE assistant manager of the Hanford tank farms.

If the sludge gets too deep in one of the double-shell Hanford tanks, flammable gases might not gradually make their way to the head space of the tank and be cleared, as expected. Instead, gases could build up deep in the sludge and then rise in a single large bubble that would increase flammable gas to higher levels than considered safe, under the potential scenario.

To empty all the C Tank Farm waste, the two double-shells being used would need to hold close to 300 inches of sludge.

Initially, Hanford officials determined that they could only defend the safety of storing up to about 172 inches of sludge in a double-shell tank, a level that has since been raised to about 190 inches of sludge, depending on the tank.

Pacific Northwest National Laboratory is studying the issue and should determine this spring if piling sludge too deep poses a credible threat of an accident, Fletcher said.

If it does not, Washington River Protection Solutions could move forward with emptying Tank C-102 and Tank C-105, which holds 132,000 gallons of waste.

However, if it's determined that deep sludge poses the risk of a sudden release of flammable gas within the tank, DOE will prepare to empty the waste in the last of the C Farm tanks into a third double-shell tank. Preparing for the transfer of waste to a third tank would take too long to allow DOE to meet the consent decree deadline for emptying all C Farm tanks, Fletcher said.

DOE also would seek an engineering fix to the problem, such as putting a mixing system into tanks that would have 300 inches of sludge to prevent a large bubble of flammable gas from forming.

w Annette Cary: 582-1533; acary@tricityherald.com; Twitter: @HanfordNews

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