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The Department of Energy has closed out a list of 28 technical issues at the Hanford vitrification plant raised by a panel of experts in 2006.
However, more testing will be needed to provide confidence that current plans and proposed changes to address issues will be adequate, said Delmar Noyes, DOE deputy project director for the vit plant.
"We recognize some uncertainty is left," he said.
The biggest challenge has been making sure that waste will be kept adequately mixed in tanks inside the vitrification plant. Bechtel National believed it would have the issue resolved by September 2009, but tests using a scaled-down version of a tank and simulated waste showed that the mixing system as designed might not be adequate.
Four months later, the Defense Nuclear Facilities Safety Board criticized the design and testing of mixing systems for some of the vitrification plant tanks. Inadequate mixing could lead to a buildup of flammable gas under some unusual circumstances or to the possibility of a nuclear criticality if too much plutonium settled and accumulated in the bottom of the tanks.
Bechtel submitted its plans for keeping the waste adequately mixed at the end of June and DOE reviewed it. Then a Technical Steering Group made up of Bechtel and DOE officials reviewed proposed actions, but could not reach unanimous agreement that the fixes would be adequate.
DOE made the call to close out the mixing issue, the last of the 28 technical issues, but also to do more testing on mixing and to place holds on certain work at the vitrification plant until testing has provided more confidence in solutions.
"We believe it will work," said DOE spokeswoman Carrie Meyer. "But we want to raise our level of confidence and that's why we are going to do additional small-scale tests for five tanks."
Those five tanks will have waste with more solids.
It also then will do testing using larger tanks than the 4-foot diameter tank now being used for testing to make sure the results hold on a larger scale and show mixing will be adequate at all 38 tanks that the expert panel questioned.
The vitrification plant is being built to turn up to 53 million gallons of radioactive waste left from the past production of plutonium into a stable glass form for disposal. The plant will include "black cells," with tanks that will be used to process waste that is so radioactive that people cannot enter the cells once operations begin.
Waste in those tanks will be kept mixed using a system with no moving parts that would require maintenance. Instead, pulse jet mixers have been developed that work like turkey basters to suck up waste and pump it back out nozzles using air pressure to keep heavier waste particles from settling to the bottom of the tanks.
Less data is available on how well pulse jet mixers work than on more conventional mixing systems, Noyes said.
Among changes proposed after doing testing are tank by tank adjustments that include adding mixers, increasing the velocity of nozzles, changing the angle of nozzles and moving suction lines lower. In addition, three tanks will have limits placed on the amount of waste or concentration of waste that can be added to them.
But for some tanks, backup systems also are proposed. Although designs are only conceptual, they call for cameras to provide a look inside the tanks and piping systems that could be used to remove solids that build up in the bottoms of tanks.
The cost of the proposed changes will not be known until a design is completed, but it would be factored into current projected costs of the plant, according to DOE.
The mixing system is expected to be one topic addressed by the Defense Nuclear Facilities Safety Board when it holds hearings in Richland on Oct. 7 and Oct. 8 on the safety of the vit plant.
In addition, Walt Tamosaitis, the former manager of research and technology for the plant, will speak at 7 p.m. today at the Red Lion Hotel in downtown Seattle about his concerns about the plant. He has filed a retaliation claim with the Occupational Safety and Health Administration, saying he was dismissed from his position for raising technical issues related to mixing and other processes at the plant.
-- Annette Cary: 582-1533; acary@tricityherald.com; more Hanford news at hanfordnews.com
