Inadequate mixing of some radioactive wastes at the Hanford vitrification plant could cause a criticality or a build-up of flammable gas that could cause an explosion, according to the Defense Nuclear Facilities Safety Board.

But the Department of Energy believes the problem can be resolved.

It initiated tests on the mixing system planned for parts of the vitrification plant after a panel of experts in 2006 identified it as one of 28 technical issues that needed to be studied. It's the last major and complex issue to be resolved for the plant, which is half-completed.

DOE has been testing the mixing systems at the M3 Mixing Test Platform installed at Mid-Columbia Engineering near the Hanford nuclear reservation. It expects to have testing completed in April, including confirming any modifications that need to be done to the mixing system.

That's ahead of a legally binding Tri-Party Agreement deadline to have the work completed in June.

At issue is keeping heavy particles in high-level radioactive waste mixed with the rest of the waste in tanks at the vitrification plant, which is being built to turn the waste into a stable glass form for disposal. The waste is left from the past production of plutonium at Hanford for the nation's nuclear weapons program.

The mixing at issue will be done in black cells, which will be too radioactive for humans to enter once the plant starts operating. The operation is planned to be done virtually maintenance-free by using a system with no moving parts that relies on pulses of air to keep wastes mixed.

At the test platform, researchers can watch inside a clear, 200-gallon acrylic tank as air-driven pulse jet mixers suck up mock waste through nozzles and shoot it back into the tank to keep particles suspended. Researchers can try different configurations, such as changing which mixers fire at what time or changing the size of nozzles, to improve mixing.

By the end of September, research had shown the pulse jet mixers should work well for 26 of the tanks, but more intensive work still was being done to analyze data for 12 tanks that researchers knew would be more difficult.

More mixing is needed in larger tanks and in tanks in the earliest steps at the pretreatment process at the vit plant, where waste is divided into high-level and low-activity waste streams.

Modifications will need to be made to the mixing system of some tanks already installed at the vitrification plant, but the number is not yet known. In addition, changes likely will need to be made to other tanks that are being manufactured.

However, DOE does not expect the changes to affect the plant's schedule, said Carrie Meyer, DOE spokeswoman. It is scheduled to begin operating in 2019.

As designed without modifications, the pulse jet mixers "lack sufficient power to adequately mix and transport the most rapidly settling particles expected to be present in the Hanford waste inventory," John Mansfield, vice chairman of the Defense Nuclear Facilities Safety Board, wrote in a letter to Ines Triay, DOE assistant secretary for environmental management.

Dense particles rich in plutonium and uranium could settle on the bottom of tanks, rather than remaining mixed throughout the waste.

"These settled particles may form a sediment layer with sufficient fissile mass in a geometry such that a criticality accident is credible," the letter said. "Furthermore, if the vessels are not well mixed, samples drawn from the vessels to ensure that such an event does not occur will not be representative."

If a layer of sediment develops on the bottom of the tanks, the layer could grow enough to retain significant quantities of flammable gas, the letter said. No instrumentation in the plant is designed to indicate the quantity of sediment and alert plant operators.

The result could be an explosion, the letter said.

Presence of a sediment layer also could interfere with level indicators, damaging components in the vit plant, the letter said.

DOE is required to submit a response to the safety board by early March.