U.S. Department of Energy officials are continuing to evaluate a decommissioning approach for the Molten Salt Reactor Experiment at the Oak Ridge National Laboratory (ORNL) in Tennessee that could involve removing above-ground structures and encasing contaminated structures below ground in a concrete-like grout.
The DOE Oak Ridge Office of Environmental Management (OREM) is beginning planning that could be part of a two-phase feasibility study that will consider entombment and other options for cleaning up the facility, said Bill McMillan, ORNL portfolio federal project director for the office. Other options would include taking no action and removing residual fuel salts at the reactor site.
The feasibility study could take three years, McMillan told the Oak Ridge Site Specific Advisory Board on Wednesday.
Among other steps, the planning process could evaluate the hazards and structures at the Molten Salt Reactor Experiment and identify what data is needed for the study. Below-ground, radioactive fuel drain tanks might need to be sampled, and DOE might have to consider what pipes must be removed from the building, McMillan said.
During the second phase of the study process, McMillan said, DOE would gather data and analyze its findings. The feasibility study could be developed at the end of the second phase, McMillan said.
The feasibility study will investigate how much of the MSRE should be entombed, if that is the approved approach; model potential long-term releases from the site, which includes isotopes that could have a high dose field for the next 300 years; evaluate the use of a material to capture fluorine generated in fuel drain tanks; and evaluate the grout materials that could be used to bury underground components, including the two fuel drain tanks and a fuel flush tank, McMillan said. He said monitoring systems will be required for long-term stewardship.
McMillan said the U.S. Nuclear Regulatory Commission has recognized entombment as a decommissioning option for nuclear facilities since the 1970s. That approach has been used elsewhere at ORNL and DOE’s Hanford Site in Washington state and Savannah River Site in South Carolina.
“We believe it is something to continue looking at,” McMillan said.
The Molten Salt Reactor Experiment operated from June 1965 to December 1969. It tested reactor fuels for use in breeder reactor technologies, McMillan said. The reactor used a molten salt fuel mixture that included uranium-235 and uranium-233 fluoride salts and a trace of plutonium. Most of its fuel has been removed, but residual salts remain in the underground drain tanks.
A 1998 federal record of decision (ROD) described an interim action meant to reduce potential risk from the salts. It called for removing fuel and flush salts from the facility by separating uranium from the highly radioactive salts and treating and storing the uranium as uranium oxide, among other activities related to the fuel salt cleanup. After the salts were removed, the tanks and associated equipment were to be managed in place, with the tanks and reactor components to be addressed as part of a subsequent decontamination and decommissioning. If entombment is now determined to be the best option, the record of decision could be revised, McMillan said.
Entombment would bury underground structures that include the fuel drain tanks, which are contaminated with radioactive fission products, including cesium-137 and strontium-90. The Energy Department has previously said parts of the Molten Salt Reactor Experiment are too radioactively “hot” for humans. There are two fuel drain tanks and one flush tank in the underground drain tank cell.
While below-ground structures could be buried in grout and covered with a slab, above-ground structures such as the high bay area above the reactor vessel and fuel drain tanks could be removed, McMillan said. An office building that supports the site could also eventually be removed, he added. McMillan said there is the one main high bay building at MSRE and three to four support facilities, including the office building.
The Energy Department doesn’t have a cost estimate for entombment yet, but McMillan said it would be less expensive than removing the fuel salts.
McMillan told the advisory board that the largest immediate hazard at MSRE is fluorine gas that is constantly being generated from the residual fuel salts in the drain tanks. Workers purge the drain tanks every six months and treat the accumulating fluorine gas, but OREM is designing a continuous purge system to replace that system. The continuous purge system could start operating by 2021. It could pump nitrogen in and vent gases like fluorine out, and it would be a passive system that would be less hazardous to workers, McMillan said.
OREM is also addressing other issues at the MSRE, including replacing aging electrical systems used by critical components and installing a new sump pump system to reliably keep water out of the building.
