The Department of Energy is evaluating multiple options for stabilizing the second radioactive waste storage tunnel for the PUREX facility at the Hanford Site, as an Aug. 1 deadline nears.
The Washington state Department of Ecology set the deadline for DOE to present a draft plan for stabilizing the second tunnel after the other PUREX tunnel was found on May 9 to have partially collapsed. The Energy Department outlined the options under consideration at a public meeting Thursday evening and might also have picked up at least one new idea to consider.
DOE met a July 1 state deadline to evaluate the structural integrity of the second tunnel, which holds irradiated equipment from the Plutonium Uranium Extraction Plant. The results of the evaluation were not good, despite the second tunnel’s more robust construction of concrete and steel rather than the wood beam construction of the first tunnel. The second tunnel also was determined to be at serious risk of collapse and in need of stabilization.
The agency is evaluating options on six initial criteria: protection of human health and the environment; capacity to allow future remedial action; ease of implementation; ease of upkeep; speed of implementation; and cost. No option under consideration rates highly on all six criteria.
The least costly option would be to pull a thick sheet of plastic over Tunnel 2, as was done at Tunnel 1 after its breach was filled with sand and soil. But the cover might only last a couple of months, said Al Farabee, representing DOE at the meeting.
Covering the length of the 1,700-foot-long tunnel with a soft- or hard-surface tent would cost more but the upkeep would be easier, according to the DOE presentation. It would provide somewhat better contamination control in the event of a collapse, but still would not provide full containment. The most expensive exterior cover being considered is a sheet-metal building. But it has the potential to trigger a collapse during construction, DOE said.
The environment and human health would be better protected by filling the tunnel with expanding foam. The foam would provide additional stability and contamination control to the tunnel interior, according to DOE. But it has some drawbacks, the department said: It could be difficult to install into the tunnel and its performance in high radiation areas long term is not known. The foam has been considered for other cleanup projects at Hanford, such as filling the glove boxes at the Plutonium Finishing Plant, but has not been used because it is a potential fire hazard, Farabee said.
Filling the tunnel with grout, as is being done at the first tunnel, is another option. The grout and waste could later be cut out of the tunnel in pieces for permanent disposal, according to DOE. But some of the approximately 50 people attending the meeting were dubious that railcars and other items in the grout could be sawed into pieces for disposal or that highly radioactive equipment could be segmented without potentially exposing workers.
The Energy Department also is looking at the possibility of initiating a controlled collapse of the tunnel, which would eliminate air space within the tunnel. The key concern is ensuring contamination control and protection of workers during the planned collapse, according to DOE. The option did not rank well based on cost, although the department has not developed specific estimates yet.
Workers also could move immediately toward retrieving waste from the tunnel, which could provide a permanent solution. But it would be an extended and technically challenging effort for facility and equipment design, construction, and implementation, according to DOE. It could take significantly more time and be much more expensive compared to the other options under consideration, the department said.
Scot Adams, a Tri-City environmental scientist and geologist familiar with the tunnel from previous work at Hanford, offered another option to DOE at the public meeting. He suggested filling the tunnel with sand or bentonite clay, which would not need to be cut out of the tunnel when a permanent cleanup method for the tunnel’s waste is picked. Farabee said DOE knows grout could be flowed into the tunnel and fill voids, based on use of grout at other facilities, including Hanford’s U Plant. But Adams said he thought it would be possible to blow a bentonite powder into the tunnel through the 17 inspection holes along its length. Adams also recommended restoring ventilation to the tunnel to sustain negative pressure. The tunnel has high levels of airborne contamination, he said.
