Veterans of the Three Mile Island cleanup said that a much larger task faced the Japanese engineers who are trying contain and secure the damaged Fukushima Daiichi reactors. And Three Mile Island took 14 years.
Lake Barrett, the senior Nuclear Regulatory Commission engineer at Three Mile Island during the early phases of the cleanup said by comparison, “it was a walk in the park compared to what they’ve got.”
The Fukushima Daiichi reactors are similar to those in Pennsylvania — “the cores are probably really similar, partially melted,” Mr. Barrett said — but engineers pointed out several key differences in the aftermath of the accidents. In Japan, four separate reactors are damaged, and fixing each one is complicated by the presence of its leaking neighbors. It will also require a major infusion of equipment to replace parts far from the reactor’s core, like pumps and switchgear that were destroyed by the tsunami.
In the short term, weather is a factor: according to engineers who managed the American cleanup, which ran from 1979 to 1993, Tokyo Electric Power has only a few weeks to patch up the three smashed secondary containments before the coming rainy season, when downpours could wash more contamination into the environment. And the company will have to carefully watch that the number of workers with the necessary skills do not burn out under the size of the task, or absorb so much radiation that they have to quit.
Still, Mr. Barrett and others say that the mess at Fukushima Daiichi can be contained, cleaned up and even securely wrapped up for long-term disposal. The plant may benefit from past experience, because it is the second major accident worldwide in a big water-cooled reactor, they say.
The first task, they agree, is to fill the reactors and the spent fuel pool with water that can be pumped out again, cooled and then returned to the reactors. That would sharply reduce the possibility of generating new hydrogen and new explosions, and would go a long way toward declaring that the plants were stable, a point that the N.R.C. observed recently that Fukushima Daiichi had not reached.
Right now the reactors are in “feed and bleed” mode, adding clean water and cooling the fuel by letting that water boil off or dribble out, but such bleeding allows radiation leakage. “Whatever you bleed is letting cesium out,” said Mr. Barrett, referring to the radioactive isotope. Cooling with recirculating water could end releases of radioactive materials, but will require new pumps and possibly new piping, experts said.
Before that new equipment can be installed, engineers will have to clean up the water in the basements of the reactor buildings, the turbine buildings and other structures. At Three Mile Island, water in the reactor building and the primary auxiliary building gave radiation doses as high as 1,000 rem an hour, said Ronald L. Freemerman, a Bechtel engineer who was the project manager of the cleanup. That meant a worker would hit the N.R.C.’s annual limit in about a minute. The water can be pumped through filters that will strain out the radioactive elements.
Engineers from Three Mile Island laid out the three next steps:
スリーマイル島のエンジニアはそれに続く3つのステップを説明して見せた。
First, decontaminate the walls and floors, to hold down the potential radiation dose. “They have to economize on how they expose these people,” Mr. Freemerman said, or the company will run out of trained workers.
Second, rebuild the secondary containments of units 1, 2 and 4, and fix or replace the heavy cranes just beneath their ceilings. That would allow workers to defuel the reactor. That step alone took five years at Three Mile Island, where no buildings had to be rebuilt.
Third, peek inside the reactor vessel and figure out what tools will be needed to remove the wrecked fuel in the core. Three Mile Island was a surprise, Mr. Freemerman said, because so much of the core had melted and flowed beneath a support made of five plates of thick steel. Another veteran of the cleanup, Michael McGough, said only then did they realize they would need new remote-controlled tools to cut through the metal, to get to the material below.
Mr. McGough’s technicians worked from a trailer outside the containment vessel, manipulating a cutting tool that was operating under about 40 feet of water. They also used long-handled picks and scoops to break apart the fused mass of ceramic fuel pellets and metal. “Basically we dug our way down through that debris until we got everything removed,” Mr. McGough said.
At Three Mile Island, technicians then painstakingly loaded debris into shielded casks, under water to shield themselves from radiation, and then brought the casks to the surface. Eventually about 150 tons of radioactive rubble was shipped to an Energy Department laboratory in Idaho Springs, Idaho, where it still sits, waiting, as all used American fuel does, for a final resting place.
Japan may have another option if the wrecked core isn’t too thoroughly mixed with other materials. It already has a reprocessing plant, where old fuel is chopped up, dissolved in acid, and then sorted, with its plutonium being removed, and the uranium sorted out for possible re-use. But that process is likely years away.
