Three Mile Island offers lessons

Three Mile Island offers treasure trove of lessons for Fukushima

March 07, 2012

By NAOYA KON / Staff Writer

THREE MILE ISLAND, Pennsylvania--When engineers inserted a camera probe into the stricken Unit 2 reactor of the Three Mile Island nuclear power plant here more than three decades ago, the images that appeared were quite unlike anything they had anticipated.

The reactor contained 177 nuclear fuel rods when the meltdown occurred in 1979. But the reactor core had hardly retained its original shape.

The camera captured images of fine, pimply and stone-like objects, according to Jack DeVine, 68, who worked for a technical division at General Public Utilities, the plant operator at the time.

Just as DeVine and other officials who were involved in post-disaster response at Three Mile Island discovered, there is no telling what the crippled reactors at the Fukushima plant will have in store.

The findings at Three Mile Island can provide plentiful lessons for the work that lies ahead at the disabled Fukushima No. 1 nuclear plant, located some 220 kilometers northeast of Tokyo.

In a recent interview, DeVine, who also helped set up an organization responsible for decontamination, said it is essential to start the work only after ascertaining the state of the reactor interiors and drawing up phased work programs leading to final decommissioning.

At Three Mile Island, unmanned robots equipped with cameras and endoscopes prowled around the interior of the reactor building as well as the reactor itself to provide a record of what had happened.

Although similar technologies are being deployed at the Fukushima facility, the conditions there, including radiation levels, are much more serious than those at the Three Mile Island plant. There is a strong possibility that new technology will have to be developed.

The Japanese government, along with Tokyo Electric Power Co., operator of the crippled Fukushima plant, jointly released a road map in December for decommissioning the plant. It defined three stages for the process of decommissioning, which is expected to take between 30 and 40 years.

In the case of Three Mile Island, it took 11 years from the time of the accident to finish the fuel removal.

The road map for Japan envisages that it will take more than double that time to achieve the same result. The fuel removal work itself is expected to take between 10 and 15 years.

At Three Mile Island, only one reactor went into a core meltdown, although that affected about 45 percent of the reactor core. The fuel stayed inside the reactor pressure vessel.

By way of contrast, meltdowns affected three reactors at the Fukushima No. 1 plant, and fuel leaked from the reactor containment vessels.

At Three Mile Island, the pressure vessel remained intact, making it possible to continue cooling the melted fuel by pumping water into it.

A major hurdle at the Fukushima No. 1 plant concerns how to fill the containment vessels with water.

Water is needed to shield against radiation and to cool down the fuel while the fuel is being removed. Assessing the extent of damage to the containment vessels and determining ways to repair them will prove key to decisions to be made in the months ahead.

During the operation to remove fuel at Three Mile Island that started in 1985, technicians stood on lead plates above the nuclear reactor and lowered a crane through openings between the plates. They were exposed to radiation doses of about 0.1 millisievert per hour, DeVine said.

In the case of Fukushima, more technicians may be required than at Three Mile Island. This is because different sorts of radiation hot spots dot the interiors of the reactor buildings, according to Roger Shaw, who helped oversee clean-up efforts at Three Mile Island in his capacity as a director of radiation protection.

It took more than four years to remove 99.5 percent of the fuel from the reactor building at Three Mile Island. The removal work finished in January 1990, after it was confirmed that the remaining 0.5 percent of the fuel, which had eluded recovery, was not going to reach re-criticality, or a sustained nuclear fission chain reaction.

At Three Mile Island, the basement of the reactor building remains inaccessible even now, with a radiation reading of about 10 millisieverts per hour. That is because radioactive water flooded the reactor building and permeated the structure during the crisis.

It cost $1 billion to remove the fuel. The Unit 2 reactor, together with Unit 1 that continues to operate, will be decommissioned in 2034.

"Given the fact there is so much rubble and other obstacles at the Fukushima No. 1 plant, more than 10 times the tasks undertaken at Three Mile Island may be waiting to be done at the Fukushima No. 1 plant," said Wataru Mizumachi, 69, chairman of the Expert Group on Occupational Radiation Protection in Severe Accident Management and Post-Accident Recovery under the International Atomic Energy Agency.

"But many lessons can still be learned from the experience in the United States, including how to develop equipment for fuel removal and how to store the fuel," added Mizumachi, who also took part in recovery work following the Three Mile Island meltdown.