Pebble-like sediment believed to be nuclear fuel debris is lifted by a special device inside the No. 2 reactor containment vessel at the Fukushima Daiichi Nuclear Power Station. (Photo courtesy of Tokyo Electric Power Co.)

TOKYO -- The government and Tokyo Electric Power Co. (TEPCO) are set to launch full-scale probes of the inside of the No. 1 through No. 3 reactors at the disaster-stricken Fukushima Daiichi Nuclear Power Station this fiscal year, in an attempt to determine which reactor to work on first to remove fuel debris -- a critical step for decommissioning the facility.

However, the interior of the No. 2 reactor, which is most likely to be the first to go through the debris removal process, has turned out to be different from what had originally been expected, underscoring the difficulties entailing the removal work. Since many companies are involved in the process, how to pass down the know-how acquired over the course of the more than 30 year-decommissioning process also poses a challenge.

"At present, it is difficult to clearly say we are going to remove all fuel debris," said Akira Ono, who leads the decommissioning project, at a regular press conference by TEPCO on March 28, while noting that the utility will not back down from its ultimate goal of full debris removal.

If TEPCO fails to take out all debris from the nuclear plant, the very premise for dismantling the facility and returning the plot to its original state will be undermined. Such a scenario would adversely affect the disaster recovery plans envisaged by the national government and the Fukushima Prefectural Government. While awareness about the difficulty of debris removal has been shared among concerned parties, the actual dismal situation had not been recognized until TEPCO conducted the first debris survey at the No. 2 reactor on Feb. 13.

In that survey, a remotely controlled special device that was injected into the No. 2 reactor's containment vessel succeeded in lifting portions of sediment accumulated at the bottom, which were believed to be fuel debris. Officials involved were relieved because they "had been worried the material would not move at all," according to Ono.

The radiation level of the material, measured at a distance of some 30 centimeters, was 7.6 sieverts per hour, far less than anticipated. If the sediment contained a good portion of nuclear fuel, the radiation doses ought to have been several hundred sieverts per hour, even eight years after the 2011 nuclear meltdowns.

This finding suggested that the sediment that TEPCO came in contact with in the survey was not the main nuclear fuel debris it was looking for. Many speculate that the surface of the sediment may mainly consist of metals including cladding tubes that used to cover nuclear fuels.

The question now is whether fuel debris exists beneath the surface of the sediment or if nuclear fuel still remains within the reactor pressure vessel, or even somewhere else. There are currently no prospects for TEPCO to ascertain an accurate distributions of debris.

The material that was lifted in the survey mostly comprised pebble-like sediment, weighing less than 1 kilogram in total. Meanwhile, fuel debris generated in the core meltdowns is estimated to total 237 metric tons at the No. 2 reactor alone and a combined 880 tons at the No. 1 through No. 3 reactors.

At the No. 2 reactor, TEPCO will conduct a more detailed survey on debris possibly in the latter half of this fiscal year and attempt to collect small amounts of samples. At the No. 1 reactor, several apparatus including a robot submarine will be used to launch a full-scale survey inside the reactor to try to collect debris this fiscal year. As for the No. 3 reactor, the power company is apparently planning to prioritize removal of spent fuel, as related devices have gone through a series of glitches.

Unlike the other reactors, the No. 2 reactor did not suffer a hydrogen explosion in the 2011 disaster. Therefore, the No. 2 reactor remains the primary candidate for the first full-scale debris removal work, which is hoped to start in 2021.

With regard to the No. 1 and No. 3 reactors, the utility has yet to be able to reach materials appearing to be debris. The decommissioning of the nuclear plant is scheduled to be completed in 2051, a full 40 years after the triple meltdowns, but a concrete path toward that goal is not yet in sight.

"We have no choice but to remove whichever debris we can," said a senior official with the Ministry of Economy, Trade and Industry.

Naoyuki Takaki, professor of nuclear engineering at Tokyo City University, commented, "There could ultimately be a decision to stop debris removal after pulling out as much debris as possible. In that case, we would have no option but to consider building a sarcophagus like the one at the Chernobyl nuclear plant."

(Japanese original by Toshiyuki Suzuki, Science & Environment News Department)