TOKYO -- By pinning down the location of melted fuel at the Fukushima Daiichi nuclear power plant, Tokyo Electric Power Co. Holdings has made modest but valuable progress on the toughest part of the decommissioning process.

The biggest hurdle to clear before the teardown can begin is confirming where the fuel is and how much is there so it can be removed. Scanning by Tepco and the government found that the bulk of unit 2's fuel is likely at the bottom of the pressure vessel.

"This is extremely important information," said Tokyo City University professor Naoyuki Takaki. "It makes it easier to decide on removal methods and the technology that has to be developed."

Tepco may be able to learn from the cleanup after the 1979 disaster at the Three Mile Island nuclear plant in the U.S., another case where melted fuel remained in the pressure vessel. Fuel debris was successfully removed from the facility after more than six years of preparation.

Discussion of removing melted fuel from units 1 to 3 at Fukushima Daiichi had centered on filling the affected containment vessels with water. Flooding just the pressure vessel, as was done at Three Mile Island, would be simpler, likely requiring less time and money.

The Fukushima Daiichi disaster was worse than the partial meltdown at Three Mile Island, however, and holes may have been punched in the pressure vessel. If the damage is minor and the holes can be plugged, the Three Mile Island lessons can still apply.

The amount of fuel involved is also much higher than in the earlier incident. Melted fuel may have solidified around control rods or other debris. Gathering more detailed information will be crucial. "It'll be important to look at conditions around the pressure vessel," said Naohiro Masuda, who heads a Tepco arm in charge of decommissioning.

The high radiation levels inside the containment vessel make it too dangerous to approach. The government and Tepco will use remote-controlled robots and cameras to examine the area around the pressure vessel. A full survey of the inside of the structure will likely start in fiscal 2018.

Tepco finally located fuel inside the No. 2 reactor five years after the disaster, and nothing has yet been confirmed at the other two units. Imaging of the No. 1 reactor in 2015 found nothing in the pressure vessel, suggesting that nearly all of the fuel broke through the structure and fell to the bottom of the containment vessel. Unit No. 3 has not yet been scanned.

This just scratches the surface of the unprecedented work required as part of a decommissioning process expected to take three or four decades. The government and Tepco will have to choose an extraction method and develop technology to tackle this daunting task. Though the aim is to start removing fuel from one of the reactors in 2021, the work that needs to be done first may not go according to plan.

(Nikkei)