Photo/IllutrationThe Fukushima No. 1 nuclear power plant (Asahi Shimbun file photo)

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Locating all the nuclear fuel debris and ascertaining the precise chain of events that led to the triple meltdown at the Fukushima No. 1 nuclear power plant was never going to be easy.

And just what a hard road lies ahead is becoming more apparent as plant operator Tokyo Electric Power Co. prepares for the day when decommissioning will start.

Over the past year, TEPCO has used remote-controlled robots to investigate the facility's wrecked interior. But the images sent back offer only a partial picture of the melted nuclear fuel and numerous obstacles to be overcome.

Of the three meltdowns in 2011, the one in the No. 1 reactor was the most severe, according to TEPCO.

It said almost all of the nuclear fuel in the No. 1 reactor likely melted through the bottom of its pressure vessel.

Images sent by a robotic probe in March 2017 showed numerous deposits in sandy condition submerged in water at the base of the reactor’s containment vessel. But no nuclear fuel debris that may lie below was visible.

More progress was made during investigations into the No. 2 reactor over the past year.

In January 2017, a remote-controlled camera hanging from a pole snapped images of dark chunks scattered in the area beneath the reactor’s pressure vessel.

It was the first confirmation of conditions in that area of any of the three reactors. Gaping holes were seen in a platform used by plant workers. The following month, a robotic probe called Scorpion was deployed, but it got stuck and was unable to transmit data.

A further probe by a remote-controlled camera in January turned up images of a nuclear fuel assembly handle near the bottom of the containment vessel. Given that fuel assemblies are about 4 meters long and the handle is mounted on the top, TEPCO said most of the nuclear fuel assemblies must have melted through the pressure vessel to land at the bottom of the containment vessel. Pebbly deposits strewn around the handle are believed to be the nuclear fuel debris, officials said.

An underwater robot used to explore the No. 3 reactor in July showed that the unit lies submerged in more water than the No. 1 and No. 2 reactors.

Images taken by the robot showed clusters of brown icicles hanging from a structure at a lower part of the pressure vessel. Similar icicles that resembled melted rocks could also be seen below.

TEPCO said this is the first time for nuclear fuel debris to be confirmed at the plant.

The government and TEPCO have set a goal of starting a test project in 2021 to retrieve nuclear fuel debris at one of the reactors.

But sticking to that plan is proving to be increasingly difficult. The robotic probes drove home the extent of damage inside the reactors. It is more serious than initially thought.

The probes also revealed a puzzling situation about the No. 2 reactor: Radiation levels at a site away from deposits of nuclear fuel debris are higher than those at a site close to them.

The government and the utility have yet to figure out how to remove the debris while keeping workers safe from radiation exposure when full-scale decommissioning starts. Also not decided is where to store the debris.

Fukushima prefectural authorities insist that the debris and other waste from the plant should be permanently stored outside of the prefecture.

But there has been no discussion to date on how to find municipalities that may agree to take the waste.

(This article was written by Chikako Kawahara and Yusuke Ogawa.)