Barely a fifth of the way into their mission, the engineers monitoring the Scorpion’s progress conceded defeat. With a remote-controlled snip of its cable, the latest robot sent into the bowels of one of Fukushima Daiichi’s damaged reactors was cut loose, its progress stalled by lumps of fuel that overheated when the nuclear plant suffered a triple meltdown six years ago this week.

As the 60cm-long Toshiba robot, equipped with a pair of cameras and sensors to gauge radiation levels was left to its fate last month, the plant’s operator, Tokyo Electric Power (Tepco), attempted to play down the failure of yet another reconnaissance mission to determine the exact location and condition of the melted fuel.

Even though its mission had been aborted, the utility said, “valuable information was obtained which will help us determine the methods to eventually remove fuel debris”.

The Scorpion mishap, two hours into an exploration that was supposed to last 10 hours, underlined the scale and difficulty of decommissioning Fukushima Daiichi – an unprecedented undertaking one expert has described as “almost beyond comprehension”.

Cleaning up the plant, scene of the world’s worst nuclear disaster since Chernobyl after it was struck by a magnitude-9 earthquake and tsunami on the afternoon of 11 March 2011, is expected to take 30 to 40 years, at a cost Japan’s trade and industry ministry recently estimated at 21.5tr yen ($189bn).

The figure, which includes compensating tens of thousands of evacuees, is nearly double an estimate released three years ago.

The tsunami killed almost 19,000 people, most of them in areas north of Fukushima, and forced 160,000 people living near the plant to flee their homes. Six years on, only a small number have returned to areas deemed safe by the authorities.

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Developing robots capable of penetrating the most dangerous parts of Fukushima Daiichi’s reactors – and spending enough time there to obtain crucial data – is proving a near-impossible challenge for Tepco. The Scorpion – so called because of its camera-mounted folding tail – “died” after stalling along a rail beneath the reactor pressure vessel, its path blocked by lumps of fuel and other debris.

The device, along with other robots, may also have been damaged by an unseen enemy: radiation. Before it was abandoned, its dosimeter indicated that radiation levels inside the No 2 containment vessel were at 250 sieverts an hour. In an earlier probe using a remote-controlled camera, radiation at about the same spot was as high as 650 sieverts an hour – enough to kill a human within a minute.

Shunji Uchida, the Fukushima Daiichi plant manager, concedes that Tepco acquired “limited” knowledge about the state of the melted fuel. “So far we’ve only managed to take a peek, as the last experiment with the robot didn’t go well,” he tells the Guardian and other media on a recent visit to the plant. “But we’re not thinking of another approach at this moment.”

Robotic mishaps aside, exploration work in the two other reactors, where radiation levels are even higher than in reactor No 2, has barely begun. There are plans to send a tiny waterproof robot into reactor No 1 in the next few weeks, but no date has been set for the more seriously damaged reactor No 3.

Naohiro Masuda, the president of Fukushima Daiichi’s decommissioning arm, says he wants another probe sent in before deciding on how to remove the melted fuel.

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Despite the setbacks, Tepco insists it will begin extracting the melted fuel in 2021 – a decade after the disaster – after consulting government officials this summer.

But Shaun Burnie, a senior nuclear specialist at Greenpeace Germany who is based in Japan, describes the challenge confronting the utility as “unprecedented and almost beyond comprehension”, adding that the decommissioning schedule was “never realistic or credible”.

The latest aborted exploration of reactor No 2 “only reinforces that reality”, Burnie says. “Without a technical solution for dealing with unit one or three, unit two was seen as less challenging. So much of what is communicated to the public and media is speculation and wishful thinking on the part of industry and government.

“The current schedule for the removal of hundreds of tons of molten nuclear fuel, the location and condition of which they still have no real understanding, was based on the timetable of prime minister [Shinzo] Abe in Tokyo and the nuclear industry – not the reality on the ground and based on sound engineering and science.”

Even Shunichi Tanaka, the chairman of Japan’s nuclear regulation authority, does not appear to share Tepco’s optimism that it will stick to its decommissioning roadmap. “It is still early to talk in such an optimistic way,” he says. “At the moment, we are still feeling around in the dark.”

‘The situation is not under control’

On the surface, much has changed since the Guardian’s first visit to Fukushima Daiichi five years ago.

Then, the site was still strewn with tsunami wreckage. Hoses, pipes and building materials covered the ground, as thousands of workers braved high radiation levels to bring a semblance of order to the scene of a nuclear disaster.

Six years later, damaged reactor buildings have been reinforced, and more than 1,300 spent fuel assemblies have been safely removed from a storage pool in reactor No 4. The ground has been covered with a special coating to prevent rainwater from adding to Tepco’s water-management woes.

Workers who once had to change into protective gear before they approached Fukushima Daiichi now wear light clothing and simple surgical masks in most areas of the plant. The 6,000 workers, including thousands of contract staff, can now eat hot meals and take breaks at a “rest house” that opened in 2015.

But further up the hill from the coastline, row upon row of steel tanks are a reminder of the decommissioning effort’s other great nemesis: contaminated water. The tanks now hold about 900,000 tons of water, with the quantity soon expected to reach 1m tons.

Tepco’s once-vaunted underground ice wall, built at a cost of 24.5bn yen, has so far failed to completely prevent groundwater from leaking into the reactor basements and mixing with radioactive coolant water.

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The structure, which freezes the soil to a depth of 30 metres, is still allowing 150 tonnes of groundwater to seep into the reactor basements every day, said Yuichi Okamura, a Tepco spokesman. Five sections have been kept open deliberately to prevent water inside the reactor basements from rising and flowing out more rapidly. “We have to close the wall gradually,” Okamura said. “By April we want to keep the influx of groundwater to about 100 tonnes a day, and to eliminate all contaminated water on the site by 2020.”