A melted-down reactor in the Fukushima nuclear power station resting precariously on corroded supports could be toppled over by a strong earthquake, new photos have revealed.
The images, which were taken by a robot, showed the reactor was resting unsteadily on a fragile frame of corroded supports.
While the interior of the damaged Dai-ichi reactors in Japan was too radioactive for humans to enter safely, a remote-controlled robotic camera has sent back pictures from reactor No 1.
The reactor was one of three which melted down in 2011, when a catastrophic tsunami caused by an earthquake knocked out the cooling systems at the plant.
The 9.0 magnitude earthquake on March 11, 2011, was so forceful it shifted the Earth off its axis and triggered a tsunami which swept over the main island of Honshu and killed more than 18,000 people.
The huge wave surged over defences at the Fukushima nuclear power plant and flooded the reactors, sparking a major disaster.
As a result there were three nuclear meltdowns, three hydrogen explosions and radioactive contamination was released.
The new photos, reported by The Times, show the concrete base on which the reactor vessel rested had been substantially dissolved by the molten reactor fuel, which leaked from the reactor core.
The vessel, which weighed 440 tonnes, was now being supported by the skeletal steel frame of the concrete base.
However, this was not believed to be enough to hold it in place if the earth shook violently.
With a magnitude of 9.0, the earthquake that caused the tsunami was one of the most powerful on record.
Aftershocks from the earthquake continue, which are weaker than the original disaster but still dangerous and posing a risk to the safety of the nuclear plant.
Just months ago, on March 16 of this year, four people died and 225 more were injured when a magnitude 7.4 earthquake struck off the coast of Fukushima.
Terrified witnesses said buildings were left shaking after what felt like two quakes, with the second lasting for around two minutes.
People in Japan are being warned that a similarly strong earthquake could strike again in the weeks following.
The tremor registered magnitude 7.3 and as high as a 6-plus on the Japanese shaking intensity scale in some areas - too strong for people to stand, according to public broadcaster NHK.
While it was not thought to have caused serious damage at the plant, it was impossible to predict when future earthquakes would strike and how destructive they would be.
Speaking of the reactor site last week, the chairman of Japan’s Nuclear Regulation Authority said: “We remain concerned about whether it will withstand a strong quake.”
The task of decommissioning has hardly begun at Fukushima, some 11 years after the disaster occurred.
Authorities have insisted they will dismantle the plant piece by piece and remove the deadly molten fuel inside.
Nothing like this had ever been attempted before, and each faltering step towards the goal seemed to be followed by some kind of setback.
Underground water has been a problem, flowing from the hills above the seaside plant, where it becomes irradiated by the ruined reactors.
This water has been collected by pumps into ever-proliferating storage tanks since the accident.
The Japanese government’s announcement that it would pour more than a million tonnes of the water, purged of some but not all of its contaminants, into the Pacific ocean, has prompted angry protests from local fishermen and from Japan’s east Asian neighbours.
Many of those affected have claimed the Fukushima disaster could have been avoided.
Prosecutors claimed the executives had access to studies predicting the risk of a nuclear calamity from a tsunami.
And in 2012 a special commission concluded Fukushima was "a profoundly man-made disaster".
