Damage from disaster so severe that clean-up expected to take decades, according to latest examination of nuclear plant

One of Japan's crippled nuclear reactors still has fatally high radiation levels and much less water to cool it than officials estimated, according to an internal examination that renews doubts about the plant's stability.

A tool equipped with a tiny video camera, a thermometer, a dosimeter and a water gauge was used to assess damage inside the number two reactor's containment chamber for the second time since the tsunami swept into the Fukushima Daiichi plant, a year ago.

The data shows the damage from the disaster is so severe the plant operator will have to develop special equipment and technology to tolerate the harsh environment, and decommission the plant. The process is expected to last decades.

The other two reactors that had meltdowns could be in even worse shape. The number two reactor is the only one officials have been able to closely examine so far.

Tuesday's examination, with an industrial endoscope, detected radiation levels up to 10 times the fatal dose inside the chamber.

Plant officials previously said more than half of the melted fuel had breached the core and dropped to the floor of the primary containment vessel, some of it splashing against the wall or the floor.

Particles from melted fuel have probably sent radiation levels up to a dangerously high 70 an hour inside the container, said Junichi Matsumoto, spokesman for the plant operator, Tepco. The figure far exceeds the highest level previously detected, of 10 sieverts an hour, which was detected around an exhaust duct shared by the number one and two units last year.

"It's extremely high," he said, adding that an endoscope would last only 14 hours in those conditions. When locating and removing melted fuel during the decommissioning process, he said, "we have to develop equipment that can tolerate high radiation".

The probe also found that the containment vessel – a beaker-shaped container enclosing the core – had cooling water up to only 60cms (24ins) from the bottom, far below the 10 metres estimated when the government declared the plant stable in December. The plant is continuing to pump water into the reactor.

Video footage taken by the probe showed the water inside was clear but contained dark yellow sediments, believed to be fragments of rust, paint or dust.

An investigation carried out in January failed to find the water surface, and provided only images showing steam, unidentified parts and rusty metal surfaces scarred by exposure to radiation, heat and humidity. Finding the water level was important to help locate damaged areas where radioactive water is escaping.

Matsumoto said the actual water level inside the chamber was way off the estimate, which had used data that turned out to be unreliable.

But the results do not affect the plant's "cold shutdown status" because the water temperature was about 50C, indicating the melted fuel is cooled.

Three Daiichi reactors had meltdowns, but the number two reactor is the only one that has been examined because radiation levels inside the reactor building are relatively low and its container is designed with a convenient slot to send in the endoscope.

The exact conditions of the other two reactors, where hydrogen explosions damaged their buildings, are still unknown. Simulations have indicated that more fuel inside number one has breached the core than the other two, but radiation at number three remains the highest.

The high radiation levels inside the number two reactor's chamber mean it is inaccessible to the workers, but parts of the reactor building are accessible for a few minutes at a time – with the workers wearing full protection.

The 2011 earthquake and a tsunami set off the worst nuclear accident since Chernobyl, sending three reactor cores into meltdown and causing massive radiation leaks.

The government said in December that the reactors had been safely cooled, and the plant had been stabilised, but experts have questioned its vulnerability.

During a recent visit by journalists, the head of the plant said it remained vulnerable to strong aftershocks and tsunamis, and that containing contaminated water and radiation remained a challenge. Radioactive water had leaked into the ocean several times already.

Workers had found a fresh leak of 120 tonnes from one of the hoses at a water treatment unit this week, with an estimated 80 litres escaping into the ocean, Matsumoto said. Officials are still investigating its impact.

Fukushima's accident has caused public distrust and concerns about nuclear safety, making it difficult for the government to start up reactors even after regular safety checks. All but one of Japan's 54 reactors are offline, with the last scheduled to stop in early May.