Nuclear engineers in Japan are preparing to move uranium and plutonium fuel rods at Fukushima, their most difficult and dangerous task since the plant's runaway reactors were brought under control two years ago.

Tokyo Electric Power (TEPCO) is expected this month to begin removing fuel rods from a pool inside a reactor building at the tsunami-hit plant, in a technically challenging operation that will test the utility's expertise after months of setbacks and glitches.

Experts say the operation is a tricky but essential step in a probably decades-long process of dismantling and recovery after the worst atomic accident in a generation.

But, they add, it pales in comparison with a much more complex task that awaits engineers in the future. They will have to remove the misshapen cores of three reactors that went into meltdown, probably relying on technology that has not yet been invented.

More than 1,500 nuclear fuel assemblies - bundles of rods - must be pulled out of the storage pool where they were being kept when a tsunami smashed into Fukushima in March 2011.

The reactor that the pool serves - No 4 - was not in operation at the time. But hydrogen from Reactor No 3 escaped into the building and exploded, tearing the roof off and leaving it at the mercy of natural hazards like earthquakes, storms or another tsunami.

TEPCO says it has not yet found any damage to the assemblies at No 4, which contain a mixture of uranium and plutonium, but will be monitoring for abnormalities.

The removal of fuel is part of regular work at any nuclear power plant, but "conditions are different from normal because of the disaster", said company spokeswoman Mayumi Yoshida.

"It is crucial. It is a first big step toward decommissioning the reactors," she said. "Being fully aware of risks, we are determined to go ahead with operations cautiously and securely."

Chunks of debris that were sent flying into the pool by explosions have largely been removed and a crane has been installed. A protective hood has been erected over the building's skeleton to contain any radioactive leaks.

A remotely controlled grabber will sink into the pool and hook onto a fuel assembly, which it will pull up and place inside a fully immersed cask.

The 4.5-meter long bundles weighing 300 kg have to be kept in water throughout the operation to keep them cool.

The cask weighing 91 metric tons will then be hauled from the pool - containing as many as 22 fuel assemblies and a lot of water - to be loaded onto a trailer and taken to a different storage pool, where the operation will be reversed.

If the rods are exposed to the air they would release radiation and could heat up, a process that if left unchecked could lead to a self-sustaining nuclear reaction. TEPCO says that is unlikely, but skeptics say that with so many unknowns in the novel operation, there is potential for a catastrophe.

"This is the first practical milestone for the project," said Hiroshi Miyano, a nuclear systems expert and visiting professor at Hosei University in Tokyo.

"Any trouble in this operation will considerably affect the timetable for the entire project," he said. "This is an operation TEPCO cannot afford to bungle."

The full decommissioning of Fukushima is likely to take decades and include tasks that have never been attempted anywhere in the world.

Meanwhile, villages and towns nearby remain largely empty, their residents unable or unwilling to return to live in the shadow of the leaking plant because of the fear of radiation.

Meanwhile, inspectors from the International Atomic Energy Agency arrived in Tokyo on Wednesday to monitor marine pollution near Fukushima as China demanded Japan provide "accurate" information on how it is handling the crisis.

China told the UN General Assembly it was worried about radioactive water leaks from the Japanese plant, which went into meltdown after being hit by a tsunami in March 2011.

Agence France-Presse

(China Daily 11/08/2013 page11)