The operator of Japan's crippled Fukushima nuclear plant is preparing to remove 400 tons of highly irradiated spent fuel from a damaged reactor building, a dangerous operation that has never been attempted before on this scale.
Containing radiation equivalent to 14,000 times the amount released in the atomic bomb attack on Hiroshima 68 years ago, more than 1,300 used fuel rod assemblies packed tightly together need to be removed from a building that is vulnerable to collapse, should another large earthquake hit the area.
Tokyo Electric Power (Tepco) is already in a losing battle to stop radioactive water overflowing from another part of the facility, and experts question whether it will be able to pull off the removal of all the assemblies successfully.
"They are going to have difficulty in removing a significant number of the rods," said Arnie Gundersen, a veteran U.S. nuclear engineer and director of Fairewinds Energy Education, who used to build fuel assemblies.
The operation, beginning this November at the plant's Reactor No. 4, is fraught with danger, including the possibility of a large release of radiation if a fuel assembly breaks, gets stuck or gets too close to an adjacent bundle, said Gundersen and other nuclear experts.
That could lead to a worse disaster than the March 2011 nuclear crisis at the Fukushima plant, the world's most serious since Chernobyl in 1986.
No one knows how bad it can get, but independent consultants Mycle Schneider and Antony Froggatt said recently in their World Nuclear Industry Status Report 2013: "Full release from the Unit-4 spent fuel pool, without any containment or control, could cause by far the most serious radiological disaster to date."
Tepco has already removed two unused fuel assemblies from the pool in a test operation last year, but these rods are less dangerous than the spent bundles.
Extracting spent fuel is a normal part of operations at a nuclear plant, but safely plucking them from a badly damaged reactor is unprecedented.
(Read more: Fukushima clean-up turns toxic for Japan's Tepco)
"To jump to the conclusion that it is going to work just fine for the rest of them is quite a leap of logic," said Gundersen.
The utility says it recognises the operation will be difficult but believes it can carry it out safely.
Nonetheless, Tepco inspires little confidence. Sharply criticised for failing to protect the Fukushima plant against natural disasters, its handling of the crisis since then has also been lambasted.
Last week, Prime Minister Shinzo Abe ordered the government to take a more active role in controlling the overflow of radioactive water being flushed over the melted reactors in Units 1, 2 and 3 at the plant.