By Aaron Sheldrick and Antoni Slodkowski: After disaster, the deadliest part of Japan's nuclear clean-up!

TOKYO, Aug 14 (Reuters) The operator of Japan's crippled Fukushima nuclear plant is preparing to remove 400 tonnes 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 earthquake hit the area. Tokyo Electric Power Co (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 put off the removal of all the assemblies successfully. "They are going to have difficulty in removing a significant number of the rods,"said Arnie Gunderson, 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 Gunderson and other nuclear experts. That could lead to a worse disaster than the March 2011 nuclear crisis at the Fukushima plant, the word'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 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. "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. Giant Frame: The fuel assemblies are in the cooling pool of the No. 4 reactor, and Tepco has erected a giant steel frame over the top of the building after removing debris left behind by an explosion that rocked the unit during the 2011 disaster. The structure will house the cranes that will carry out the delicate task of extracting fuel assemblies that may be damaged by the quake, the explosion or corrosion from salt water that was poured into the pool when fresh supplies ran out during the crisis. The process will begin in November and Tepco expects to take about a year removing the assemblies, spokesman Yoshikazu Nagai told Reuters by e-mail. It's just one instalment in the decommissioning process for the plant forecast to take about 40 years and cost $11 billion.