The operator of Japan's crippled Fukushima nuclear plant will as early as this week begin removing 400 tons of highly irradiated spent fuel in a hugely delicate and unprecedented operation fraught with risk.

Carefully plucking more than 1,500 brittle and potentially damaged fuel assemblies from the plant's unstable Reactor No. 4 is expected to take about a year, and will be seen as a test of Tokyo Electric Power Co's ability to move ahead with decommissioning the whole facility - a task likely to take decades and cost tens of billions of dollars.

If the rods - there are 50-70 in each of the assemblies, which weigh around 300 kg (660 pounds) and are 4.5 meters (15 feet) long - are exposed to air or if they break, huge amounts of radioactive gases could be released into the atmosphere.

The hazardous removal operation has been likened by Arnie Gundersen, a veteran U.S. nuclear engineer and director of Fairewinds Energy Education, to trying to pull cigarettes from a crushed pack.

When the time comes, extracting spent fuel from the plant's other reactors, where radiation levels are much higher because of core meltdowns, will be even more challenging. Reactors No. 1 and No. 3 sustained heavier damage than No. 4 as a result of the March 2011 earthquake and tsunami that knocked out power and cooling at the Fukushima station, triggering three meltdowns that sent a plume of radiation into the air and nearby Pacific Ocean.

The urgency to clear Reactor No. 4 of the fuel assemblies is because of the risk in having spent fuel stored at such a height - some 18 meters above ground level - in a building that has buckled and tilted and could collapse if another quake strikes.

Also, if the pool housing the fuel assemblies is punctured and the water drains away, there could be a fire that releases more radiation than during the 2011 disaster, threatening Tokyo, some 200 kms (125 miles) to the south.

"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," independent consultants Mycle Schneider and Antony Froggatt wrote in a recent World Nuclear Industry Status Report.

Challenging Test

Tokyo Electric, or Tepco, has shored up Reactor No. 4 and erected a giant steel frame over the top of the building which lost its roof in the 2011 explosion. It says the building can withstand shaking similar to the 2011 quake.

The utility has removed the larger debris left from that explosion from the pool that has been cooling the fuel assemblies for the past two and a half years. As the water used to cool the rods has had to be pumped in from the ocean, there is a risk that some may have corroded from the seawater.

Tepco has already removed two unused fuel assemblies from the pool in a test operation at Reactor No. 4 last year, but these rods are less dangerous than the spent bundles. Extracting spent fuel is a normal part of a nuclear plant's operations, but there is little normal about Fukushima today.

Widely criticised for a series of missteps in its handling of the post-disaster clean-up, Tepco says it recognises the operation will be difficult, but believes it can carry it out safely. The utility has struggled to stop radioactive water overflowing from another part of the Fukushima facility, and experts have questioned whether it should still be in charge of the clean-up and decommissioning.

Tepco says the assembly removal process will begin around mid-November, withholding the actual date for what it says are security reasons.

"I agree with doing Unit 4 first as it may give them some experience as to how to approach the more difficult jobs at Units 1-3," said Dale Bridenbaugh, a former General Electric engineer and manager, who has previously worked at Japanese nuclear plants including Fukushima Daiichi's Reactor No. 1. "It will also provide a pathway for use in deciding how to move the remaining fuel and debris from Units 1-3."

Trusting Tepco

The steel frame that now perches above the damaged reactor holds the cranes that will pluck the 1,331 used radiated fuel assemblies, which are packed tightly together, and another 202 unused assemblies also stored in Reactor No. 4's cooling pool. The cranes and equipment normally used to extract used fuel from the reactor's core were destroyed in the disaster.

The fuel assemblies are held in a 10 x 12 meter concrete pool, the base of which is on the fourth story of the complex. The assemblies - which contain plutonium, one of the most toxic substances known - are under 7 meters of water.

"They must be handled one by one, very carefully," Shunichi Tanaka, the chairman of Japan's Nuclear Regulation Authority, said after approving the operation recently.

The assemblies must first be pulled from their storage racks and individually placed in a heavy steel chamber - kept all the while under water to prevent overheating. The chamber, which weighs around 90 tons when filled and shields the radiation pulsating from the rods, is then removed from the pool, lowered to ground level and transported by trailer to a common storage pool in an undamaged building about 100 meters away.

Teams of six will operate in 2-hour shifts, with as many as three rotations each day, manually guiding and operating the cranes that will transfer the rods, Tepco said.

Spokesman Yoshikazu Nagai said Tepco will carry out a test operation for moving the heavy chamber later on Wednesday.

"A lot of debris fell into the fuel pool as a result of the March 2011 hydrogen explosion. The large pieces of debris have been removed," Takashi Hara, a Tepco employee in charge of the fuel removal, told reporters during a recent plant tour. "If, for some reason, the water levels drop, the fuel would quickly heat up," he added.

The operation to remove all the fuel would normally take about 100 days, and Tepco initially planned to take two years before halving that timeframe in recognition of the urgency.

"We are all worried ... Every day we read news about the plant, and we are aware of their plans to remove the spent fuel rods," said Ichiro Kazawa, 61, a former real estate manager from the nearby town of Hirono. He lost his home to the tsunami and now lives in temporary housing.

"Everybody's concerned and just hoping there will be no major accidents. No one here trusts Tokyo Electric