TOKYO (Reuters) - Three of the spent fuel assemblies due to be carefully plucked from the crippled Japanese nuclear plant at Fukushima in a hazardous year-long operation were damaged even before the 2011 earthquake and tsunami that knocked out the facility.
The plant's operator, Tokyo Electric Power Co, or Tepco, said the damaged assemblies - 4.5 meter high racks containing 50-70 thin rods of highly irradiated used fuel - can't be removed from Fukushima's Reactor No. 4 using the large cask assigned to taking out more than 1,500 of the assemblies.
One of the assemblies was damaged as far back as 1982, when it was mishandled during a transfer, and is bent out of shape, Tepco said in a brief note at the bottom of an 11-page information sheet in August.
In a statement from April 2010, Tepco said it found two other spent fuel racks in the reactor's cooling pool had what appeared to be wire trapped in them. Rods in those assemblies have pin-hole cracks and are leaking low-level radioactive gases, Tepco spokesman Yoshikazu Nagai told Reuters on Thursday.
The existence of the damaged racks, reported in a Fukushima regional newspaper on Wednesday, came to light as Tepco prepares to begin decommissioning the plant by removing all the spent fuel assemblies from Reactor No. 4.
"The three fuel assemblies ... cannot be transported by cask," Tepco spokeswoman Mayumi Yoshida said in an emailed response to queries on Thursday, referring to the large steel chamber that will be used to shift the fuel assemblies from the pool high up in the damaged reactor building to safe storage.
"We are currently reviewing how to transport these fuel assemblies to the common spent fuel pool," she said.
Tepco is due within days to begin removing 400 tones of the dangerous spent fuel in a hugely delicate and unprecedented operation fraught with risk.
Having to deal with the damaged assemblies is likely to make that task more difficult and could jeopardize a 12-month timeframe to complete the removal that many have already called ambitious.
Three reactors suffered core meltdowns at the Fukushima Daiichi plant north of Tokyo after the March 2011 disaster that triggered explosions and forced the evacuation of 160,000 people from nearby towns and villages.
Tepco, which has floundered in trying to bring the plant under control in the two and a half years since the disaster, is now moving to full decommissioning at the six-reactor facility.
The most urgent task is to remove the fuel assemblies from the unstable Reactor No. 4, which due to their height - about 18 meters above ground level - are more vulnerable to any new earthquake. The operation is seen as a test of Tepco's ability to move ahead with decommissioning the whole facility - a task likely to take decades and cost tens of billions of dollars.
Lake Barrett, a former U.S. nuclear regulator who is advising Tepco, visited the Fukushima site on Wednesday and endorsed preparations for the removal of the assemblies.
"While removal of the fuel is usually a routine procedure in operating a power plant, the damage to the reactor building has made the job more complex," he said, adding he was "genuinely impressed by the thoroughness of the effort and Tepco's contingency planning."
Tepco has said the assembly removal process will begin around mid-November, but has not given an exact date, citing what it says are security reasons.
The assemblies must first be lifted from their storage frames in the pool and individually placed in a steel cask - kept all the while under water to prevent overheating. The cask, weighing around 90 tones when filled, will then be hoisted by crane from the pool, lowered to ground level and transported by trailer to a common storage pool about 100 meters away.
Additional reporting by James Topham; Editing by Ian Geoghegan