By Yoko Kubota
TOKYO (Reuters) - The operator of Japan's troubled Monju prototype fast-breeder reactor said Friday it had completed the potentially risky retrieval of a 3.3-tonne device that had fallen into a reactor vessel, although a restart of the long-idled project remained in doubt as worries mount over nuclear safety.
The Japan Atomic Energy Agency still needs approval from national and local authorities to resume operations at the project, originally launched to promote Japan's energy independence.
The retrieval, completed in the early hours of Friday, took about eight hours and involved more than 20 workers, JAEA said.
"The next step is to inspect the device and see what the malfunctions were, and also to check whether any parts had fallen from it," said a JAEA spokesman. "We will have to retrieve any pieces that have fallen off."
Located on the coast of the Sea of Japan 400 km (250 miles) west of Tokyo, the 280-megawatt Monju reactor is designed to burn plutonium refined from spent fuel at conventional nuclear reactors to create more fuel. The government hoped it would help to reduce Japan's reliance on imports for its energy needs.
But the prototype, named after the Buddhist deity symbolizing wisdom, was shut for 14 years to May 2010 after a liquid sodium leak from a cooling system. Three months after restarting, Monju had to halt again after the 3.3 tonne fuel loading device fell into the reactor vessel and got stuck.
Removing the device was a potentially risky operation because Monju's fuel coolant can catch fire when it comes into contact with air.
The JAEA planned to resume power generation tests by next April, but an agency spokesman said that may be pushed back given the protracted crisis at Tokyo Electric Power Co's Fukushima Daiichi nuclear plant and a need to check the project's own safety standards.
Both national and local governments must approve any resumption of testing, but local governments have come under pressure from residents wary about nuclear safety in the wake of the Fukushima disaster, the world's worst atomic crisis in 25 years.
Fast-breeder reactors have been beset by technical problems and many countries have abandoned costly programs to develop them. Some critics oppose them because the high-purity plutonium they produce can be used to make nuclear weapons.
But if Japan gives up on the 900 billion yen ($11 billion)project, it faces tough choices on what to do with spent fuel rods from its 54 nuclear reactors, which are now generally stored in pools at the reactor sites.
That practice severely compounded the problems at the Fukushima complex.
A medium-term storage facility for nuclear waste is under construction in the village of Mutsu in northern Japan. A longer-term project to build a uranium enrichment and reprocessing plant at nearby Rokkasho, which dates back to the early 1990s, has faced repeated delays and technical difficulties.
($1 = 80.545 Japanese Yen)
(Additional reporting by Shinichi Saoshiro; Editing by Edmund Klamann)