* Reactor leaks 8 tonnes of water but none escapes outside
* Incident follows smaller leaks last weekend
* Timing bad as govt seeks consensus to restart reactors
* Tepco stabilising reactors so it can start dismantling (Adds specialist comment, details, background)
TOKYO, Feb 2 More than 8 tonnes of radioactive water leaked from a reactor at Japan's Fukushima nuclear plant but none reached outside the reactor building, Tokyo Electric Power Co said on Thursday as it strives to ensure damaged reactors are stable enough for work to start on dismantling them.
Experts said the incident, which follows smaller leaks last weekend, is not a big setback to getting the plant under control but the timing is awkward for the government as it strives to win public acceptance for the restart of reactors elsewhere to avoid a summer power crunch.
The Fukushima Daiichi plant, on the coast 240 km (150 miles) northeast of Tokyo, was wrecked by the March 11 earthquake and tsunami, triggering reactor meltdowns and radiation leaks that caused mass evacuations and widespread contamination.
The leak discovered late on Tuesday was from a pipe at the plant's No.4 reactor that may have burst after freezing due to cold weather, said a spokesman for the utility, known as Tepco. He said leaking water would go into a drain leading to a storage facility.
It contained only a tiny amount of radiation compared with the huge amount of water used to cool the reactors in the aftermath of the March disaster, much of which is still being treated at the plant to lower its radiation level, he said.
The No.4 reactor was shut for maintenance when the tsunami struck, and no fuel rods are inside the reactor vessel.
"I wouldn't say this is a positive development. But it isn't something that would further stoke safety concerns over other nuclear plants," said Kenji Sumita, honorary professor at Osaka University.
"If it hadn't been for the Fukushima disaster, an incident like this could have gone unreported."
Shattered trust in the safety of nuclear energy has prevented the restart of reactors shut for routine maintenance, straining power supply and threatening blackouts. Only three of Japan's 54 nuclear reactors are now operating. Without approval for restarts, all of them could be shut by the end of April, boosting the use of fossil fuels and adding over $30 billion a year to the nation's energy costs, a government estimate said.
U.N. nuclear experts gave their backing on Tuesday to stress tests aimed at showing Japan's nuclear plants can withstand the sort of disasters that devastated Fukushima Daiichi.
The government announced on Dec. 16 that reactors at the plant had reached a state of cold shutdown, a milestone in cleanup efforts and a pre-condition for allowing about 80,000 residents evacuated from a 20-km (12 miles) radius of the Daiichi plant to return home.
Resource-poor Japan had aimed to increase the share of nuclear power to more than half of its electricity supply by 2030 before the disaster, but now looks to reduce its reliance on nuclear power and raise the role renewable sources such as wind and solar power.
Tepco initially estimated the size of this week's leak at six litres but later revised it to 8.5 cubic metres, and is looking into how it can prevent similar incidents.
It follows the discovery and plugging of smaller leaks at the same reactor last weekend. (Reporting by Mark Bendeich and Kiyoshi Takenaka; Editing by Michael Watson)