quake or tsunami

Did quake or tsunami cause Fukushima meltdown?

Japan's nuclear safety agency today rejected a claim in British newspaper The Independent that the earthquake itself, not the subsequent tsunami, destroyed cooling systems leading to meltdowns at the Fukushima Daiichi nuclear plant.

"It is not correct," a spokesman for Japan's nuclear safety watchdog, the Nuclear and Industrial Safety Agency (NISA), told New Scientist.

The claim made in The Independent contradicts public reassurances from the Tokyo Electric Power Company (TEPCO) , the company that owns the plant, that its facility stood up to the quake as it should, but was overwhelmed by the tsunami. If the quake did cause the damage, it could call into question the resilience of TEPCO's other nuclear installations in Japan. TEPCO and Japan's nuclear industry as a whole have been criticised for attempting to cover up accidents in the past.

The paper reported that workers said they had seen cooling-water pipes bursting as they were evacuating from the nuclear plant following the quake at 2.52 pm on 11 March – before the tsunami struck about 45 minutes later.

It also quoted nuclear engineers who concluded from data released by TEPCO that coolant systems must have failed shortly after the quake.

Meltdown inevitable

"There was already so much damage to the cooling system that a meltdown was inevitable," Mitsuhiko Tanaka, a former nuclear plant designer, is quoted as saying.

Tanaka said that according to TEPCO's own data, emergency water-circulation equipment started up automatically shortly after the quake. "This only happens when there is a loss of coolant," he told The Independent. Likewise, between 3.04 pm and 3.11 pm, water sprayers in the containment vessel of reactor unit 1 were activated; Tanaka says this is a failsafe for when all other cooling systems have failed.

So by the time the tsunami struck at 3.37 pm, "the plant was already on its way to melting down", says the newspaper.

The Independent also quotes the results of a NISA visit to Fukushima nine days before the quake. It says that NISA warned TEPCO about its failure to inspect critical machinery at the plant, including recirculation pumps.

No damage

NISA's spokesman said that the agency's press release about its visit on 2 March may have been misunderstood. "There was no damaged piping in the Fukushima Daiichi nuclear power plant, as claimed in the article," he said.

What the press release actually said was that some of TEPCO's periodic equipment checks were behind schedule, said the spokesman.

NISA also rejected the central claim of the article: that the quake, not the tsunami, caused the critical damage leading to meltdown. "It is not correct," said the spokesman. "Before the tsunami hit, the cooling system was operated by diesel generators in the plant [to compensate for] a loss of external power sources after the earthquake."

So not until the tsunami swept away the diesel generators did the cooling system fail, ultimately causing meltdowns.

Viennese backup

NISA's version of events was backed up yesterday by the International Atomic Energy Agency in Vienna, Austria, which sent a fact-finding mission to Fukushima in May.

An IAEA spokesman said that a report from the mission – led by Mike Weightman the UK's chief inspector of nuclear installations – contains detailed accounts of the failure of cooling systems in the early hours of the disaster which challenge the idea that the quake caused the damage, as claimed in The Independent.

Meanwhile, TEPCO said on Wednesday that overall radiation released from the three damaged Fukushima reactors is now a 10-millionth of peak levels recorded on 15 March, just after the accident.

Wednesday also saw reactor 3 of the Tomari nuclear plant in Hokkaido become the first of Japan's nuclear installations since the disaster to resume full commercial operation.