The nuclear power station at Kashiwazaki-Kariwa pumped radioactive particles into the air for nearly three days after Monday’s massive Niigata earthquake, The Times has learned.

The scandal-hit utility-group that runs the controversial plant, the Tokyo Electric Power Company (Tepco), admitted earlier today that iodine had escaped from the exhaust pipe of one of seven reactors at the plant, which was near the epicentre of the quake.

The company was unable to explain why it took engineers until Wednesday afternoon to locate and staunch the leak, but said that the radioactive particles had been blown into the atmosphere from the turbine, which continued running after the reactor automatically shut-down, against the recommendation of the official safety manual.

The blunder, which is currently not thought to pose a risk to human life, joins a catalogue more than 50 other safety failings discovered at the plant since the earthquake. Tepco has been heavily criticised for its sloppy management of the quake aftermath and for initially under-reporting the extent of the radiation leaks that occurred.

Vital reactor data on the minutes immediately after the quake has already been lost by Tepco.

The company that runs the plant at Kashiwazaki admitted that it has still not been able to investigate whether there has been damage to the fuel-rods or control-rods in the reactor’s central core. Tepco said today that it had no idea of when it would be able to even begin such checks.

It also emerged that none of Tepco’s nuclear plants give employees a manual on how to fight fires themselves. Monday’s quake caused a transformer to ignite, but the blaze was not dealt with for nearly two hours.

In a further blow to the credibility of the Japanese nuclear industry, it emerged today that Washington had offered US help in investigating the reactor at Kashiwazaki. Japan has already been told by the International Atomic Energy Agency to investigate the problems at the plant in a “transparent” manner – a comment seen as highly critical of Japan’s long history of nuclear-related cover-ups.

The massive 6.8 magnitude quake, which shook Niigata on Monday morning and has left thousands of homes uninhabitable, was three times more powerful than the designers of the nearest nuclear power plant – Kashiwazaki-Kariwa – had prepared for or even imagined.

The unfolding crisis at Kashiwazaki has renewed calls for the immediate closure of all the five atomic reactors at Hamaoka – an old plant which it has now emerged is sited directly on top of a geologically active fault, about 100km west of Tokyo. Tepco had previously firmly denied in court that this was the case.

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