The Tokyo Electric Power Co. (TEPCO) covered up an incident in which air from a reactor discharge pipe was tainted with leaked plutonium and other radioactive substances, an anti-nuclear activist group has said.

TEPCO has been scandalized for hiding the discovery of cracks at its nuclear plants, however the leakage of plutonium and other chemicals had not been revealed.

Members of the Osaka-based anti-nuclear group told the press Wednesday that a TEPCO informer sent them e-mail in late September and later copies of secret inside documents detailing the leakage.

The internal documents show that nuclear substances were detected at the outlet of a 120-meter-long discharge pipe attached to two reactors in the Fukushima No. 1 nuclear plant from 1979 to 1981.

TEPCO officials have admitted they detected the leaked radioactivity, but tried to justify their failure to announce the findings.

"We didn't report the leakage to national government authorities because the concentration of radioactivity at the edge of the plant was only less than one 10,000th of the government regulation amount," one of the officials said. "Outside the plant, the amount of radioactivity discharged was also less than the minimum level."

The level of radioactivity detected was up to 30 times the lowest detectable value of a Geiger counter.

The official explained that several fuel rods at the reactors cracked during the period allowing nuclear substances to leak and contaminate the air in the reactor pipe.

Kunikazu Noguchi, a radioactivity expert at Nihon University, said that TEPCO must voluntarily explain what exactly was leaked and to what extent workers were exposed to the nuclear substances.

"Generally no counters can detect such radioactivity because normally the amount (found at the outlet of discharge pipes) is below the minimum detectable level of counters at nuclear power plants," Noguchi said. "Therefore, the fact that such substances were detected is abnormal."

The nuclear substances found are generally called "alpha nuclides" as they discharge alpha rays.

This is the first revelation of alpha nuclides leaking into the air at a nuclear power plant, although such substances were leaked at a reprocessing factory in Tokai, Ibaraki Prefecture, during a major accident in 1997. (Mainichi Shimbun, Oct. 24, 2002)