Tokyo Electric Power Co. said Wednesday it has observed a neutron beam, a kind of radioactive ray, 13 times on the premises of its crippled Fukushima No. 1 nuclear plant.
Tepco said the neutron beam measured about 1.5 km southwest of the plant's Nos. 1 and 2 reactors over three days from March 13 and is equivalent to 0.01 to 0.02 microsieverts per hour. This is not a dangerous level of radiation, it added.
The utility said it will also measure uranium and plutonium, which could emit a neutron beam.
In the 1999 criticality accident at a nuclear fuel processing plant run by JCO Co. in Tokai, Ibaraki Prefecture, uranium broke apart continually in nuclear fission, causing a massive amount of neutron beams.
In the latest case at the Fukushima No. 1 plant, a criticality accident has yet to happen.
But the measured neutron beam may be evidence that uranium and plutonium leaked from the plant's nuclear reactors and spent nuclear fuel have discharged a small amount of neutron beams via fission.