NISA doubles early fallout estimate


NISA on Monday more than doubled its estimate of the radioactive material ejected into the air in the early days of the Fukushima nuclear crisis to 770,000 terabecquerels.

The nuclear safety agency also issued its own assessment of the cores in reactors 1, 2 and 3 at the Fukushima No. 1 power plant, assuming that all of them melted, and said it was possible the meltdowns in units 1 and 2 happened faster than the time frame estimated by Tokyo Electric Power Co.

The assessment by the Nuclear and Industrial Safety Agency is expected to be reflected in Japan’s report on the accident at a ministerial meeting being hosted by the International Atomic Energy Agency later this month.

In April, Japan raised the severity level of the crisis to 7, the maximum on the International Nuclear Event Scale, putting it on par with the 1986 Chernobyl disaster.

At the time, NISA believed that 370,000 terabecquerels of radioactive material had been ejected from reactors 1, 2 and 3. That was revised Monday after NISA found that more material escaped from reactor 2 than thought.

Level 7 accidents correspond to the external release of material equal to tens of thousands of terabecquerels of radioactive iodine 131. One terabecquerel equals 1 trillion becquerels.

NISA said the melted fuel in reactor 1 fell to the bottom of the pressure vessel and damaged it at about 8 p.m. on March 11, about five hours after the quake. In reactor 2, a similar event took place at about 10:50 p.m. March 14, it said.

However, Tepco says the pressure vessel in reactor 1 was damaged on the morning of March 12, and the pressure vessel in reactor 2 in the early hours of March 16.

A NISA official said the assessments vary due to different water injection assumptions.