More than twice as much radioactivity was spewed into the atmosphere on March 11 from the crippled Fukushima No. 1 nuclear power plant than previously estimated, Japan's regulator of nuclear industry safety said in its latest report.
More than twice as much radioactivity was spewed into the atmosphere on March 11 from the crippled Fukushima No. 1 nuclear power plant than previously estimated, Japan's regulator of nuclear industry safety said in its latest report.
The Nuclear and Industrial Safety Agency said June 6 that the pressure vessel at the No. 1 reactor was damaged on the night of March 11, the day of the magnitude-9.0 temblor and tsunami and 10 hours earlier than the plant operator originally reported. The damage resulted from a meltdown, NISA said.
The agency report, based on a review of plant operator Tokyo Electric Power Co.'s analysis of data on the accident, said the equivalent of 770,000 terabecquerels (1 tera is 1 trillion) of radioactive iodine-137 was released into the air.
That figure is more than double TEPCO's previous estimate of 370,000 terabecquerels and in excess of the 630,000 terabecquerels calculated by the Nuclear Safety Commission of Japan, a panel within the Cabinet Office, based on readings in areas surrounding the plant.
NISA said it raised the emission amount on the presumption that the accident had created gaps as big as 50 square centimeters in the outer containment vessel and 300 square centimeters in the suppression pool of the No. 2 reactor after the temperature in its containment vessel soared above 138 degrees, the ceiling it was designed for.
The accident was a maximum level 7 on the International Nuclear Event Scale, matching the severity rating of the 1986 meltdown at Chernobyl, which released 5.2 million terabecquerels.
NISA also said that meltdowns at the No. 1 and No. 2 reactors caused damage to their pressure vessels earlier than the TEPCO report said.
The No. 1 reactor's pressure vessel was damaged around 8 p.m., about five hours after the Great East Japan Earthquake hit at 2:46 p.m. on March 11. The damage followed soon after the fuel rods melted and fell to the bottom of the pressure vessel.
TEPCO had reported the damage occurred at around 6 a.m. on March 12.
As for the No. 2 reactor, NISA said that the damage caused by its meltdown occurred at around 10:50 p.m. on March 14, 29 hours earlier than TEPCO concluded.
In contrast, the damage to the pressure vessel of the No. 3 reactor came at around 10:10 p.m. on March 14, 13 hours later than TEPCO said it had occurred.
NISA said its estimate for the No. 3 rector is based on a reconstruction of pressure behavior, which it said is closer to what actually happened than the TEPCO analysis.
The agency said the reactor core at the No. 1 reactor became exposed from around 5 p.m. on March 11.
NISA's conclusion on the timing of the reactor core exposure, it said, is consistent with the immediate record of a surge in radioactivity levels that was written at 5:50 p.m. on the whiteboard of the central control room and a jump in radioactivity levels at the building housing the turbine after entry to the reactor building was prohibited.
(This article was written by Eisuke Sasaki and Tatsuyuki Kobori.)