An explosion rocked the quake-hit Fukushima No. 1 nuclear power plant on March 12, injuring four workers, the Nuclear and Industrial Safety Agency (NISA) said.
An explosion rocked the quake-hit Fukushima No. 1 nuclear power plant on March 12, injuring four workers, the Nuclear and Industrial Safety Agency (NISA) said.
The explosion, which occurred at 3:36 p.m., followed a NISA report earlier in the day that nuclear fuel in the No. 1 reactor had apparently begun to melt.
While concerns grew over a possible nuclear meltdown, Chief Cabinet Secretary Yukio Edano said Saturday night the explosion damaged only the reactor’s outer building and did not affect the inner containment vessel.
“The reactor core’s containment vessel was not affected at all,” he said.
He said the explosion was caused when hydrogen, from water vapor, was combined with oxygen.
The plant’s operator, Tokyo Electric Power Co., and NISA were looking into the cause of the explosion and the current state of the reactor core in Fukushima Prefecture. The prefecture said the explosion caused the ceiling of the No. 1 reactor building to collapse.
High levels of radioactive substances, such as cesium and iodine produced from uranium fission, have been detected within the plant site. But Edano said the levels actually went down after the explosion.
If a nuclear meltdown should occur, it could lead to an explosive reaction and release large quantities of radioactive substances into the air.
Edano told a news conference earlier March 12 that radioactivity is being monitored and is believed to be staying within the range expected from the release of radioactive steam to reduce pressure within the containment vessel.
He stressed that the government is analyzing information and studying measures to take with nuclear energy experts.
The government later instructed the prefecture to expand the area of evacuation to a 20-kilometer radius of the Fukushima No. 1 plant. Neighborhood residents within a 10-km radius of the plant had earlier been told to evacuate. About 800 residents were still leaving as of 6 p.m.
The government instructed residents within a 10-km radius of the Fukushima No. 2 plant to evacuate earlier March 12.
TEPCO has had difficulty removing heat from the reactor core as the emergency core cooling system (ECCS) failed to work after the March 11 magnitude-9.0 quake shut down the reactor.
After 4 p.m., TEPCO reported that high levels of radioactivity were detected near the No. 1 reactor, at 1,015 microsieverts per hour.
That amount exceeds the maximum allowable exposure a year for an ordinary citizen of 1,000 microsieverts. Edano said the figure was observed before the explosion.
According to NISA officials, the pressure within the No. 1 reactor’s containment vessel at the No. 1 plant sharply fell after the explosion was heard.
They said it was not known whether the decrease was a result of the operation to release steam from the vessel to reduce its pressure.
The water level within the pressurized vessel kept coming down, exposing almost 1.7 meters of the 4-meter fuel rods as of 5:28 p.m. TEPCO has been pouring in water, but has failed to keep pace.