The cooling system failed at another reactor at the troubled No. 1 Fukushima nuclear power plant Sunday, as the government acknowledged that a meltdown may have already hit the quake-damaged site.
The cooling system failed at another reactor at the troubled No. 1 Fukushima nuclear power plant Sunday, as the government acknowledged that a meltdown may have already hit the quake-damaged site.
"We are dealing with the situation as though a core meltdown has occurred," Chief Cabinet Secretary Yukio Edano said at a Sunday morning news conference.
While government officials continued to urge the public to remain calm, Tokyo Electric Power Co., the operator of the plant, has been forced to resort to a number of unorthodox measures, including pumping in seawater, to cool reactor cores and prevent radiation from leaking out.
TEPCO is now dealing with problems at two reactors at the No. 1 Fukushima plant and three reactors at its No. 2 Fukushima plant, both in Fukushima Prefecture, north of Tokyo.
Radiation levels exceeding legally established safety standards were recorded at the boundary to the No. 1 Fukushima site, creating an emergency situation that TEPCO reported to the Ministry of Economy, Trade and Industry on Sunday.
TEPCO officials said nuclear fuel rods in the core of the No. 3 reactor had become exposed above the cooling water level.
While TEPCO officials were unsure if a core meltdown had occurred, they said water mixed with boric acid was injected into the core at 9:25 a.m. Sunday. By 11:30 a.m., the water level in the core had again risen above the fuel rods.
Edano later Sunday provided more details. He said that after the fuel rods were exposed for a short period, radiation levels around the plant reached a high level of 1,557.5 microsieverts. But the insertion of seawater to the core reduced the radiation level to 184.1 microsieverts after about an hour.
However, Edano noted that insufficient cooling of the core may have caused excess hydrogen to accumulate in the building that houses the core containment vessel.
That buildup runs the risk of triggering an explosion like the one that destroyed the building housing the No. 1 reactor at 3:36 p.m. Saturday, Edano said.
The government's top spokesman said that, like Saturday's blast, an explosion at the No. 3 reactor building would not likely damage the core containment vessel nor release large levels of radioactive materials.
He added that no new evacuation measures were necessary due to the latest problems.
Officials of the Nuclear and Industrial Safety Agency (NISA) said Sunday morning that TEPCO officials had informed them that water supply mechanisms to cool the core of the No. 3 reactor all stopped at 5:10 a.m. Sunday.
If coolant within the core continues to evaporate and generate steam, the accumulated pressure could eventually rupture the containment vessel and release large amounts of radioactive materials into the atmosphere.
To avert that scenario, TEPCO began releasing steam containing low levels of radioactive materials from 9:20 a.m. into the atmosphere through a vent tower about 120 meters high, officials said.
During efforts to lower pressure in the containment vessel, officials will continue work to restore the cooling mechanism, they said.
Steam containing radioactive material had already been released from the No. 1 reactor from Saturday afternoon to reduce pressure that had built up within that reactor's containment vessel.
After Friday's massive earthquake, the external electric power supply to the No. 1 Fukushima plant was cut off, and emergency generators failed to work. That stopped the emergency core cooling system.
Another pumping mechanism was used to cool the heat remaining in the No. 3 reactor core, but it stopped working apparently because the batteries had run out.
TEPCO officials started using fire engine tanks to pump in more seawater to cool the No. 3 reactor core while continuing efforts to restore the water supply mechanism.
TEPCO has already been pouring 30 tons of seawater an hour since Saturday night to cool the No. 1 reactor of the No. 1 Fukushima plant, NISA officials said Sunday morning.
Boric acid, which absorbs neutrons, making it difficult for a chain reaction to occur within the reactor core, has been added to the seawater.
Although another cooling mechanism was used for the No. 1 reactor after the emergency core cooling system failed, cesium was detected outside of the core, leading officials to speculate that a core meltdown may have occurred after core temperatures rose excessively.
After the detection of cesium, a radioactive substance created through nuclear fission of uranium fuel rods, the decision was made to pump in seawater and boric acid to prevent damage to the core container, NISA officials said.
Despite widespread fears in Japan of a nuclear accident, a NISA official said, "The situation is not one that should cause concerns among the public."
Since last September, the No. 3 reactor has been using a mixed-oxide fuel that contains plutonium oxide mixed with uranium oxide rather than uranium fuel rods.