The crisis at the No. 1 Fukushima nuclear power plant was quickly running out of control Tuesday, after two explosions, a fire and dangerous levels of radiation compounded the problems at four of its six reactors.
The crisis at the No. 1 Fukushima nuclear power plant was quickly running out of control Tuesday, after two explosions, a fire and dangerous levels of radiation compounded the problems at four of its six reactors.
Chief Cabinet Secretary Yukio Edano said radiation levels reached as high as 400 millisieverts around the reactors, a level that "clearly would affect people's health."
The unit used by Edano is a thousand times higher than the microsieverts that had been used in previous announcements.
Prime Minister Naoto Kan also addressed the nation concerning developments at the Fukushima nuclear plant, asking residents living within a radius of 20 to 30 kilometers of the plant to remain indoors.
Kan repeated an earlier instruction to have residents living within a 20-kilometer radius of the plant to evacuate.
Kansei Nakano, the chairman of the National Public Safety Commission, said that as of 7 a.m. Tuesday, only 96 people in a hospital remained within a 20-kilometer radius of the nuclear plant. Most were bed-ridden patients, but the Ground Self-Defense Force was planning to evacuate those people.
Edano also explained that a fire had broken out at the No. 4 reactor of the No. 1 Fukushima plant on Tuesday. The reactor was not operating when Friday's earthquake struck.
The fire was doused before noon.
Edano said the fire may have been caused by an explosion of accumulated hydrogen produced by spent nuclear fuel rods stored at the No. 4 reactor.
Officials of Tokyo Electric Power Co. (TEPCO), operator of the No. 1 Fukushima plant, confirmed that an explosion occurred at the No. 4 reactor at 6 a.m. Tuesday. The roof on the fifth floor of the building housing the reactor was damaged, and the fire was confirmed near the northwest part of the fourth floor of the building.
Two holes measuring 8 meters square each were formed in the wall of the northwestern section of the outer building housing the No. 4 reactor, officials said Tuesday evening.
Although the No. 4 reactor was not operating, spent fuel rods were stored in a pool on the fifth floor of the building housing the reactor.
Because the spent fuel rods still emit heat, water was circulated in the pool to cool the rods. However, Friday's Great East Japan Earthquake caused a power outage to the water circulating system.
The pool temperature is normally maintained at under 40 degrees. It had risen to about 85 degrees at 4:18 p.m. Monday.
There is the possibility that water in the pool evaporated, exposing the fuel rods and leading to the production of hydrogen.
However, the amount of heat emitted was likely low because considerable time had passed since the reactor's operations were stopped.
For that reason, TEPCO officials said they did not know if the spent fuel rods were actually exposed above the water level.
Edano also said that radiation levels measured at 10:22 a.m. were 400 millisieverts per hour near the No. 3 reactor and 100 millisieverts near the No. 4 reactor.
Workers in emergency situations are only allowed to remain in a site with measurements of 400 millisieverts for 15 minutes.
"We believe effects from (the fire) at the No. 4 reactor were large. The figures clearly show levels that could affect human health," Edano said.
An explosion was also heard at 6:14 a.m. Tuesday at the No. 2 reactor of the No. 1 Fukushima plant. Officials of the Nuclear and Industrial Safety Agency received a report about the explosion from TEPCO officials.
There is a possibility that the suppression pool under the core containment vessel was damaged in the blast.
Radiation levels around the No. 2 reactor rose quickly after the explosion.
TEPCO officials said they evacuated employees at the nuclear power plant who were not directly involved in pumping in water to cool the reactors, leaving 50 workers at the No. 1 Fukushima plant.
NISA officials said the suppression pool is filled with coolant and is located under the containment vessel. The suppression pool serves to keep pressure from rising within the pressure container by cooling and changing steam back into water.
The suppression pool also serves as the water source for the reactor's emergency core cooling system, which has been rendered inoperable.
After the explosion, pressure within the suppression pool fell rapidly from the normal 3 atmospheres (atm) to 1 atm, which is almost the same as the outside air pressure.
The drop in pressure may indicate that a hole was opened to the outside atmosphere, meaning water and steam containing high concentrations of radioactive materials may have leaked to the outside atmosphere.
A monitoring post in a west-southwest direction from the No. 2 reactor recorded radiation levels of 73.2 microsieverts per hour at 6 a.m. Tuesday. At 6:50 a.m., after the explosion, the radiation level reached 583.7 microsieverts per hour and rose further to 965.5 microsieverts per hour at 7 a.m.
At a news conference around 4 p.m., Edano said the radiation level of 8,217 microsieverts recorded in front of the main entrance to the plant at 8:30 a.m. rose to 11,930 microsieverts by 9 a.m.
However, at 12:30 p.m., the radiation level had fallen to 1,362 microsieverts. Edano said that by 3:30 p.m., the level had dropped further to 596.4 microsieverts.
About the falling radiation levels, Edano said: "We are slightly relieved. The situation is not one of highly concentrated radioactive materials being continuously emitted from the No. 4 reactor (where a fire broke out)."
At the same time, he added, "Because the extinguishing (of the fire) was only to the exterior, we have to carefully examine what happened internally."
It was not immediately known what caused the explosion at the No. 2 reactor, but there is the possibility that the process was similar to what occurred before the hydrogen blasts that struck the No. 1 and No. 3 reactors.
Edano said: "Currently, water is being pumped (for cooling purposes) in a stable manner to the No. 1 and No. 3 reactors. While water is also being pumped into the No. 2 reactor, we will have to follow the situation somewhat longer before we can determine if it is stable."
There is still the possibility that the core containment vessel above the suppression pool has not been damaged because the pressure in the vessel has not changed.
The problems at the No. 1 Fukushima nuclear plant increased radiation levels around Japan.
Iwaki in Fukushima Prefecture south of the nuclear plant had readings about 400 times normal.
Higher than normal measurements were also recorded in Kita-Ibaraki, Tokyo's Shinjuku Ward and Yokosuka, Kanagawa Prefecture.
However, prefectural government officials said the figures did not represent an immediate threat to people's health.
At 4 a.m. Tuesday, a measurement in Iwaki recorded a radiation level of 23.7 microsieverts per hour. An individual exposed to that level of radiation for 24 hours would be exposed to an amount equivalent to a single stomach X-ray.