White smoke started spewing from the stricken No. 3 reactor at the Fukushima No. 1 nuclear power plant around 8:30 a.m. on March 16.
White smoke started spewing from the stricken No. 3 reactor at the Fukushima No. 1 nuclear power plant around 8:30 a.m. on March 16.
There is the possibility that water in the pool where nuclear fuel rods are stored may be boiling.
High levels of radiation were measured near the Fukushima plant on March 16. There are growing concerns that the high radiation levels are due to damage to the core containment vessel at the No. 2 reactor.
Radiation levels of 10,000 microsieverts (10 millisieverts) per hour were recorded at the main entrance to the Fukushima plant at about 10:40 a.m.
Referring to the smoke billowing from the No. 3 reactor, Chief Cabinet Secretary Yukio Edano said at a news conference around 11 a.m. on March 16: “We have received a report that steam is being emitted from the containment vessel of the No. 3 reactor. The temporarily high radiation levels may have been due to the steam (that contains radiation).”
An official of the Nuclear and Industrial Safety Agency said on March 16, “There is a possibility that highly concentrated radioactive materials may have leaked to the outside atmosphere due to damage to the suppression pool of the containment vessel of the No. 2 reactor.”
Officials of Tokyo Electric Power Co., which operates the Fukushima plant, said the white smoke from the No. 3 reactor was probably evaporating water from the storage pool for nuclear fuel rods.
TEPCO officials said about 500 nuclear fuel rods stored in the pool were put there about nine months ago.
The loss of electric power to the Fukushima plant as well as the failure of the emergency generators after the March 11 Great East Japan Earthquake cut off the power source needed to supply cooling water to the storage pool.
The boiling of the pool water likely caused the steam emitted from the reactor.
If the hot nuclear fuel rods are exposed to air, the metal alloys covering them could weaken. As a result, there would be a much bigger danger of damage to the fuel rods.
If that occurred, large amounts of radioactive materials would be released.
The storage pool is located outside of the containment vessel and within the steel-reinforced concrete building housing the reactor.
The outer building was damaged on March 14 by a hydrogen explosion.
TEPCO officials said on March 16 that neutron rays were detected March 15 near the main entrance to its Fukushima No. 1 nuclear power plant.
Measurements by a monitoring car detected 0.02 microsievert per hour of neutron rays at 1:30 a.m. on March 15 and 0.01 microsievert 10 minutes later.
Neutron rays were also detected on March 14 after a hydrogen explosion occurred at the No. 3 reactor of the Fukushima plant. Neutron rays were not detected again until early March 15.
Neutron rays are emitted when nuclear fission occurs, but it was unclear whether this had really happened on March 15.
TEPCO officials also said an employee discovered a fire at the No. 4 reactor at about 5:45 a.m. on March 16.
At 6:15 a.m., plant officials checked on the No. 4 reactor from a higher vantage point and said no fire was visible.
The high radiation levels at the reactor building made it difficult for workers to approach it.
Meanwhile, fire-fighting units arrived at the scene before 9 a.m. and were considering how to deal with the fire.
A Ground Self-Defense Force helicopter was unable to dump water to extinguish the fire on March 16 due to the high levels of radiation over the reactor.
The GSDF said it will try again on March 17.
The March 16 fire occurred near the re-circulating pump located in the No. 4 reactor building, the same location as a fire the morning of March 15.
TEPCO officials had announced on March 15 that the fire that morning had burned itself out. The basis of that announcement was that no smoke was coming from the building. However, it now seems likely that the March 15 fire continued to smolder inside the building.
The No. 4 reactor was taken out of service in late November for a regular inspection. The March 15 fire created two 8-meter square holes in the steel-reinforced concrete wall of the reactor building.
Spent nuclear fuel rods at high temperatures are stored within the reactor building. There is a high possibility that highly concentrated radioactive materials from the fuel rods are being emitted into the outside atmosphere.
The spent fuel rods are stored in a pool, but the high temperature of the fuel rods continually evaporates the water. It is therefore necessary to provide a continuous supply of water to the pool.
The Fukushima plant lost all outside electric power during the massive quake on March 11 and its emergency generators failed.
The loss of power appears to have stopped the water supply to the reactors, leading to water evaporating from the storage pool. The water temperature in the pool on March 14 was 84 degrees.
TEPCO officials are considering asking the U.S. military to drop boric acid from a helicopter over the reactor building. The aim would be to suppress nuclear fission in the spent fuel rods.
A plan to spray water onto the No. 4 reactor from a truck outside of the plant site is also being considered.
According to officials at TEPCO’s Fukushima office, radiation levels at the main entrance to the nuclear plant was 628 microsieverts per hour at 8 a.m. on March 16 and 20 microsieverts per hour in Fukushima city.
Meanwhile, the Institute for Science and International Security of the United States released a statement on March 15 that said the situation at the Fukushima plant was now closer to a level 6 on the International Nuclear and Radiological Events scale, which ranks events from 1 to 7.
It said the situation at Fukushima “may unfortunately reach a level 7.”
The Chernobyl nuclear accident of 1986 was assessed as a level 7 accident.