A second meltdown likely occurred in the No. 3 reactor at the Fukushima No. 1 nuclear power plant, a scenario that could hinder the current strategy to end the crisis, a scientist said.
In that meltdown, 10 days after the March 11 Great East Japan Earthquake, the fuel may have leaked to the surrounding containment vessel, according to a report by Fumiya Tanabe, a former senior researcher at what was then the government-affiliated Japan Atomic Energy Research Institute.
His report will be announced at next month's meeting of the Atomic Energy Society of Japan.
Under Tokyo Electric Power Co.'s road map to deal with its crippled nuclear plant, reducing temperatures at the bottom of the core pressure vessel is one objective for bringing the accident under control. But if the fuel burned through the pressure vessel surrounding the No. 3 reactor and dropped into the containment vessel, that plan would be affected.
The No. 3 reactor was in a state of dry boil for about six hours until cooling water was pumped into the core from 9:25 a.m. on March 13.
Around 11 a.m. on March 14, the reactor building was hit by a large hydrogen explosion that was likely caused by a core meltdown, which led to fuel falling to the bottom of the pressure vessel.
According to data released by TEPCO, about 300 tons of water was pumped into the No. 3 reactor core daily until March 20, which likely cooled the fuel into a large clump.
However, between March 21 and 23, only about 24 tons of water was pumped in, while on March 24, about 69 tons entered the reactor.
One possible cause for the decline in water volume was that pressure within the pressure vessel increased, making it more difficult for water to enter the vessel.
According to Tanabe, who analyzed the nuclear accident at Three Mile Island in the United States when he was a researcher at the Japan Atomic Energy Research Institute, the volume of water pumped in on those days was only between 11 and 32 percent of the amount needed to remove decay heat from the nuclear fuel in the core.
In such a situation, the fuel could reach high enough temperatures to begin melting again in just one day.
Tanabe also estimates that the second meltdown led to the release of large amounts of radioactive materials, and that much of the fuel fell through the pressure vessel to the surrounding containment vessel.
The fuel is now believed to have formed another clump after being cooled.
Achieving a cold shutdown with the fuel sufficiently cooled would mark the completion of the second step of TEPCO's road map for dealing with the Fukushima nuclear accident. The central government has compiled its own road map based on TEPCO's objectives.
However, Tanabe said: "In deciding if a cold shutdown has occurred, the location where temperatures are measured will depend on where the melted fuel is. A thorough analysis should be conducted on what has really occurred in the reactor core."
Even officials of TEPCO and the central government acknowledge that they do not know the specific condition of the core at the No. 3 reactor. But their view until now has been that the melted fuel has settled at the bottom of the pressure vessel.
One factor used by Tanabe in speculating that a second meltdown occurred is the increase in radiation levels from the morning of March 21 in areas downwind from the Fukushima No. 1 plant, such as the Fukushima No. 2 nuclear power plant as well as the Kanto region municipalities of Kita-Ibaraki, Takahagi and Mito.
Initially, officials of the Nuclear and Industrial Safety Agency explained that the higher radiation levels were caused by radioactive materials falling to the ground with the rain.
But there is also the possibility that additional radioactive materials emitted from the second meltdown may have been blown by the wind.
Between 1 a.m. and 3 a.m. on March 21, the pressure within the pressure vessel of the No. 3 reactor core increased sharply to about 110 atmospheres, likely caused by an explosion within the pressure vessel due to a lack of cooling of the fuel. That was probably the start of the second meltdown, Tanabe said.
As for the sudden pressure increase, Tanabe points to the possibility that the clump of melted fuel in the pressure vessel may have fallen apart due to a lack of cooling. The magma-like substance with high temperatures may have leaked out of the vessel and emitted large amounts of steam when it came in contact with water.
At the No. 3 reactor building, black smoke spewed from the reactor building on the afternoons of March 21 and March 23. Tanabe said the smoke may have been the result of what is referred to as a core-concrete reaction, when melted fuel comes in contact with the concrete of the containment vessel. Such a reaction typically occurs when insufficient cooling follows a core meltdown.
TEPCO officials said the black smoke was probably caused by rubber or lubricant oil catching fire.
They acknowledged the possibility that some of the fuel may have fallen into the containment vessel, but they did not explain how the fire started.
Kunihisa Soda, a former commissioner at the Nuclear Safety Commission of Japan who is a specialist on severe accidents at nuclear plants, said the possibility of a second meltdown could not be ruled out. But he said that accurately estimating the level of possibility is impossible because of uncertainties surrounding the validity of the released data.
Tanabe also suggested that TEPCO officials think about what steps should be taken to monitor whether the fuel that has fallen into the containment vessel is being properly cooled.
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