Thursday, March 29, 2012

Unit 3 2nd Meltdown Theory: Fukushima Event

This article by Asahi basically says that a researcher believes that after the initial explosion which was thought to scatter piles of radioactive fallout over Japan, on March 21 there was another meltdown as water slowed to only a trickle of what it took to cool, exploded and spilled hot fuel onto the water pool on the floor, which created another steam plume and created more radioactive fission products which explain the black smoke, steam, and higher levels of radiation that were detected around the plant. Very, very bad if true, and it looks like the guy is probably right.

Report suggests second meltdown at reactor at Fukushima plant

August 08, 2011

By TOMOOKI YASUDA / Staff Writer
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.

The plant blew up with a massive hydrogen (or some say steam explosion) not long after they started to put water in it after it had been running without any water. Perhaps this water is what produced the massive amount of hydrogen:

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.

This section says that after the first meltdown and explosion, they had no problem pumping lots of water, but between Mar 20 and 23, the water slowed to a trickle, only 11-32% of what was needed to keep it from melting again, so they believe that it must have melted again and released all kinds of nasty radioactive badness, and the fuel probably melted through the bottom of the pressure vessel (big pressure cooker teapot) onto the containment floor, as is the case in Unit 2. That's really bad.

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.

This section says that increased radiation levels downwind led this fellow to believe there was another meltdown. This is really bad, and largely unreported just how much radiation from unit 3 got away even after the initial explosion.

Asahi'[s diagram shows fuel spilled from RPV pot onto floor, and hole(s) in containment  releasing radiation down wind.

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.

Here it says that the pressure increased dramatically, which indicates there was probably an explosion as the fuel heated up quickly, and a cloud of steam as fuel hit water at the bottom of the containment.:

Steam rising out of the edges of the reactor well

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.

Black smoke on March 21st

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