http://www.independent.co.uk/news/world/asia/suicide-squads-paid-huge-sums-amid-fresh-fears-for-nuclear-site-2256741.htmlBy David McNeill in Tokyo
Wednesday, 30 March 2011
"The radioactive core in one reactor at Fukushima's beleaguered nuclear power plant appeared to have melted through the bottom of its containment vessel, an expert warned yesterday, sparking fears that workers would not be able to save the reactor and that radioactive gases could soon be released into the atmosphere.
Richard Lahey, who was a head of reactor safety research at General Electric when the company installed the units at Fukushima, said the workers, who have been pumping water into the three reactors in an attempt to keep the fuel rods from melting, had effectively lost their battle. "The core has melted through the bottom of the pressure vessel in unit two, and at least some of it is down on the floor of the drywell," he said.
The damning analysis came as it emerged that workers at Japan's stricken nuclear plant are reportedly being offered huge sums to brave high radiation in an attempt to bring its overheated reactors under control. The plant's operator, the Tokyo Electric Power Company, is hoping to stop a spreading contamination crisis which could see another 130,000 people forced to leave their homes.
Radiation has already found its way into milk, vegetables and tap-water and is leaking into the sea around the complex. Government tests found yesterday that small quantities of plutonium, one of the world's most dangerous elements, have seeped into soil outside the plant......................."
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http://barryonenergy.wordpress.com/2011/03/29/nuclear-or-petroleum-%E2%80%9Cit-melted-the-concrete%E2%80%9D/"Nuclear or Petroleum – “It Melted the Concrete” (the plutonium factor)
Three weeks after the Japanese nuclear power plant disaster began, yesterday’s headlines read; “Toxic plutonium seeping from Japan’s nuclear plant.” It’s unfathomable, the danger this presents to Japan and portions of the world, today and tomorrow. Plutonium breaks down very slowly, so it remains dangerously radioactive for hundreds of thousands of years. “If you inhale it, it’s there and it stays there forever,” said Alan Lockwood, a professor of Neurology and Nuclear Medicine at the University at Buffalo and a member of the board of directors of Physicians for Social Responsibility, an advocacy group.
The dangers of plutonium are analyzed in detail in a Lawrence Livermore National Laboratory Report that is available on the web at www.llnl.gov/csts/publications/sutcliffe/118825.html
Key facts are:
• Plutonium is toxic both because of its chemical effects and because of its radioactivity. The chemical toxicity is similar to that of other “heavy metals” and is not the cause for the widespread fear.
• Ingestion. For acute radiation poisoning, the lethal dose of plutonium is estimated to be 500 milligrams (mg), i.e. about 1/2 gram. A common poison, cyanide, requires a dose 5 times smaller to cause death: 100 mg. Thus for ingestion, plutonium is very toxic, but five times less toxic than cyanide. There is also a risk of cancer from ingestion, with a lethal doze (1 cancer) for 480 mg.
• Inhalation. For inhalation, the plutonium can cause death within a month (from pulmonary fibrosis or pulmonary edema); that requires 20 mg inhaled. To cause cancer with high probability, the amount that must be inhaled is 0.08 mg = 80 micrograms. The lethal dose for botulism toxin is 0.070 micrograms = 70 nanograms, a factor of
• How easy is it to breathe in 0.08 mg = 80 micrograms? To get to the critical part of the lungs, the particle must be no larger than about 3 microns. A particle of that size has a mass of about 0.140 micrograms. To get to a dose of 80 micrograms requires 80/0.14 = 560 particles.
• In contrast, the lethal dose for anthrax is estimated to be 10,000 particles of a similar size.
• Botulism: 70 nanograms, injested, is the lethal dose. 100 micrograms per kilogram? 100 ng per human?....................................."
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http://mitnse.com/2011/03/17/on-worst-case-scenarios/On prediction of “worst case” scenarios (MIT Department of Nuclear Science and Engineering)
"Meltdown
The term meltdown describes melting of the zirconium alloy cladding, and uranium oxide (or mixed oxide, in the case of Unit 3) fuel pellets. These two structures are the first two barriers to fission release, since radioactive fission products normally exist as either solids within the fuel pellet, gases within pores in the fuel pellet, or gases that escape the pellet but remain in the cladding. When a reactor is shut down, these fission products continue to decay, generating heat. This amount of heat is produced at first at 7% of its initial rate, and then decreases as the isotopes responsible for generating it decay away. If this decay heat is not removed by cooling water, the fuel and cladding increase in temperature.
At temperatures above 1200 C, the corrosion reaction which is constantly ongoing in the zirconium cladding accelerates dramatically. The reaction’s products include zirconium oxide, hydrogen (for more on this hydrogen, see our post “Explanation of Hydrogen Explosion at Units 1 and 3), and heat. This heat continues to both fuel the corrosion reaction, and to prevent the fuel rods from being cooled. Because of the self-catalyzing nature of this reaction, safety systems are usually actuated in such a fashion as to provide a large margin of safety to the clad reaching 1200 C.
If multiple failures prevent these actions from being taken, as was the case at Three Mile Island, the fuel rods heat up until the uranium oxide reaches its melting point, 2400-2860 C (this figure depends on the makeup and operating history of the fuel). At this point, the fuel rods begin to slump within their assemblies. When the fuel becomes sufficiently liquid, slumping turns to oozing, and the “corium” (a mixture of molten cladding, fuel, and structural steel) begins a migration to the bottom of the reactor vessel. If at any point the hot fuel or cladding is exposed to cooling water, it may solidify and fracture, falling to the bottom of the reactor vessel.
A similar sequence of events takes place if cooling to spent fuel pools is not maintained, but at a reduced rate of progression..................."
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