http://www.scientificamerican.com/article.cfm?id=nuclear-fuel-fukushima<snip>
The spent fuel pools are of significant concern, Marvin Resnikoff, a radioactive waste management consultant, said in a Wednesday press briefing organized by the nonprofit organization Physicians for Social Responsibility. Resnikoff noted that the fuel pools are thought to have contained the following amounts of fuel, according to The Mainichi Daily News:
• Reactor No. 1 fuel pool: 50 tons of nuclear fuel
• Reactor No. 2 fuel pool: 81 tons
• Reactor No. 3 fuel pool: 88 tons
• Reactor No. 4 fuel pool: 135 tons
• Reactor No. 5 fuel pool: 142 tons
• Reactor No. 6 fuel pool: 151 tons
• Also, a separate facility fuel pool on ground level contains 1,097 tons of nuclear fuel; and some 70 tons of nuclear materials are kept on the grounds in dry storage.
The reactor cores themselves contain less than 100 tons of fuel, Resnikoff noted.
The fuel had been moved from Reactor No. 4's core to its spent fuel pool recently, so "that fuel is relatively fresh and hotter, thermally," Resnikoff explained. "So it's not surprising that when the water
no longer circulating that the water was actually boiled off in a zirconium exothermic reaction, that the zirconium burned" (which occurs at about 1,800 degrees C).
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Hundreds of millions of curies of radioactivity