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*** Fukushima *** and other nuclear-----updates and links
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[quote:Southern OR:MV8xNTM3MTQ2XzM1NzY3MjYwXzg2M0NEOUEy] Fukushima Accident 2011 In unit 1 most of the core - as corium comprised of melted fuel and control rods - was assumed to be in the bottom of the RPV, but later it appeared that it had mostly gone through the bottom of the RPV and eroded about 65 cm into the drywell concrete below (which is 2.6 m thick). This reduced the intensity of the heat and enabled the mass to solidify. Much of the fuel in units 2 & 3 also apparently melted to some degree, but to a lesser extent than in unit 1, and a day or two later. In mid May the unit 1 core would still be producing 1.8 MW of heat, and units 2 & 3 would be producing about 3.0 MW each. In unit 2, water injection using the steam-driven back-up water injection system failed on Monday 14th, and it was about six hours before a fire pump started injecting seawater into the RPV. Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen, hence the delay. Meanwhile the reactor water level dropped rapidly after back-up cooling was lost, so that core damage started about 8 pm, and it is now provisionally understood that much of the fuel then melted and probably fell into the water at the bottom of the RPV about 100 hours after the scram. Pressure was vented on 13th and again on 15th, and meanwhile the blowout panel near the top of the building was opened to avoid a repetition of unit 1 hydrogen explosion. Early on Tuesday 15th, the pressure suppression chamber under the actual reactor seemed to rupture, possibly due to a hydrogen explosion there, and the drywell containment pressure inside dropped. However, subsequent inspection of the suppression chamber did not support the rupture interpretation. Later analysis suggested that a leak of the PCV developed on Tuesday 15th. In Unit 3, the main back-up water injection system failed at 11 am on Saturday 12th and early on Sunday 13th, water injection using the high pressure system failed also and water levels dropped dramatically. RPV pressure was reduced by venting steam into the wetwell, allowing injection of seawater using a fire pump from just before noon. Early on Sunday venting the suppression chamber and containment was successfully undertaken. It is now understood that core damage started about 9 am and much or all of the fuel melted on the morning of Sunday 13th and possibly fell into the water at the bottom of the RPV, or was retained on the core support plate within the shroud. http://www.world-nuclear.org/info/fukushima_accident_inf129.html (updated 10 January 2013) I see there was discussion again on needing verification of melt through. I found this to be a nice summary. [/quote]
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