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View Diary: Physicist: Nuclear crisis may be close to "point of no return" (560 comments)

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  •  Meltdown Does Not Require Criticality? (1+ / 0-)
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    glitterscale

    "Shutdown" presumes that there is a "nuclear reaction" that needs to be "shut down".  

    I've seen this threshold referred to as "criticality" or "re-criticality", I think.

    There is a growing need for those who can speak to this issue with reliable expertise to clarify it.

    My layperson's understanding is that the heat produced by drop in water level that leaves the fuel assemblies uncovered (uncoolled except by air flow which is minimal especially in dense pack configurations) can result in increases in heat from and to the fuel assemblies sufficient to result in those assemblies and their coverings partially melting.  Speculation started problably a week or more ago that this had occurred in one or more reactors' spent fuel pools and possibly within the reactors themselves.  Application of sea water was intended to cut that melting off and prevent further melting or other potential results of continued over-heating.

    I do not believe this necessarily requires them to have ignited and burned.

    Even if melting and/or burning has occurred or is occuring, I do not believe that this necessarily means that the fuel either in the reactors or in the spent fuel pools has reached a critical point at which a nuclear reaction has re-started (in the case of fuel in the reactor) or started (in the case of fuel in the spent fuel pools.)

    Sometimes when life hands you lemons, you should throw them back.

    by Into The Woods on Mon Mar 28, 2011 at 01:20:22 PM PDT

    [ Parent ]

    •  defined (4+ / 0-)

      critical means operating at a sustained level of power production

      subcritical means the power level is decreasing (i.e. shutdown)

      super critical means the power level is increasing

      prompt-critical means that fast neutrons are sustaining the reaction (i.e. bomb)

      in a shutdown reactor immediately after powering down 1% of the total energy of full power is still produced by latent radioactivity.  This decay heat is what needs to be removed from the reactors and the spent fuel pools to prevent meltdown,  this amount of heat is easily enough to cause a meltdown which we have seen.

      it is yet unclear how badly the containment vessel in #3 is breached.

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