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Originally Posted by geos
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Actually, the most feared event to a Nuclear Engineer (what's a Nuclear Scientist doing in a operating commercial reactor?
Scurry on back to your research reactor, egghead.) would be a supercritical event followed very quickly by an uncontrolled power excursion. This happens too fast for even computer-controlled insertion of negative reactivity and is only stopped by "a change in core geometry." Which is a euphemism for your reactor blowing itself to hell and gone. Well, not really gone, but in itty bitty pieces all over the place.
There's loads of different types of containment to breach. You can lose containment on trace radioactive steam or hydrogen gas (like when TMI vented or Fukishima blew the roof off several containment buildings). You can lose containment of core coolant when you have a LOCA (TMI again). You can lose containment of fuel material when you expose the fuel rods, then re-flood the core (or-hey-cooling pools) with cold water, shattering the Zr cladding due to thermal stress. (TMI again, and probably where the Plutonium external to the core at Fukishima is coming from).
Containment loss is bad, no doubt. People get fired and plants get fined breach of containment, but nobody dies. You need a power excursion (Like the one which caused a steam explosion which jettisoned control rods from an experimental reactor and impaled a guy working on top of the reactor at Rocketdyne way back when) or an industrial accident (crane tips over, guy falls off a catwalk) before you can start talking loss of life.
I got all excited too about "core breaches" at Fukishima, but the thermal readings throughout the event indicate far too low core temperatures for it to be un-contained fuel melt or anything which could ever lead to re-criticality. Most likely it's simply seals failing and the core getting a little leaky due to the thermal stresses over the past few days. Nothing so sexy as an actual melt-out, happily. I'd be very surprised if any of the cores have more than a few feet of melt once they peel them open maybe a decade from now. Though knowing the Japanese, it'll probably take more like 2 years.