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Originally Posted by ICER
We have reactors that are meltdown proof? I find that hard to believe when you consider the amount of heat a reactor can give off. Can you provide an example or an explanation as to what makes them meltdown proof? I would be interested in knowing.
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So a basic reactor contains fuel, moderator, coolant, and control.
In the first reactors, all four were seperate. The moderator was graphite and the coolant was water. If you lost your coolant, the reaction continued, the graphite slagged, possibly caught fire, and you generally had a bad day (incidentally, molten burning graphite is still an effective moderator, so the reactor continues to operate, though wildly out of it's design parameters)
Then someone noted that water can function as a moderator. So, if you lose your coolant, the reactor automagically shuts down. They also figured out that if you adjust the fuel to moderator ratio (often refered to as metal to water ratio), you can change the way the core responds to changes in temperature. So rather than having the core become more reactive as temperature increases (leading to runaway supercriticality requiring operator intervention), you could have a negative temperature coefficient of reactivity... the hotter the core gets, the less reactive it becomes. So, for any given reactor design, you could work out some theoretical power level where it would be self-regulating without coolant. For the most part, the temperatures involve are far beyond the failure points for the core materials... ergo, melt down.
Newer designs and newer materials have brought the stable thermal output within the limits.
They're not really "meltdown proof"... It's just that lack of supervision of these plants will result in them shutting down after a while. You could still get them to meltdown, but you would have to take concious active steps to produce the proper conditions. Simple operator error wouldn't cut it.