In article ,
[email protected]
says...
Boiling water reactors run saturated steam in the primary loop. This
means that both water and steam coexist at the operating temperature and
the core does not boil dry. This is why pumps are needed in the cooling
system, to maintain the necessary pressure, otherwise they could just
put the reactor in a huge pool of water.
If the core is allowed to boil dry then someone has screwed up horribly
and the reactor will melt down long before anyone can try to cool it
with massive quantities of water. Chernobyl went from apparently (to
the operators) normal operation to pieces of the core raining down from
the sky in under a minute.
The defueling procedure is done with the reactor in cold shutdown. But
I'm not sure how defueling enters into anything under discussion--right
now they're just trying to get the reactors into cold shutdown, they're
not trying to defuel tnem.
The latent heat isn't really the issue--generally speaking if you remove
cooling from something its latent heat does not cause the temperature to
rise, and the reactor is not hot enough to melt itself without some kind
of reaction running. The decay products at the moment of shutdown
typically produce energy equal to something like 10 percent of the power
output of the reactor--if it's an 800 megawatt plant like
Fukushima 3 then that's 80 million or so watts of heat. An hour after
shutdown it's down to about 2 percent or for Fukushima 3 maybe 16
million watts. After a week it's down to maybe 2 million. Dissipating
those millions of watts is the real problem with an emergency shutdown.
Generally there are backup generators. In this case the generators
started but then got drowned out by the tsunami.
