First off, don’t use the word “critical” when speaking of reactors unless you know what it means. The definition of critical is “the same number of neutrons were born in this generation as were born in the previous generation.”
Translation: reactor power is stable
FYI subcritical means reactor power is decreasing and supercritical means reactor power is increasing.
All of the reactors at Fukushima are subcritical. The heat created in a shutdown reactor is not from fission of the fuel, but decay of the fission products.
Second, the breakdown of the cladding at high temperatures is not a nuclear reaction; it’s a chemical reaction between the zirconium and water. It produces zirconium dioxide and hydrogen gas.
I forget what temperature that happens since I’ve never melted a core much less created a water-metal reaction in a reactor.
The danger of this reaction is not the heat, but the pressure and explosive gas created.
Third, water moderated reactors such as these GE BWRs have a negative temperature coefficient of reactivity. Cooling the reactor with water will *help* the reactor go critical, not prevent it.
Cooler water —> denser water —> more effective at “slowing down” neutrons from “fast” to “thermal” —> more likely U-235 and Pu-239 to absorb neutron and fission
You are correct about boron. It’s one of the best neutron sponges out there. (Hafnium and xenon are other notables)
Thanks - as you can tell, I’m not a nuke (but I play on on the internet). :-)
This whole episode has been a learning experience for me, and for some reason, I find it somewhat fascinating. Not because of the danger (percieved and real), but because of the series of problems and the way they are slowly being dealt with.
I’ve never melted a core either. Closest thing was a little copper a couple of times, but that wasn’t necessarily intentional either.