It's the concern over the fuel rods themselves that are of concern. If the reactor vessel is 300 C, then the fuel within is certainly hotter. The heat, as I understand it, damages and can break down the plating on the rods in such a way that at some point, a nuclear reaction can occur which would produce an enormous amount of heat.
The idea is to cool the fuel down to prevent it from going "critical".
Ideally, the entire fuel assembly should be covered with water (normally, circulating through a heat exchanger of some sort as part of its circulation, to remove excess heat). They're trying to achieve that with sea water, and they're adding boron or boric acid (not sure which) to further reduce and block neutron flow so that the fuel is much less likely to go critical.
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)