Hmm. My (admittedly layman's) understanding of nuclear physics is that in the absence of a neutron "brake" (eg., the control rods), the reaction will occur uncontrollably. In fact, even with the "brake" fully applied (the control rods fully inserted, absorbing a large percentage of stray neutrons), the nuclear fuel still generates an enormous amount of decay heat (7-10% of full capacity). So much so that they still need to be cooled by being completely immersed in water. And that water gets so hot that if it were not in a sealed pressure vessel, it would be boiling. In fact, in cases where the water level dropped and the water did in fact boil, even the tiny "voids" in the water (the bubbles) were enough to compromise the cooling effect of the water, and overheating occurred.
Decay heat for a uranium or MOX reactor starts off at 6.6% of the previous sustained power level and drops off on an exponential function from there, to 1.5% after an hour, down to 0.2% after a week.
The decay curve function:
The fission chain reaction in these plants relies on thermal neutrons, neutrons produced by a fission of uranium or plutonium and slowed by a 'moderator' until they are moving so slowly that they can be caught and absorbed by another uranium or plutonium nucleus. There's a thing called the '
absorption cross-section' that indicates how readily the neutron is captured. Fast moving neutrons are not readily captured by the nuclear fuel in these reactors, and need to be slowed down.
In the Chernobyl plant, the moderator that slowed down neutrons was the graphite blocks the fuel was embedded in. That meant that unless the control rods could be fully inserted, the nuclear chain reaction would continue.
The water-moderated reactors, used for almost all nuclear power applications currently, rely on the water to slow the neutrons. With the water removed, the fast neutrons rapidly escape the reactor core and are absorbed in the surrounding neutron shield. (Typically a jacket including boron, which has a high absorption cross-section for neutrons but doesn't become radioactive.)
With the water removed, the nuclear chain reaction that supports continued fission stops.
My point is that even with the control rods fully inserted, the reaction is still occurring. If the cooling agent (water) drops below the tops of the fuel rods, they overheat and melt, even when only producing "decay" heat. Thus, the reaction is "sustained," even after the geometry of the fuel rods disintegrates into a puddle of molten uranium.
The fission reaction is stopped at this point, as the water 'moderator' is removed. The control rods, metal alloys that absorb neutrons so as to damp the chain reaction, sill in the event of a core meltdown become part of that puddle in the bottom of the reactor vessal.
As I understand it (and again, I'm no physicist), the only requirements for a sustained fission reaction is proximity/density of fuel, and absence of a neutron absorber. In a molten state, at the bottom of the reactor chamber, I would expect that both of these conditions would be met, and the reaction would indeed be sustained indefinitely.
Proximity of fuel, and presence of a neutron flux sufficient to be absorbed and maintain a chain reaction, to be specific. Removing the moderator results in a loss of the neutron flux. The remaining fast neutron flux is insufficient to maintain a chain reaction. The core being collapsed and spread out over a reactor vessel bottom doesn't provide the needed proximity. (The fuel region looks like a disk, rather than a concentrated sphere, and is laced with neutron absorbing material from the control rods.)
Note that when water was eventually injected for cooling, it included a charge of boric acid. The boron atoms are potent neutron absorbers, 'poisoning' any neutron chain reaction. This is done to prevent the coolant from acting as a moderator for a partial meltdown configuration that might otherwise sustain a local chain reaction in part of the core.
There are reactors, such as the fast breeder design, that can maintain a chain reaction using a fast neutron flux. These use a different fuel mix, and special materials to reflect and concentrate the neutron flux in the core. A precise core geometry is also needed to keep the reaction going. The conditions to sustain this reaction simply do not exist in a thermal neutron reactor such as the Fukushima plant uses.
I'm not sure what you mean here - you seem to be saying the water helped facilitate the fission reaction. My understanding is that the water actually discourages the nuclear reaction (by absorbing stray neutrons, preventing them from perpetuating more reactions), in addition to providing a cooling effect to avoid meltdown.
Pure water does facilitate the reaction, by slowing neutrons so that that can be absorbed by the fuel. Borated water, and to some extent ordinary sea water will damp the reaction by absorbing neutrons.
Sure, but I believe that that "decay heat" is itself a fission reaction, and in the absence of a cooling agent, will generate enough heat to reduce the fuel to a molten state.
Decay heat is not a
fission reaction. Decay heat comes from radioactive decay processes. Often the products of a fission reaction are two atoms, with roughly similar masses about half that of the fissioned atom. (That is, fission typically breaks a big atom in half, so we get two lighter atoms, and maybe a couple of free neutrons.) These fission product atoms often have an extra neutron or two in their nucleus compared to stable atoms of the same element. These atoms will 'decay' by emitting an electron from their nucleus, effectively trading a surplus neutron in for a proton and becoming more stable.
This process results in the release of heat, but it is not a chain reaction. As shown above, the decay heat from these secondary nuclear reactions drops off pretty quickly.
To be fair, we're talking about dramatically different amounts of nuclear fuel here. The amount of raw nuclear material in the Hiroshima and Nagasaki bombs were about the size of a softball. The nuclear reactors in question contain much, much more fuel than that. Also, the bombs were only able to fission about 10% of their material before being dissipated too broadly for the reaction to continue. Left undisturbed, the reactor core material will continue to fission to a much higher percentage, unless we can extract it and separate it from itself.
Again, I'm not a physicist, everything I just wrote could be completely wrong.
Actually, the reactor core contains a much more dilute form of the active nuclear material (Uranium-235) than a weapon, and as I have indicated above, the fission reaction stops in the absence of the moderator, or the insertion of control rods. (It also stops pretty quickly if one simply stops managing the reaction at all courtesy of a thing called the negative temperature coefficient of reactivity, but that is beyond the scope of the current topic.)
Oh, I do have a degree in physics, and taught reactor plant operation for two and a half years. I think I was fairly successful at it, as Nords and Gumby never did manage to vaporize central Idaho...
* Graphs, formulae, and links courtesy of Wikipedia. It's nice that someone else scanned in the texts and graphs.