If rods containing boron and not containing hydrogen were put in and among the assemblies, that would put a neutron absorber and remove H2O from the vicinity. The risk of criticality is greatest with the fuel that has not been irradiated. It is small with the spent fuel because the U235 has been consumed and neutron absorbing fission products have accumulated.

Boron can also be dissolved in the water (and very likely is). It is poor practice to rely on boron in the water because if the fuel breaks up, the pieces can fall to the bottom and there is less water in the pile, then there is also less boron. If you mechanically mix boron containing stuff (like pyrex glass or boron carbide), then even if stuff breaks and sinks there is still plenty of boron around.

If there is a criticality event, what that means is that the power released by the critical mass starts increasing exponentially. The size of the exponent depends on how many neutrons get generated and how many get absorbed or lost before they cause fission. Criticality is when for every neutrons that causes fission, more than one neutron causes another fission. The average number of neutrons produced per fission is ~2.4 or so. If 1.4 of those neutrons are lost or absorbed, then the system is critical. If 1.5 are lost then the system is subcritical. If 1.3 are lost, then 1.1 cause fission and the system is supercritical. Criticality is at exactly 1.00000

The neutrons produced during fission are high energy (MeV). These are so fast that they diffuse out of the critical mass quickly. Hydrogen slows them down by the scattering. Hydrogen is the best moderator because hydrogen and a neutron have the same mass (momentum and energy are conserved, so high Z elements don’t moderate very well).

Neutron production is not completely instantaneous. There are some “delayed” neutrons, but these are relatively few. It is these delayed neutrons that allow reactors to be controlled. If you maintain the number of neutrons produced to be greater than 1.000 but only by the number of delayed neutrons, then the power of the reactor will increase, but only due to the delayed neutrons. You can make this time constant as slow as you want because the delayed neutrons have a very long tail. If you insert a neutron absorber the number of neutrons drops immediately so you can stop things quite fast.

In the event of criticality due to moderated neutrons, if the moderator is lost, then the average velocity of neutrons goes up and more are lost. This is the logic behind the “negative void coefficient”, where a loss of coolant causes a decrease in reactivity, reactor power and a loss of criticality. This is typical behavior for light water reactors. The light water is the moderator, if a bubble forms there is less moderator, the velocity of neutrons goes up and more are lost so less are produced.

This is the type of criticality most likely to occur. There could be a boiling of the water, a void would form, the reaction would be quenched, the bubble would collapse, criticality would be restored and this could go on for a long time. This would generate very substantial circulation of water. Unless that flow of water damaged the fuel assemblies, that wouldn’t lead to the much more dangerous type of criticality, criticality due to fast neutrons. This type of criticality can occur in melted fuel. Melted fuel doesn’t have hydrogen in it because it is so hot. There is no limit to how hot a reactor can get other than the boiling point of the melted fuel. Once the fuel starts to boil, then its density goes down and more neutrons are lost from its surface and it becomes subcritical.

If the system is subcritical immersed in water, it will not become critical if the water is removed. Water lowers the amount of fissile material required for criticality by acting as a moderator.

Water does keep the fuel cool, and if the fuel gets too hot, then it can melt, collapse or break apart. How quickly that would happen depends on how much decay heat the fuel is producing. The fuel elements are in zirconium tubes. Zirconium can take pretty high temperatures, but it is susceptible to oxidation by air, steam, or even nitrogen.

Gundersen told Reuters of an incredibly dangerous “criticality” that would result if a chain reaction takes place at any point, if the rods break or even so much as collide with each other in the wrong way. The resulting radiation is too great for the cooling pool to absorb – it simply has not been designed to do so.

http://americablog.com/2013/11/fukushima-high-risk-tepco-work-reactor-4-started.html

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