"Could you explain to me how the hell fuel in Reactor 4 storage pond at Fukushima 1 went critical?"
I can. The term 'critical' is used to describe the point at which fission events are being caused by the neutrons emitted by other fission events. Also called a 'chain reaction.'
Reactor fuel emits neutrons (actually even 'depleted' fuel emits neutrons see half life). Unused (and spent) fuel is stored in storage ponds filled with water because water is an excellent neutron moderator. Thus any neutrons that are emitted by the natural decay, or by internal fission events of the material are absorbed in the water and converted into a (relatively) small amount of heat.
Air is a poor damper of neutrons, and in fact the chance of a neutron hitting an atom in a gas is greatly reduced.
When the water in a fuel storage pond is removed, the naturally occurring neutrons can reach other fuel that is stored in that pond. If they are sufficiently energetic they can cause atoms in the fuel they hit to fission. If they do cause fission then heat is generated and another neutron (note: it maybe 2 I admit I've forgotten the exact reaction with U235) is emitted which might cause another fission event.
Criticality is controlled by moderating the probability that a neutron will create a fission reaction. If you want to store uranium you can do this by either spreading it far apart or by placing it in an environment that moderates neutron flux. Since the latter is as 'simple' as putting it in water, that is the usual option.
The caveat is that your spent fuel will continue to generate heat, if you don't remove that heat eventually the water will boil. If the water boils sufficiently to allow fuel to be exposed to air, the result can be that the fuel passes its criticality threshold. If you can't keep water cool then you can replace it with something that moderates and doesn't boil.
No, the point isn't lost. And reactor ponds are not the 'best' way to store old fuel, they never have been. They were originally only needed during the refueling cycle and the spent fuel was to be removed off site.
Spent fuel you're not ever going to use again can be melted into boro-silicate glass and not only will it never go critical again, it can't leach into the environment, etc etc. If you've read the Yucca mountain documents I believe they make a pretty good case for 10K yr life expectancy for keeping the fuel out of circulation.
But can you explain why that reactor pond at that time (presumably lost water and) went critical???
The waste was just sitting around an already shut reactor at the time of the Tsunami. After the Tsunami, the workers were doling out water to the other reactors and they just forgot about the cool ponds? Did the pond just spring a leak? The reactors lost power after the Tsunami but we can presume they have it back now.
Just much, why aren't isn't the fuel pallets in these ponds spaces so far apart that one could never interact with another under any circumstances?
"But can you explain why that reactor pond at that time (presumably lost water and) went critical???"
No, but I can speculate.
We "know" [1] that the ponds were boiling at one point. As boiling is the point at which water becomes vapor, if nothing else the ponds will eventually boil dry.
We also know that the spent fuel pools require cooling and that all power has been cut to the plant so there is no cooling.
Temperature rise in the pond will be a function of how many fuel rods are present and the amount of U235 in those rods (are they 'new' waiting to be installed, or 'old' waiting to be disposed? I don't know and its not clear from what I've read what there relative age is.) The uranium is decaying to lead [2] and as it decays, and its decay products decay. It generates heat. Not enough heat to be a useful source of energy, but certainly enough heat to keep the water boiling.
My speculation is that since the other electrical infrastructure has been severely damaged around the plant (and they do not have power back yet [3]) that the equipment that was responsible for circulating water in the pond through convection coolers isn't running and natural convection has either been disrupted or is insufficient to keep the water temperature down.
This combines to allow the water to boil, and if the spent fuel rods in the water were to be uncovered, there is sufficient U235 remaining in the rods present for the group to reach crititcality.
"why aren't isn't the fuel pallets in these ponds spaces so far apart that one could never interact with another under any circumstances?" because the contingency plans did not include a triple failure of main power, diesel backup, and battery backup. I suspect in the future two things will be true, one there will be stronger limits on how many rods you can keep in the pond, and there will be a passive cooling requirement.
"Reports from the scene indicate the water in the cooling pond is boiling vigorously and engineers fear it will soon boil away, exposing the fuel rods, which would allow them to melt. "
I can. The term 'critical' is used to describe the point at which fission events are being caused by the neutrons emitted by other fission events. Also called a 'chain reaction.'
Reactor fuel emits neutrons (actually even 'depleted' fuel emits neutrons see half life). Unused (and spent) fuel is stored in storage ponds filled with water because water is an excellent neutron moderator. Thus any neutrons that are emitted by the natural decay, or by internal fission events of the material are absorbed in the water and converted into a (relatively) small amount of heat.
Air is a poor damper of neutrons, and in fact the chance of a neutron hitting an atom in a gas is greatly reduced.
When the water in a fuel storage pond is removed, the naturally occurring neutrons can reach other fuel that is stored in that pond. If they are sufficiently energetic they can cause atoms in the fuel they hit to fission. If they do cause fission then heat is generated and another neutron (note: it maybe 2 I admit I've forgotten the exact reaction with U235) is emitted which might cause another fission event.
Criticality is controlled by moderating the probability that a neutron will create a fission reaction. If you want to store uranium you can do this by either spreading it far apart or by placing it in an environment that moderates neutron flux. Since the latter is as 'simple' as putting it in water, that is the usual option.
The caveat is that your spent fuel will continue to generate heat, if you don't remove that heat eventually the water will boil. If the water boils sufficiently to allow fuel to be exposed to air, the result can be that the fuel passes its criticality threshold. If you can't keep water cool then you can replace it with something that moderates and doesn't boil.