"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. "
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.
[1] http://www.latimes.com/news/nationworld/world/la-sci-0316-ja...
"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. "
[2] http://scifun.chem.wisc.edu/chemweek/radiation/radiation.htm...
[3] http://www.iaea.org/newscenter/news/tsunamiupdate01.html
"Attempts to return power to the entire Daiichi site are also continuing."