"...structures which have subsequently fallen into the fuel pool likely means that there could well be an uncontrolled, if weak, fission reaction currently going on in the reactor 4 SFP (where the water temperature is currently 90 degrees)..."
"According to TEPCO, radioactive iodine-131 amounting to 220 becquerels per cubic centimeter, cesium-134 of 88 becquerels and cesium-137 of 93 becquerels were detected in the pool water. Those substances are generated by nuclear fission."
It is not chain rreaction - the numbers clearly contradict a possibility of it being ongoing there!
A chain reaction at 90 degrees, which would emit into the water just some couple of dozens Bq/cm3 after a month (which most probably got there with the spraying operation - one would of course expect dozens of Megabequerels/m3 of Cs-137 there from fallout), and having such an izotope signature??
The isotope signature in the water clearly shows the chain reaction stopped more than month ago. For ongoing chain reaction the I-131 ratio to Cs-137 would need to be >35 and to Cs-137 >20.