"It is also likely that the water in the fuel cooling pond was supersaturated with hydrogen. When the explosion occurred, the hydrogen in solution in the cooling pond water would have frothed up, burned, and deflagrated, sending the fuel rods out of the top of the holding pond. Visualize a bottle of Coke dropping; the resultant the carbon dioxide coming out of solution and shooting out of the end; now instead of carbon dioxide picture it as burning hydrogen shooting out"
Empirical lab research by the University of Tokyo and the Japanese Atomic Energy Agency now confirms the generation of hydrogen in such fuel pools. They attribute the explosion in Fukushima 4 to hydrogen generated in fuel pools. The POTRBLOG team suspects that unlike Fukushima 3, the Fukushima 4 explosion did not manifest a large BLACK mushroom cloud because the Fukushima 4 explosion triggered before the fuel pools could become supersaturated with Hydrogen and Oxygen.This early triggering, as compared to Fukushima 3, would have limited damage to the roof line of the Fukushima 4 building.
For POTRBLOG's full analysis on the Unit 3 explosion see:
Was the Fukushima Daiichi #3 EXPLOSION a detonation or deflagration?