Pool at Stricken Reactor #4 Holds Answers to Key Safety Questions

Of all the terrible news from the crippled Fukushima Daiichi nuclear power plant, reports about the spent fuel storage pool for reactor #4 may be among the most disconcerting for scientists. The pool held the entire complement of fuel rods from the reactor9s core, which had been emptied 3 months before the 11 March earthquake and tsunami struck. And yet on 15 March the building exploded, apparently fueled by hydrogen, leaving nuclear engineers to speculate about the source. Adding to the confusion are reports of fires in the pool, a worst-case scenario that had never before occurred in a working nuclear plant. Unraveling the mysteries surrounding the #4 pool will require discovering why water levels there fell so quickly and whether the 230 tons of spent nuclear fuel melted in addition to catching on fire. * With reporting by Dennis Normile in Tokyo.