The threat of a fission explosion at the Fukushima power facility emerged today when the roof of the number three reactor exploded and fears that a spent fuel pool, located over the reactor, has been compromised. The pool, designed to allow reactor fuel to cool off for several years, was constructed on top of the Fukushima reactors instead of underground. As of 2010, there were 3450 fuel assemblies in the pool at the number three reactor. The destruction of the number three reactor building has experts concerned about whether the spent fuel storage pool, which sits just below the roof, could have survived intact the hydrogen explosion. The explosion was much more severe than Saturday’s blast at the number one reactor.