Explosion rocks Japanese nuclear power plant; 5 reactors in peril

In what may become the most serious nuclear power crisis since the Chernobyl disaster, the explosion followed large tremors at the Fukushima Daiichi No. 1 reactor Saturday afternoon, injuring four workers who were struggling to get the quake-stricken unit under control.
Earlier, Japan's Nuclear and Industrial Safety Agency had warned that the reactor, whose cooling system had been crippled by the giant earthquake on Friday, could be nearing a meltdown and that two radioactive substances, cesium and radioactive iodine, had already been detected nearby.
The full extent of the blast remained unclear, but footage on Japanese television showed that the walls of the building housing the reactor crumpled, leaving a skeletal metal frame, according to the Associated Press. The building was likely the containment structure surrounding the reactor vessel.