"TOKYO — Japan’s nuclear crisis verged toward catastrophe on Tuesday after an explosion damaged the vessel containing the nuclear core at one reactor and a fire at another spewed large amounts of radioactive material into the air, according to statements from Japanese government and industry officials.
In a brief morning address to the nation Tokyo time, Prime Minister Naoto Kan pleaded for calm, but warned that radiation had already spread from the crippled reactors and there was “a very high risk” of further leakage.
The sudden turn of events, after an explosion Monday at one reactor and then an early-morning explosion Tuesday at yet another — the third in four days at the plant — already made the crisis at the Fukushima Daiichi Nuclear Power Station the worst nuclear accident since the Chernobyl reactor disaster a quarter century ago.
Engineers at the plant, working at tremendous personal risk, on Tuesday continued efforts to cool down the most heavily damaged unit, reactor No. 2, by pumping in seawater. According to government statements, most of the 800 workers at the plant had been withdrawn, leaving 50 or so workers in a desperate effort to keep the cores of three stricken reactors cooled with seawater pumped by firefighting equipment, while crews battled to put out the fire at the No. 4 reactor, which they claimed to have done just after noon on Tuesday.
But late Tuesday, Japan’s nuclear watchdog said a pool storing spent fuel rods at that fourth reactor had overheated and reached boiling point and had become unapproachable by workers at the plant. The fire earlier Tuesday morning was sparked by a hydrogen explosion generated by rising temperatures at the fuel pool, which released radioactivity directly into the atmosphere."
"The Risks Of Radiation Exposure" (Toronto Globe & Mail)
"New Blast at Japanese Nuclear Plant" (Financial Times) 
Japan Asks for U.S. Help as 3rd Blast Rocks Earthquake-Hit Fukushima Plant" (New York Daily News)