From the NY Times:
WASHINGTON — As the scale of Japan’s nuclear crisis begins to come to light, experts in Japan and the United States say the country is now facing a cascade of accumulating problems that suggest that radioactive releases of steam from the crippled plants could go on for weeks or even months.
The emergency flooding of stricken reactors with seawater and the resulting steam releases are a desperate step intended to avoid a much bigger problem: a full meltdown of the nuclear cores in reactors at the Fukushima Daiichi Nuclear Power Station. On Monday, an explosion blew the roof off the second reactor, not damaging the core, officials said, but presumably leaking more radiation.
Later Monday, the government said cooling systems at a third reactor had failed. The Kyodo news agency reported that the damaged fuel rods at the third reactor had been temporarily exposed, increasing the risk of overheating. Sea water was being channeled into the reactor to cover the rods, Kyodo reported.
So far, Japanese officials have said the melting of the nuclear cores in the two plants is assumed to be “partial,” and the amount of radioactivity measured outside the plants, though twice the level Japan considers safe, has been relatively modest.