Possible 'worse than meltdown' scenario brewing at Japanese nuclear plant

A nuclear crisis at the quake-hit Fukushima No. 1 nuclear plant deepened Tuesday as fresh explosions occurred at the site and its operator said water in a pool storing spent nuclear fuel rods may be boiling, an ominous sign for the release of high-level radioactive materials from the fuel.Tokyo Electric Power Co. said water levels in the pool storing the spent fuel rods at its Fukushima No. 1 nuclear plant's No. 4 reactor may have dropped, exposing the rods. The firm said it has not yet confirmed the current water levels or started operations to pour water into the facility.Unless the spent fuel rods are cooled down, they could be damaged and emit radioactive substances. The government's Nuclear and Industrial Safety Agency urged TEPCO to inject water into the pool soon to prevent heating of the fuel rods.
More from the NYT, apparently it's a VERY bad thing:

The pools, which sit on the top level of the reactor buildings and keep spent fuel submerged in water, have lost their cooling systems and the Japanese have been unable to take emergency steps because of the multiplying crises.

By late Tuesday, the water meant to cool spent fuel rods in the No. 4 reactor was boiling, Japan’s nuclear watchdog said. If the water evaporates and the rods run dry, they could overheat and catch fire, potentially spreading radioactive materials in dangerous clouds.

If any of the spent fuel rods in the pools do indeed catch fire, nuclear experts say, the high heat would loft the radiation in clouds that would spread the radioactivity.

“It’s worse than a meltdown,” said David A. Lochbaum, a nuclear engineer at the Union of Concerned Scientists who worked as an instructor on the kinds of General Electric reactors used in Japan.