In Stricken Fuel-Cooling Pools, a Danger for the Longer Term, New York Times, March 14, 2011:

Emphasis Added

…Experts now fear that the pool containing those rods from the fourth reactor has run dry, allowing the rods to overheat and catch fire. That could spread radioactive materials far and wide in dangerous clouds.

The pools are a worry at the stricken reactors at the Fukushima Daiichi plant because at least two of the three have lost their roofs in explosions, exposing the spent fuel pools to the atmosphere. By contrast, reactors have strong containment vessels that stand a better chance of bottling up radiation from a meltdown of the fuel in the reactor core.

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

A spokesman for the Japanese company that runs the stricken reactors said in an interview on Monday that the spent fuel at the Fukushima Daiichi and Daini plants had been left uncooled since shortly after the quake. …

The threat is considered so severe that at the start of the crisis Friday, immediately after the shattering earthquake, Fukushima plant officials focused their attention on a damaged storage pool for spent nuclear fuel at the No. 2 reactor at Daiichi, said a nuclear executive who requested anonymity because his company is not involved in the emergency response at the reactors and is wary of antagonizing other companies in the industry.

Brookhaven study of uncovered spent fuel

A 1997 study by the Brookhaven National Laboratory on Long Island described a worst-case disaster from uncovered spent fuel in a reactor cooling pool. It estimated 100 quick deaths would occur within a range of 500 miles and 138,000 eventual deaths.

The study also found that land over 2,170 miles would be contaminated and damages would hit $546 billion.

That section of the Brookhaven study focused on boiling water reactors — the kind at the heart of the Japanese crisis.

Worse than a meltdown says Expert

“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. “The reactor is inside thick walls, and the spent fuel of Reactors 1 and 3 is out in the open.” …

Depending on the freshness of the spent fuel, Mr. Lochbaum said, the water in an uncooled pool would start to boil in anywhere from days to a week. The water would boil off to a dangerous level in another week or two. …

Once most of the fuel is exposed, he said, it can catch fire.

How long will radioactivity last?

If the spent fuel is a few months old, most of the iodine 131 — one of the most dangerous radioactive byproducts in spent fuel — will have decayed into harmless forms.

But the cesium 137 in the spent fuel has a half-life of 30 years, meaning it would take about two centuries to diminish its levels of radioactivity down to 1 percent.

It is cesium 137 that still contaminates much land in Ukraine around the Chernobyl reactor, which suffered a meltdown in 1986.

Read the New York Times report here.

Spent nuclear fuel pool catches fire, plant operator says, Deutsche Presse-Agentur, March 15, 2011, 9:32 am GMT:

Water in a pool storing spent nuclear fuel at the No. 4 reactor at the Fukushima 1 nuclear power plant may be boiling after the fuel had caught fire earlier, the Tokyo Electric Power Co. (TEPCO) said Tuesday. …