Fukushima

Lethal radioactive particles, notably strontium-90 and caesium-137, would stream into the atmosphere and be dispersed according to the vagaries of the weather, winds and currents. Even worse, were this process to unfold at Reactor No 4 (or at any of the others, of course, it's just that No 4 is the most vulnerable), it might become impossible to prevent similar processes eventually unfolding in all the other spent fuel repositories at the site (substantial as it is, the load at No 4 is little more than 10% of the total).

It has been a standard practice in the nuclear industry to avoid consideration of all of these possibilities, based on the assumption that there will be "lots of time" to react to any emergency involving the spent fuel pool, as it will normally take days for the spent fuel to reach the melting point and it will be a "simple matter" to refill the pools with water if necessary.

This ignores the fact that major structural damage may make it impossible to approach the spent fuel pool due to the lethal levels of gamma radiation emanating from the spent fuel once the protective shielding of the water is gone.

To judge by various estimates found here and there on the net, the damage from such an ultimately uncontrolled spent fuel fire hardly bears thinking about.

Based on U.S. Energy Department data, assuming a total of 11,138 spent fuel assemblies are being stored at the Dai-Ichi site, nearly all, which is in pools. They contain roughly 336 million curies (~1.2 E+19 Bq) of long-lived radioactivity. About 134 million curies is Cesium-137 - roughly 85 times the amount of Cs-137 released at the Chernobyl accident as estimated by the U.S. National Council on Radiation Protection (NCRP).[author's emphasis]

Viewed in this light, the light of what might have been, March 2011 starts to look like a win. For days (perhaps weeks) after the 11th, things teetered on the brink. Had just one spent fuel pool seriously cracked, had some pumps failed for an hour or two too long, it might have been all over; there may then have been no way to indefinitely keep the fuel assemblies covered with water and the whole thing would have fed on itself.

As it was, all that escaped into the air was a comparative smidgen. Enough, mind you, to cause the evacuation of 160,000 people and create a 20 km exclusion zone which is still mostly in force. Thankfully, the evacuations were ordered early enough to avoid any significant radiation damage to residents surrounding the plant.

The curious thing is, in one sense radioactive materials are more manageable than I'd thought. Particle emissions, for example, are for the most part easily blocked, sometimes with as little as a sheet of paper. Gamma rays are altogether more vicious, although even they can be shielded against with sufficient depth of water, concrete, lead or steel. The real trouble starts when radioactive particles get out into the environment. Then, they very quickly end up in living creatures where they wreak their damage directly. Once they've escaped and been scattered by wind, water and rain, the deed is done, much of it irrevocable.

And the effects, well, they go on, and on. Thousands, tens of thousands, hundreds of thousands of years. Much further into the future than Neanderthal man lies in the past. Have we taken leave of our senses, perhaps, to take such risks, however slight, for so little gain?