Many of you will have seen this set of photographs of the region surrounding Chernobyl in Ukraine, some from archives and others taken by an unnamed Ukrainian woman who motorcycles through the area. (Since the combination of an attractive woman, a powerful motorcycle and an apocalyptic landscape constitutes some kind of pathetic geek fantasy, the original site was swiftly slashdotted; the above link is to a mirror I've made.)
Some time ago I pondered the question of `nuclear terrorism', and particularly the risk of an attack on a nuclear power station using a hijacked aeroplane. It remains unclear to me whether this is a realistic attack, but I think there's some chance that the containment structure of a reactor could be severely enough damaged by a crashing aeroplane to vent the reactor to the atmosphere. (The Chernobyl accident ocurred when an explosion inside the reactor vented its contents to the atmosphere.)
The answer -- as if you didn't know it -- is `a bloody big mess'. Here's a map of the southeastern UK, with the exclusion zones around Chernobyl overlaid on the area around the Sizewell power station:
(This is an approximate and pessimistic scenario. Obviously the fallout might be blown east, out to sea, rather than onto the land. After the Chernobyl accident, the wind changed while the fallout plume was still in the air, and the fallout settled both to the northeast and southwest of the reactor. In the above map, I've assumed an offshore wind which does not change direction while the fallout settles. I picked Sizewell because it's local. The data come from this site on Chernobyl, but unfortunately I've had to composite the above map manually because that site doesn't provide raw data on radiological contamination. The exclusion and evacuation zones are related to particular levels of contamination, chiefly with caesium-137, which is the major contaminant over a scale of a few years -- its halflife is about thirty years. The immediate radiological hazard would have been iodine-131, with a one-week halflife. The underlying map image is produced from the Ordnance Survey Get-a-map service. Image reproduced with kind permission of Ordnance Survey and Ordnance Survey of Northern Ireland.)
As I said above, I'm not sure whether a nuclear power station containment dome could be seriously damaged by an airliner crash. In 1988, Sandia National Laboratories in the US performed a test to address this question, by taking the fuselage of an old F4 fighter aeroplane and driving it into a concrete slab at 480 mph using a bunch of rockets. After the September 11th 2001 terrorist attacks, they published footage from the test on their website. The results of this test suggested that such a slab would be penetrated to a depth of about 2cm by the aeroplane fuselage, or about 6cm by the engines (which have very heavy axles, by comparison to the rest of the aeroplane). There are more details in this FAQ response; basically, the body of the aeroplane does very little damage (2cm penetration) but the engines go a bit further (6cm). Commercial aeroplanes are much heavier and have larger engines, of course. In my view the risk can't be ruled out, and effective countermeasures -- siting anti-aircraft missiles at nuclear sites -- are sufficiently cheap to be worthwhile in any case.