UK atomic weapons part 2 - popcorning
In part 1 I described how in order to reshape and compress the plutonium pit in order to get a 300-ton yield the conventional explosives must be very accurately detonated, but what happens if the explosive is not detonated accurately?
The details are complex and the knowledge needed to give accurate answers is well beyond what's publicly available, but for high yields the supercritical mass must be formed very quickly so that it doesn't blow itself apart before it has time for a lot of the mass to undergo fission - in the 300 ton case "a lot" is about1% of the plutonium.
If a critical mass of formed more slowly, perhaps because the explosives were not detonated correctly, then a small yield "fizzle" can happen. This isn't very likely, as there isn't a lot of extra plutonium in a modern primary, and it has to be shaped into a sphere and also be compressed before a critical mass can be assembled, but it is perhaps possible. This is the other main cause of single point sensitivity, but what would the yield of such a low-order detonation be?
Well it can't be more than 300 tons, because getting 300 tons is the maximum that can be done with accurate detonation, but it may be ten kilos, a ton or even ten tons depending on the speed of assembly and the degree of supercriticality. More than ten tons or so yield is very unlikely, bordering on impossible.
Could this set off another low-order detonation in a nearby warhead? It's possible, although unlikely. How unlikely? Only the MOD could answer that, it depends on classified details of the design.
Lets look at a worst-case popcorning scenario, where the 48 warheads in a sub or in a store do set each other off in low order detonations. Note that a single medium-order 300-ton detonation will be enough to disrupt the remaining warheads.
If each warhead yielded the maximum10 tons then the total would be 480 tons, but for two too-long-to-explain-here reasons 350 tons is a better maximum figure, and even this is extremely unlikely - even 50 tons is pushing probability.
However lets say the maximum credible total yield for a single point failure or popcorn event is 350 tons - now this isn't spare change by any means.
The effects of sub-kiloton explosions are a bit different to multi-kiloton explosions, and it is quite possible to be killed by prompt radiation without suffering lethal blast or burn damage (though you will still get knocked ass-over-tit). That's what the 100 sieverts at 1 km figure is about. Think "neutron bomb", although neutron bombs are designed to give off lots of neutrons, and typically have yields in the low kiloton range.
However, any remotely likely popcorning event can only take place where there are several warheads close together, ie either in a sub or in a bunker, and the prompt radiation will be attenuated by the hull of the sub and perhaps the water, or the walls of the bunker. The raw radiation will consist of neutrons and gamma rays of approximately equal lethality.
The gamma rays will be attenuated by the hull of the sub by a factor of at least 500, and would not be a great concern at 1 km. The neutrons are much more of a worry, but a few feet of water or bunker walls would stop them from being lethal at 1 km.
In general, at 1 km distance from a popcorning event I'd be far more worried about contamination from plutonium than prompt radiation.
Is a popcorning event possible at all? For some early warheads undoubtedly the answer is yes, but could today's UK warheads popcorn? I don't think so, but that depends on classified design details I have no access to. The official line is "It's perhaps possible but extremely unlikely, we can't prove it but it may well be impossible". I'd agree in general, though perhaps not on exactly how unlikely.