[Hugh R]

I'm not a nuclear engineer or even anything close. But I did stay at a Hilton last night (not a Holiday Inn). When the Plutonium fissions, it essentially burns (think combustion, but not really) and in the process emits alpha, beta and gamma radiation (think heat from combustion). That is what blasted those observers. Since the fission is not 100% perfect some residual Plutonium, and radioactive daughters, are left over which will drift downwind and continue to emit alpha, beta and gamma radiation. Those radioactive solids are relatively dense, especially Plutonium, and they settle out into the food chain. The radioacitve daughters like Thorium and radioactive (heavy) Iodine, Potassium and Carbon are easily picked up by the body; as was seen with the Japanese nuke accident when people in the USA were trying to buy Iodine supplements for fear of thyroid cancer. There is possibly a measurable statistical increase in cancer in the World from that (as RWebb correctly noted, gubmit security clouds those findings). The exposure/risk drops off relative to a gaussian plume dispersion model. What that means is think of smoke coming from your barbeque, and how that smoke disipates as you get farther away from the BBQ. Its the same sort of dispersion but on a global scale. Which is why they see high concentrations of radiation at the nuke plants in Japan, but not much in Hawaii and even less in Los Angeles. Maybe someone else can explain it better.