Forty percent of the radiation produced by a groundburst nuclear explosion comes later from the delayed fallout. Fallout dispersion is influenced primarily by wind direction, but also by the size and type of weapon, height of explosion, type of soil, rainfall and the nature of the terrain. The intensity of fallout declines with time. Generally, the decay rate is such that at 7 hours the radioactivity of an area is 1/7 that of 1 hour, and after 49 hours it is 1/100 that of 1 hour. The areas affected are shown in Table 1. Bombs over 1 Mt would be of a fission/fusion type, hence the theoretical amount of radioactivity released is only 50% of a pure fission bomb. Within a fallout contour, hot spots occur sporadically which are far more radioactive than the rest of the zone. Size of weapon
75t
1Mt
10Mt
Downwind distance (miles/km)
31/50
60/97
274
Maximum width (miles/km)
2.2/3.5
8.8/14
46/74
Area (square miles/km2)
54/140
415/1075
6140/15,903
Downwind distance (miles/km)
12/19
27/43.5
76/122
Maximum width (miles/km)
0.1/0.16
4/6.4
27/43.5
Area (square miles/km2)
9.5/24.6
85/220
1600/4144
3 G/hr
10 G/hr
Table 1: Area of land affected downwind from a nuclear explosion. Accumulated fallout is very important as far as animals (and humans) are concerned, as animals usually do not have access to fallout shelters in which to hide – neither do most humans. Fallout with an initial radiation dose of 1 Gy/hr would still be emitting 0.5 Gy/hr after 7 days. Table 2 shows approximate dimensions of areas receiving different accumulated doses of radiation after the first two weeks of a 1 Mt, 50% groundburst bomb.These are the areas, especially in the 4.5 and 10 Gy range, where animals are most likely to be affected by radiation. If a bomb was dropped on a nuclear reactor, the amount of radiation released would be greater, and each dose contour would spread wider. In the event of a major accident at a nuclear reactor, as happened at Chernobyl, large areas of land would also be contaminated by radiation, including gamma-emitters.
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