Particle size and chemical fractionation

The ratio of activities 137Cs/1311 is much higher in reactor than in bomb fission products. The release of 137Cs from Chernobyl was 10 times greater than that from the Nevada tests, though only about 4% of the release in H tests. 

The particle size of a fission aerosol, and the distribution of fission products between particulate and vapour phases, depends on the mechanism of release to the atmosphere. In a weapons explosion, some physicochemical fractionation of radionuclides may occur, particularly if the explosion is near the ground. Everything in the vicinity is vapourised by the heat of the explosion, but within less than a minute the fireball cools to a temperature in the range 1000-2000°C, and refractory materials such as metal oxides and silicates condense to form particles (Glasstone Dolan, 1977). Refractory fission products, and plutonium, are incorporated in these particles. 

Less-refractory fission products condense later onto the surface of the particles. Those with gaseous precursors, for example Sr and 137Cs, condense as they are formed by decay of their parent nuclides. The 

At distances of about 100 km from ground zero of the Nevada tests, the particle size of fallout ranged from a few jum to a few 100 jum. Fission products with volatile precursors were enhanced by about a factor two compared with refractory fission products. The fractionation was greater the smaller the particles (Hicks, 1982). 

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