least two pathways from 238 U,via production from ‘-ourity in 2398p device materials. 235 U, or as an unburned It is also produced as a decay product . eg 5 25 2 24 of short-lived transamericium isotopes, 254 NG 250e, Fm, $46c¢, cf een, The isotcpic compe-ftian of some plutonium fuels can be found in Keller (op.cit.), and Poet and Martel] (1972). are commonly reported together (e.g. The concentrations of 239 Pu and 240 Pu 2394240 Pu) since alpha spectroscopy measurements cannot distinguish between their energies. Plutonium-238 is the next most commonly reported alpha-emitting plutonium isotope in fallout. 2394240, The 238 . Pu:"~" Pu activity ratio measured in samples which have accumulated the isotioe from fallout is about 20:1 (Noshkin, 1972). 3.2 Introduction of Radioactivity in Fallout The composition, structures and origins of radioactive fallout particles produced in the nuclear testing program at the Pacific proving ground were investigated by Schell (1959) and Adams, Fairlow and Schell (1960). The formation of fallout particles were then, and have since been, shown to be governed by the interactions of the condensing vaporized device and soil materials in the cooling fireball and by entrainment and/or impaction pro- cesses with non-vaporized materials swept into the fireball at later times. Because of their extreme dilution, the individual radioactive elements are never concentrated sufficiently to condense as individual metal or oxide particles but only condense on to the particles which are being formed by the major vaporized constituents and by the soil materials (Adams et al.,op.cit.). A sianificant effort has been directed toward describing the fractionation of radionuclides following nuclear explosions. Quantitatively, fractiona- tion of volatile and refractory radioelements was first demonstrated by Freiling (1962). Heft (1970) reported: For land Surface and subsurface detonations, the (particle size] distribution functions may be expressed as linear combinations of two or three log-normal distribution functions. Each component corresponds to a particle group