C0169 33 or slightly acid media. Determination of the soluble fraction is therefore an important problemand solubility studies have been reported on fallout from several _ of the shots at Operations CASTIE and REDWING. For CASTIE fallout, it was found that the soluble fraction was strongly dependent on the detonation enviroment, being around 0.05 for land shots and 0.58-0.73 for shots fired from a barge (Reference 15). The solubility in seawater of the fallout from the reef shot Tewa, Operation KEDWING, was investigated in two ways: by leaching of particles placed on top of a glass wool column and by centrifuging a suspension of the fallout material (Reference 13). The soluble fractions found by these two methods were 0.08 and 0.18, respectively. An ultrafiltration method was used for deter- mining the solubility of fallout from the land shot, Zuni. About 25 per cent of the total ganma activity and Np?3? were soluble in seawater and 5 per cent of the total gamma activity was soluble in rainwater. Recent investigations (Reference 60) have shown that biological availability is analogous to solubility in 1 N HCl. hy Bomb debris from large tests is 99 per cent soluble in 1 N HCl, independent of shot envirorment. ‘ 1.2.2 Cloud Development. During the later stages of existence of the fire- ball, it is transformed into a vortex ring whose rotational velocity persists up to the maximcloud altitude, at least for the larger shots. The vortex contains the fission products, environmental material and bomb components which were present in the fireball and is the site where the radioactive fallout par- ticles are generated. The cloud contimies to rise until its buoyency is reduced to zero by adiabatic expansion, entrainment 233