Recent investigationa (Reference 16) have shown that biological availability ls analogous to solubility in 1 N HCl. Debris from megaton-range bursts is 99 percent soluble in 1 N HCl, tndependent of shot environment. 1.2.2 Cloud Development. During the later stages of existence of the fireball, it is transformed into a vortex ring whose rotational velocity persists up to the maximum cloudaltitude, at least for the larger shots. The vortex contains the fission products, environmental material, and bomb components that were present in the fireball and is the site where the radioactivefallout particles are generated. The cloud continues to rise until its buoyancy is reduced to zero by adiabatic expansion, entraining of cold air, and loss of energy in overcoming atmospheric drag (References 17 through 19). The diameter of the ring increases rapidly during the ascent, and the cloud spreads out laterally to a large area as its upward velocity decreases. For smaller yields the cloud stops at the tropopause or below, but for megaton~range yields the top may penetrate several thousand feet into the stratosphere. The time to maximum altitude is somewhat less than 10 minutes. A knowledge of the distribution of activity and particles within the stabilized cloud is needed for the establishment of a rational fallout model; however, the collection of a suitable set of samples that could be used to determine these quantities experimentally presents a formidable operational problem that has not yet been solved. Several distributions have been assumed in an effort to matchthe fallout patterns on the ground, but it is not known how closely these models correspond to the actual structure of the cloud. Considering the method of formation, it might be anticipated that the activity would be greatest in an anchor ring centered on the axis of the | cloud. Some evidence for this structure was obtained during Operation Redwing with rockets with telemetering ionization chambers (Reference 20). 1.2.3 Transport and Distribution. During the ascent of the nuclear cloud, the particles are acted on by body forces and by the vertical currents in the rising air. Someof the large parti- cles will be heavy enough so that they will have a net downward velocity even though the cloud as a whole is moving upward. They will contribute to the fallout in the immediate vicinity of ground zero (Reference 21). During this time, volatile fission products may be fractionated from less volatile fission products by a kind of fractional distillation process within the hot cloud. Once the upward motion has ceased, the particles in the cloud will begin to settle out at rates determined by their density, dimensions, and shapes and by the viscosity and density of the air (Reference 22). The terminal velocities for small spheres can be accurately calculated when the dependenceof the drag coefficient on Reynold’s number is known. Irregular or angular particles will fall more slowly than spheres of the same weight, but their velocities cannot be estimated as well because of uncertainty in the shape factors (Reference 23}. The particles that make up the local fallout follow trajectories to the surface governed by their fall races and by the mean wind vector between their points of origin in the cloud and the ground level. Locations can be specified by reference to a surface coordinate system made up of height lines and size lines. The height lines are the loci of the points of arrival of all particles originating at given heights on the axis of the cloud. The size lines connect the arrival points of particles of the same size from different altitudes. Time and space variation of the winds will change the magnitude and direction of the mean wind vector, and vertical motions in the atmosphere will alter the falling rates of the particles. Corrections for these effects can be made when adequate meteorological data is available. The local fallout, as defined here, will be down in 4.5 gate of particles ranging from about 25-micron diameter shots the majority of this will be in the troposphere, but portion will be deposited in the stratosphere. Hence, in days or less, leaving aloft an aggre~ down to submicron size. For small for megaton-range yields a large prodiscussing worldwide fallout, it is de- sirable to consider it as subdivided into two classes identified as tropospheric (or intermediate) fallout and stratospheric (or delayed) fallout (Reference 24). 14