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DESCRIPTION OF UNDERGROUND BURSTS 59gases and vapors can thus supply addi- crater depends on the energy yield of thetional energy to the spalled layers, so detonation and on the nature of the exthattheir upward motion is sustained for cavated medium. In general, for equivatime or even increased. This effect is alent conditions, the volume of thereferred to as "gas acceleration." crater is roughly proportional to the2.93 The ground surface moving yield of the explosion.upward first assumes the shape of a 2.94 The relative extents to whichdome. As the dome continues to in- spalling and gas acceleration contributecrease in height, cracks form through to the formation of a throwout craterwhich the cavity gases vent to the at- depend to large extent on the moisturemosphere. The mound then disinte- content of the rock medium. In rockgrates completely and the rock frag- containing a moderately large propormentsare thrown upward and outward tion of water, the cavity pressure is(Fig. 2.93). Subsequently, much of the greatly increased by the presence ofejected material collapses and falls water vapor. Gas acceleration then playsback, partly into the newly formed an important role in crater formation. Incrater and partly onto the surrounding dry rock, however, the contribution of"lip." The material that falls back im- gas acceleration to the upward motion ofmediately into the crater is called the the ground is generally small and may"fallback," whereas that descending be unobservable.on the lip is called the "ejecta." The 2.95 As in an underwater burst, partsize of the remaining (or "apparent") of the energy released by the weapon inFigure 2.93.Shallow underground burst.
60 DESCRIPTIONS OF NUCLEAR EXPLOSIONSa shallow underground explosion ap- tamination over a large area, the extentpears as an air blast wave. The fraction of which depends upon the depth ofof the energy imparted to the air in the burst, the nature of the soil, and theform of blast depends primarily on the atmospheric conditions, as well as upondepth of burst for the given total energy the energy yield of the explosion. A dryyield. The greater the depth of burst, the sandy terrain would be particularly consmaller,in general, will be the propor- ducive to base surge formation in antion of shock energy that escapes into underground burst.the air. For a sufficiently deep explo- 2.97 Throwout crater formation ission, there is, of course, no blast wave. apparently always accompanied by abase surge. If gas acceleration occurs,BASE SURGE AND MAIN CLOUD however, a cloud consisting of particlesof various sizes and the hot gases2.96 When the fallback from a escaping from the explosion cavity genshallowunderground detonation de- erally also forms and rises to a height ofscends to the ground, it entrains air and thousands of feet. This is usually refinedust particles which are carried ferred to as the "main cloud," to disdownward.The dust-laden air upon tinguish it from the base surge cloud.reaching the ground moves outward as a The latter surrounds the base of the mainresult of its momentum and density, cloud and spreads out initially to athereby producing a base surge, similar greater distance. The main cloud andto that observed in shallow underwater base surge formed in the SEDAN testexplosions. The base surge of dirt par- (100 kilotons yield, depth of burial 635ticles moves outward from the center of feet in alluvium containing 7 percent ofthe explosion and is subsequently car- water) are shown in the photograph inried downwind. Eventually the particles Fig. 2.97, taken six minutes after thesettle out and produce radioactive con- explosion.Figure 2.97.Main cloud and base surge 6 minutes after the SEDAN underground burst.
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