The Fallout Fraction When radimetive materials are released to the atmosphere during a prompt massive venting of an underground nuclear detonation, part of the radioactive debris falls rather rapidly to the earth's surface and is commonly referred to as local fallout. The remaining radioactivity, that in gaseous form or associated with very small particles, is carried to much greater downwind distances. That portion of the total activity produced by the. nuclear explosion which is deposited in the local fallout pattern is referred to as the fallout fraction. Withincertain limite the fallout fraction is a function of the scaled distance of the device beneath the earth's surface. It is also a function of other variables such as the water content of the medium surrounding the device. Experimental data show that for devices detonated within the range of scaled depths fran about 15 w/3 to 150 w/3 the associated fallout fractions range from about 80% downward to a few percent, respectively. These data form the basis for an empirical relationship which provides useful estimates of the fallout fractions to be expected fram detonations in this rance of scaled depths. Data also indicate that for larger scaled depths of burial, the fallout fraction approaches zero as the scaled depth asymptotically approaches a value of about 350 w/3 sometimes referred to as the asymptote of no venting. Unfortunately, experience has also shown that unexpectedly large fallout fractions have indeed occurred at even greater scaled depths of burial. Data exist which perhaps indicate that a reasonable upper limit to the fallout fraction which might result from an accidental massive venting of an underground detonation designed for complete containment is on the