- 19 verified by actual field experiments. Several years of experience in the weapons program and extensive studies into containment failure mechanisms has resulted in a great deal of knowledge of the phenomenology involved. The debris resulting from a venting of radioactivity to the atmosphere can be categorized by the physical nature of the release: That resulting from seepage or that resulting from a "prompt" dynamic release. In the usual underground explosion a column-shaped volume of broken or crushed rock, termed a chimney, is formed as the initial cavity created by the explosion collapses. The volatile radionuclides produced by the explosion diffuse with cavity gases into the void spaces formed by the collapsed rock. This chimney material acts as a filter sO that the only radioactive material which can seep to the surface to reach the atmosphere consists of noble gases and a relatively small amount of iodine. The amount of rudioactivity released by seepage is a very small fraction of that formed and can be measured only by very sophisticated laboratory equipment and exacting analytical techniques. The Commission is continuing its efforts to define containment models which will predict more accurately the effects of various types of rock Materials and various chemical techniques designed to reduce the amounts of volatile radionuclides produced. The possibility of seepage of radio- activity to the atmosphere is considered for every underground nuclear test designed for containment. Calculations of the number of curies of radio- activity thit credibly could be released to the atmosphere under an accident