Chapter 3—Containing Underground Nuclear Explosions ¢ 47 Photo creat David Graham Fast acting closure. Controlled Tunnel Purging the explosion (called ‘‘gas sampling’’), and sealing the drill back holes (called *‘cement back"’) Controlled tunnel purgingis an intentional release of radioactive material to recover experimental equipment and ventilate test tunnels. During a controlled tunnel purging, gases from the tunnel are filtered, mixed with air to reduce the concentration, and released over time when weatherconditions are favorable for dispersion into sparsely populated areas. The containment of underground nuclear explosions is a process that has continually evolved through learning, experimentation. and experience. The record of containment illustrates the various types of releases and their relative impact. Operational Release Containment Evaluation Panel Operational releases are smallreleases of radioactivity resulting from operational aspects of vertical drill hole tests. Activities that often result in operational releases include: drilling back down to the location of the explosion to collect core samples (called **drill back’’), collecting gas sampies from RECORD OF CONTAINMENT The Containment Evaluation Panel defines suc- cessful containment as no radioactivity detectable offsite and no unanticipated release of activity onsite. By this definition, the CEP has failed to predict unsuccessful containment on four occasions since 1970: