Chapter 3—Containing Underground Nuclear Explosions ¢ 47 Photo creat. David Graham Fast acting closure. Controlled Tunnel Purging Controlled tunnel! purgingis an intentional release of radioactive material to recover experimental equipment and ventilate test tunnels. During a the explosion (called ‘gas sampling’’), and sealing the drill back holes (called ‘‘cement back’’) RECORD OF CONTAINMENT controlled tunnel purging, gases from the tunnel are filtered, mixed with air to reduce the concentration, and released over time when weather conditions 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 containmentillustrates the various types of releases and their relative impact. Operational Release Containment Evaluation Panel Operational releases are small releases of radioactivity resulting from operational aspects of vertical driil 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 samples from The Containment Evaluation Panel defines successful 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: