MIGHTY OAK containment review

Description/Abstract

MIGHTY OAK was conducted on April 10, 1986, below the surface of Rainier Mesa in the T-tunnel complex. Containment of MIGHTY OAK was satisfactory in that there was no accidental release of radioactivity to the atmosphere whatsoever. However, the tunnel grout in the near-cavity region was not sufficiently constrained. Failure of this plug led to the subsequent failure of the mechanical closure systems and to the eventual flow of cavity gas, plug material, and other debris down the line-of-sight (LOS) pipe to the experiment chamber and the tunnel complex beyond. Because of the resulting damage to equipment and facilities, a significant portion of the MIGHTY OAK containment-related diagnostics data was lost. Consequently, little direct information is available concerning why the ground-motion/LOS pipe closure process failed to form an adequate plug. However, we have made an extensive review of relevant information available, including the containment system design, site geologic structure and materials property data, as-built installations, and previous experience. As a result, we have identified three design modifications that together we feel would have prevented the observed failure. These include: (1) using a mechanical LOS closure and abutment that has been strengthened to provide better confinement of the grout plug; (2) using alternating lengths of strong and weak grout to fill the LOS drift between the cavity and the MAC to avoid excessive motion (and loss) of the weak grout; and (3) using grout materials whose composition is such that that their behavior will be reliable and reproducible. Additionally, we recommend improving the capabilities of the mechanical LOS closures so they better complement the ground-motion-induced closure. Other, more general recommendations are included in the following report. 3 figs.