The hydrodynamic yield of an underground nuclear explosion in a small cavity has been recalculated using computer modeling. We have considered explosions in spherical cavities having scaled radii, R/W1/3, less than 3.5 m/kt1/3 (R is the cavity radius in meters and W is the explosion energy in kilotons) in quartz (SiO2) using an available equation of state (Ree, 1976). These calculations show, at a scaled radius of ≈1.5 m/kt1/3, a maximum hydrodynamic yield substantially greater than that of an explosion in a tamped cavity (enhanced coupling) as measured by the shock time of arrival in the near field. These results are in qualitative agreement with those of earlier investigators who utilized the reduced displacement potential to evaluate coupling. We found, however, that the coupling coefficient (ratio of yield for cavity model to yield for tamped model) is significantly reduced when an improved quartz equation of state is employed. We also calculated enhanced coupling in two additional geologic materials, salt and tuff. All of the calculations mentioned above (including our initial ones) were simplified through the omission of significant thermal radiation contributions to energy and pressure in the cavity and the diffusion by photon transport of energy from the explosion cavity into the adjacent rock. When we take these processes into account, we find that for a cavity radius of approximately 1.5 m/kt1/3, near the peak of enhancement for our models with no radiation, there is instead decoupling of about 15% in SiO2, 20% in tuff, and about 15% in salt.

At the largest scaled cavity radius considered, 3.42 m/kt1/3, there is a decoupling of 40% in SiO2, 35% in tuff, and 35% in salt. For all three media the coupling coefficients decrease monotonically as the cavity radius increases. This result is different from previous ones in which the radiation terms were neglected. The radiation processes not only provide another method for vaporization of the rock but also change the effective gamma of the cavity gas as a result of the reduced cavity temperature and the addition of radiation pressure and energy terms. We have also found that the major effect on coupling is due to the contribution of radiation to the equation of state. Computer calculations which omitted radiation diffusion showed that the major change in effective coupling is due to the radiation energy and pressure modifications of the equation of state and to the corresponding changes in the effective gamma of the cavity gas. Thus, while the radiation diffusion is responsible for minor changes in the coupling curves, the main trend of these curves does not depend on diffusion. ¿ American Geophysical Union 1989