Dealing with Decoupled Nuclear Explosions Under a Comprehensive Test Ban Treaty

The detonation of nuclear explosions in huge underground cavities so as to muffle or decouple the seismic waves they generated has been debated for more than 35 years. This paper reviews the history of the decoupling concept, assesses what countries have the technological capabilities to carry out such a test of a given yield, and evaluates several decoupling scenarios. I conclude that testing with huge decoupling factors, DF, is feasible for yields of a few kilotons (kt) or larger only in cavities in salt domes. Past nuclear explosions conducted in salt that are large enough for the full decoupling of explosions with yields ≥ 0.5 kt are concentrated in only a few areas of Kazakhstan and Russia. The existence of all cavities of that size that were created by past explosions is known since the events that created those cavities must be at least 20 times larger in yield than the size of a fully decoupled event that can be detonated in them. Monitoring of cavities created in that way that may remain standing should be relatively easy at the 1 kt level if appropriate verification measures are put in place. While large cavities can be created in salt by solution mining, no country is known to have evacuated the brine from such a cavity and then conducted a decoupled nuclear explosion in it. Air-filled cavities in salt suitable for significant decoupled testing are stable over only a very narrow range of depths from about 200 m to a maximum of 900 to 1300 m. Most areas of thick salt deposits in the Former Soviet Union and the U.S. are typified by efficient transmission for seismic waves and low natural seismic activity. The scaled cavity radius of 20 m cited in the literature for full decoupling in granite is poorly determined, probably is too small, and has resulted in overestimates of the potential to employ cavities in hard rock for decoupled nuclear testing. For cavities in hard rock, lack of any known experience in conducting decoupled nuclear testing in them, insuring containment in the presence of large differences in principal stresses and the presence of joints and other inhomogeneities on a scale of 1 to 100 m, and the excavation of such a large cavity without being detected are factors that make clandestine decoupled testing of a few kt or larger very unlikely for sites in hard rock, even for countries with considerable testing experience. Decoupled testing of large DF in any media at such yields by countries lacking containment experience would be difficult to carry out in secrecy.