Credible Scenarios to Evade Detection of Nuclear Explosions by the CTBT's Verification System
Dr Yury Khokhlov
The CTBT verification system known as the International Monitoring System (IMS) is designed to detect non-evasively conducted nuclear explosions down to at least 1 kt detonated in the atmosphere, underwater or underground.
Any attempts to conduct a clandestine nuclear test in the atmosphere or underwater are likely to be detected by one or more elements of the IMS, e.g. the radionuclide or hydroacoustic system. The detection of any signals, whether they are anomalous or not, could lead to a request for an on-site-inspection (OSI). For a number of reasons the best environment in which to attempt a clandestine test is underground. Any seismic signals generated by such an explosion may be buried in the background noise and hence not detected or obscured by one of the many thousands of earthquakes which occur each year.
A number of possible methods of conducting a clandestine test underground and evading detection have been proposed over the years and techniques have been developed to defeat them, e.g. multiple explosions to simulate earthquake-like characteristics. However, the most practical one is that known as decoupling in which an explosion is conducted underground in a cavity so that the seismic waves are effectively muffled and as a result may not be detected. What is required to achieve decoupling is a cavity filled with an energy-absorbing medium so that the pressure from the shock wave on the cavity wall is made equal to, or less than, the overburden pressure. The most effective energy-absorbing medium is air or a very porous material. Theoretical simulation studies have shown that the maximum decoupling of an air-filled cavity occurs when the radius of the cavity is equal to or greater than 30m/kt1/3 where m is in metres and kt is in kilotonnes. At low frequencies, say ~1 Hz a decoupled explosion conducted in a cavity excavated in salt will reduce the seismic waves generated by a factor of the order of 100. This means a 100 kt nuclear explosion would generate seismic waves equivalent to a 1 kt explosion. If the cavity size is reduced then the decoupling factor decreases and the explosion becomes partially decoupled.
The experimental evidence for decoupling of nuclear explosions is limited, it is however, convincing. Both the Soviet Union and the United States have proved that nuclear explosions detonated in salt cavities can be decoupled by factors of between 40 and 70 although there is evidence that at higher frequencies (around 15-20 Hz) the decoupling factor is significantly less but to detect such frequencies would require a seismic station quite close to the epicentre, say within 500 kms.
To conduct a decoupled explosion would entail considerable financial expense to construct a suitable cavity underground. To prevent any surface evidence of an underground explosion the nuclear device would need to be placed in a cavity at considerable depth, of the order of 1 km or more for even a 1 kt explosion. (To prevent any surface evidence the relationship between depth and explosion yield is of the order of: Depth = 1000m W1/3 where W is the yield in kt). Even at these depths the natural fissures surrounding the cavity may allow radioactive noble permeate to leak to the surface and be detected by the IMS radionuclide system.
A potential evader must decide what yield he wishes to conduct evasively. Also what cavity volume is required for that yield and at what depth must the cavity be to achieve full decoupling. He must be sure that the cavity created will survive at that depth for long enough to emplace the device and conduct the test. In some rock-types the cavity may collapse or may fill with water. These are some of the problems the evader must address. Furthermore could such activities be conducted without being detected by another nation's NTM? Clearly there are many uncertainties for a potential evader to consider. Such technical evidence, detected by NTM could be used to request the CTBTO Executive Council for an OSI.
Of course the evader could consider using an existing cavity created by a nuclear explosion conducted before the moratorium or the CTBT came into force. This is probably a riskier enterprise than constructing a new cavity. Most nuclear explosion generated cavities occur at shallow depths with extensive fracturing radiating out from the cavity many of which will be close to the surface increasing the risk of releasing noble-gases to the atmosphere.
Within the provisions of the Treaty there is a procedure for resolving ambiguous signals detected by the IMS and these are given in the Table below.
Confidence-building measures	Each State Party on a voluntary basis shall provide the Technical Secretariat with notification of any chemical explosion ³ 300 t TNT and provide to the Technical Secretariat information related to its national use of all chemical explosions ³ 300 t TNT
Detection of ambiguous events and standard event screening	International Monitoring System and co-operating national facilities, International Data Centre, national technical means
Clarification the fact whether a nuclear explosion has been carried out and gathering any facts to assist in identifying any possible violator	On-site inspection