design sophistication are much less likely to succeed in concealing significant tests. This is in part because of the importance of test experience in constructing cavities that can achieve seismic decoupling without leaking radioactivity, and in part because considerable weapon-design experience is required to achieve low yields. Countries with lesser prior test experience and/or design sophistication would also lack the sophisticated test-related expertise to extract much value from such very-low-yield tests as they might be able to conceal. They could lay some useful groundwork for a subsequent open test program in the event that they left the CTBT regime or it collapsed, but they would not be able to cross any of the thresholds in nuclear-weapons development that would matter in terms of the threat they could pose to the United States.

Undetected evasive testing under a CTBT would be limited to a level of about 1 to 2 kilotons and probably would be much less due to difficulties involved in evasive testing, particularly for states without extensive nuclear testing experience and availability of the required geological formations. They should properly be concerned that the yield of the test device might exceed that planned. To avoid this, an evader might conduct a series of “creep up” tests—which would increase the probability of detection and would be costly in terms of nuclear materials. The inability to test at yields above 1 to 2 kt would preclude the demonstration of boosted fission weapons, of primary nuclear explosives for driving the thermonuclear secondaries of strategic weapons, and the demonstration of thermonuclear weaponry. Possible evasive hydronuclear tests, which might escape detection by seismic means, would serve primarily to determine whether nuclear weapons are safe against accidental detonation at a single point; such tests in violation of the CTBT would not impair U.S. security interests and they would be costly in terms of the expenditure of plutonium.

In relation to two of the key “comparison” questions posed at the beginning of this chapter about the implications of potential clandestine testing, then, we conclude as follows:

  • Very little of the benefit of a scrupulously observed CTBT regime would be lost in the case of clandestine testing within the considerable constraints imposed by the available monitoring capabilities. Those countries that are best able to successfully conduct such clandestine testing already possess advanced nuclear weapons of a number of types and could add little, with additional testing, to the threats they already pose or can pose to the United States. Countries of lesser nuclear test experience and/or design sophistication would be unable to conceal tests in the numbers and yields required to master nuclear weapons more advanced than the ones they could develop and deploy without any testing at all.

  • The worst-case scenario under a no-CTBT regime poses far bigger threats to U.S. security interests—sophisticated nuclear weapons in the hands of many more adversaries—than the worst-case scenario of clandestine testing in a CTBT regime, within the constraints posed by the monitoring system.

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