Foreign Relations of the United States, 1961–1963, Volume VII, Arms Control and Disarmament

305. Paper Prepared for the Kennedy-Macmillan Talks⁰

Birch Grove, England, June 30, 1963.

Question One. What is the maximum size of underground nuclear tests which the USSR could repeatedly carry out without significant fear of detection?

Answer. By testing in carefully chosen soft media, the USSR could test up to perhaps 3 KT and risk only a 10% chance of detection of individual tests by seismic means. The risk by detection by physical means would be larger if a series of tests were carried out, and the risk of detection by other kinds of intelligence would also increase. Substantially larger tests, up to perhaps 25¹KT, could be carried out with small risk of seismic detection by accomplishing them in spherical underground cavities of a few hundred feet in diameter. But the construction of such a cavity would run the risk of being detected by other means.

Question Two. What can be learned technically from such small underground tests and what will be the military significance of the increased knowledge?

Answer. Increases of several fold in the yield-to-weight ratio could be accomplished for weapons in the low kiloton range. The fraction of fissionable material in thermonuclear weapons could be reduced. Weapons could be developed with enhanced radiation yield and also with deliberately suppressed radiation yield. Some weapons effects tests could be carried out, specifically on the effects of radiation on warhead components and some studies of hardened structure response. The following weapons effects cannot be studied² underground: Electro-magnetic pulse, blackout, megaton-range ground shock, and full-scale interactions of re-entry vehicles.

The necessity of carrying out these tests clandestinely would substantially increase both the time scale and the cost of the program.

Studies of these sorts would permit development of a wide range of low-yield tactical nuclear weapons. Underground tests could also lead to improvements in the warheads of anti-ballistic missiles. However, the warhead is only one component in the necessarily complex ABM system, and the offensive forces have enough retaliatory means at their disposal that the possible improvements of ABM warheads do not appear to be of great military importance. Underground testing cannot be expected to lead to operationally meaningful improvements in strategic warheads.³

Question Three. Will atmospheric tests be required to prove out developments made by underground testing?

Answer. Atmospheric tests will not be required for development of tactical nuclear weapons. Nor are they necessary for improving some components of strategic weapons. However, atmospheric testing would be required: (1) For proving out significant new designs of strategic weapons which might be based on developments made by underground tests; (2) to test the operational effectiveness of anti-ballistic missile systems; (3) for a range of significant weapons effects tests.

0. Source: Kennedy Library, National Security Files, Kaysen Series, Test Ban Inspections. No classification marking. Prepared by a U.S.-U.K. team consisting of McNaughton, Sir Solly Zuckerman, Frank Long, and Archibald Duncan Wilson, Assistant Under Secretary of State in the Foreign Office.
1. A handwritten marginal note reads “50.”
2. A handwritten marginal insertion reads: “to any significant extent.”
3. An additional handwritten sentence in the margin reads: “JCS says extrapolated ideas could be tested in outer space (!)”