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Disarmament Diplomacy

Issue No. 66, September 2002

News Review

US Scientific Panel Defends CTBT

On July 31, a committee of distinguished US nuclear scientists issued a report on 'Technical Issues Related to the Comprehensive Test Ban Treaty'. The report was requested in January 2001 by retired General John Shalikashvili, former Chair of the Joint Chiefs of Staff, following his report to Congress on 'Findings and Recommendations Concerning the Comprehensive Nuclear Test Ban Treaty' (see Disarmament Diplomacy No. 53, December 2000/January 2001). It was conducted by an eleven-member National Academy of Sciences (NAS) committee chaired by John Holdren, Professor of Environmental Science and Public Policy in the Department of Earth and Planetary Sciences, Harvard University. Although the committee was not directed to submit an overall recommendation on whether the US should ratify the CTBT, it was requested to explore the possible national security advantages and drawbacks of doing so. The scientists unanimously concluded that the positive results of accession would far outweigh any negative impact. While the study can be guaranteed not to affect the anti-CTBT position of the current administration, it was enthusiastically received by treaty advocates as an important reminder of the long-term national security interest of the United States in helping to secure a global prohibition of nuclear testing.

The key findings and arguments of the committee, as set out in the Executive Summary to the 96-page report, were as follows:

"Confidence in the Nuclear-Weapon Stockpile and in Related Capabilities

We judge that the United States has the technical capabilities to maintain confidence in the safety and reliability of its existing nuclear-weapon stockpile under the CTBT, provided that adequate resources are made available to the Department of Energy's (DOE) nuclear-weapon complex and are properly focused on this task. The measures that are most important to maintaining and bolstering stockpile confidence are (a) maintaining and bolstering a highly motivated and competent work force in the nuclear-weapon laboratories and production complex, (b) intensifying stockpile surveillance, (c) enhancing manufacturing/remanufacturing capabilities, (d) increasing the performance margins of nuclear-weapon primaries, (e) sustaining the capacity for development and manufacture of the non-nuclear components of nuclear weapons, and (f) practicing "change discipline" in the maintenance and remanufacture of the nuclear subsystem. ...

Confidence in the safety and reliability of stockpiled nuclear weapons depended far more on activities in the first five [of these] categories...than on nuclear testing even when numbers and kinds of nuclear tests were essentially unconstrained. (The sixth category did not play a large role in the past, because weapons were generally replaced by new tested designs before cumulative changes could become a concern.) Most US nuclear tests were focused on the development of new designs; the other major roles of testing were exploring weapon physics and investigating weapon effects. The so-called stockpile confidence tests were limited to only one per year and - with two exceptions (involving weapon types retired soon after the tests) - they involved new-production units, so they would be better described as "production verification" tests. Even in the absence of constraints on nuclear testing, no need was ever identified for a program that would periodically subject stockpile weapons to nuclear tests.

Stockpile stewardship by means other than nuclear testing, then, is not a new requirement imposed by the CTBT. It has always been the mainstay of the US approach to maintaining confidence in stockpile safety and reliability. The fact that older nuclear designs are no longer being replaced by newer ones means, however, that the average age of the nuclear subsystems in the stockpile will increase over time beyond previous experience. ... This means that the enhanced surveillance activities that are part of the current SSP [Stockpile Stewardship Program] will become increasingly important. But that would be so whether nuclear testing continued or not. Nuclear testing would not add substantially to the SSP in its task of maintaining confidence in the assessment of the existing stockpile. ...

Although a properly focused SSP is capable, in our judgment, of maintaining the required confidence in the enduring stockpile under a CTBT, we do not believe that it will lead to a capability to certify new nuclear subsystems for entry into the stockpile without nuclear testing - unless by accepting a substantial reduction in the confidence in weapon performance associated with certification up until now, or a return to earlier, simpler, single-stage design concepts, such as gun-type weapons. ...

In the event that quantity replacements of major components of the nuclear subsystem should become necessary, prudence would indicate the desirability of formal peer reviews. Evaluation of the acceptability of age-related changes relative to original specifications and the cumulative effect of individually small modifications of the nuclear subsystem should also be subject to periodic independent review. Such reviews, involving the three weapon laboratories [Los Alamos, Lawrence Livermore and Sandia] and external reviewers, as appropriate, would evaluate potential adverse effects on system performance and the possible need for nuclear testing. ...

Some have asserted, in the CTBT debate, that confidence in the enduring stockpile will inevitably degrade over time in the absence of nuclear testing. Certainly, the aging of the stockpile combined with the lengthening interval since nuclear weapons were last exploded will create a growing challenge, over time, to the mechanisms for maintaining confidence in the stockpile. But we see no reason that the capabilities of those mechanisms - surveillance techniques, diagnostics, analytical and computational tools, science-based understanding, remanufacturing capabilities - cannot grow as fast as the challenge they must meet. ... It seems to us that the argument to the contrary...underestimates the current capabilities for stockpile stewardship, underestimates the effects of current and likely future rates of progress in improving these capabilities, and overestimates the role that nuclear testing ever played (or would ever be likely to play) in ensuring stockpile reliability.

Capabilities for Monitoring Nuclear Testing

Detection, identification, and attribution of nuclear explosions rest on a combination of methods, some being deployed under the International Monitoring System (IMS) established under the CTBT, some deployed as National Technical Means (NTM), and some relying on other methods of intelligence collection together with openly available data not originally acquired for treaty monitoring. The following conclusions presume that all of the elements of the IMS are deployed and supported at a level that ensures their full capability, functionality, and continuity of operation into the future.

In the absence of special efforts at evasion, nuclear explosions with a yield of 1 kiloton (kt) or more can be detected with high confidence in all environments. ...

The capabilities to detect and identify nuclear explosions without special efforts at evasion are considerably better than the "one kiloton worldwide" characterization that has often been stated for the IMS. If deemed necessary, these capabilities could be further improved by increasing the number of stations in networks whose data streams are continuously searched for signals.

In the history of discussions of the merits of a CTBT, a number of scenarios have been mentioned under which parties seeking to test clandestinely might be able to evade detection, identification, or attribution. With the exception of the use of underground cavities to decouple explosions from the surrounding geologic media and thereby reduce the seismic signal that is generated, none of these scenarios for evading detection and/or attribution has been explored experimentally. And the only one that would have a good chance of working without prior experimentation is masking a nuclear test with a large chemical explosion nearby in an underground mine. The experimentation needed to explore other approaches to evasion would be highly uncertain of success, costly, and likely in itself to be detected. ...

In the case of cavity decoupling, the experimental base is very small, and the signal-reduction ("decoupling") factor of 70 that is often mentioned as a general rule has actually only been achieved in one test of very low yield (about 0.4 kt). The practical difficulties of achieving a high decoupling factor - size and depth of the needed cavity and probability of significant venting - increase sharply with increasing yield. And evaders must reckon with the high sensitivity of the global IMS, with the possibility of detection by regional seismic networks operated for scientific purposes, and with the chance that a higher-than-expected yields will lead to detection because their cavity was sized for a smaller one.

As for mine-masking, chemical explosions in mines are typically ripple-fired and thus relatively inefficient at generating seismic signals compared to single explosions of the same total yield. For a nuclear explosion that is not cavity-decoupled to be hidden by a mine explosion of this type, the nuclear yield could not exceed about 10 percent of the aggregate yield of the chemical explosion. A very high yield, single-fired chemical explosion could mask a nuclear explosion with yield more comparable to the chemical one, but the very rarity of chemical explosions of this nature would draw suspicion to the event. Making a nuclear yield even as large as a kiloton in a mine would require combining the cavity-decoupling and mine-masking scenarios, adding to the difficulties of cavity decoupling already mentioned.

Taking all factors into account and assuming a fully functional IMS, we judge that an underground nuclear explosion cannot be confidently hidden if its yield is larger than 1 or 2 kt. ...

The task of monitoring is eased (and the difficulty of cheating magnified), finally, by the circumstance that most of the purposes of nuclear testing - and particularly exploring nuclear-weapon physics or developing new weapons - would require not one test but many. ...

It can be expected, in future decades, that monitoring capabilities will significantly improve beyond those described here, as instrumentation, communications, and methods of analysis improve, as data archives expand and experience increases, and as the limited regions associated with serious evasion scenarios become the subject of close attention and better understanding. Of course, the realization of this expectation depends on continued US public and policy maker recognition of the importance of this country's capacity to monitor nuclear testing, with concomitant commitments of resources to the task.

Potential Impact of Foreign Testing on US Security Interests and Concerns

The potential impact on US security interests and concerns of the low-yield foreign nuclear tests that could plausibly occur without detection in a CTBT regime can only be meaningfully assessed by comparison with two alternative situations - the situation in the absence of a CTBT, and the situation in which a CTBT is being strictly observed by all parties. ...

In the reference case of no CTBT at all, the nuclear-weapon states party to the Non-Proliferation Treaty (NPT) would be able to test without legal constraint in the underground environment (except for the 150-kt limit agreed to by the United States and Russia under the bilateral Threshold Test Ban Treaty), and non-parties to the NPT would similarly be able to test without constraint. Non-nuclear-weapon states party to the NPT would be constrained legally from testing. In this circumstance:

  • China and Russia might use the option of testing to make certain refinements in their nuclear arsenals. In the case of Russia, it is difficult to envision how such refinements could significantly increase the threats to US security interests that Russia can pose with the previously tested nuclear-weapon types it already possesses.
  • In the case of China, further nuclear testing might enable reductions in the size and weight of its nuclear warheads as well as improved yield-to-weight ratios. Such improvements would make it easier for China to expand and add multiple independently targetable re-entry vehicle (MIRV) to its strategic arsenal if it wanted to, and changes in these directions would affect US security interests. But China could also achieve some kinds of improvements in its nuclear weapons without nuclear testing, and if it wanted to do so it could achieve considerable expansion and MIRVing of its arsenal using nuclear-weapon types it has already tested.
  • India and Pakistan could use their option of testing, as non-parties to the Non-Proliferation Treaty, to perfect boosted fission weapons and thermonuclear weapons... (Of course they might also do this under a CTBT that they had not signed, but the absence of a CTBT and the resumption of testing by others would make it politically much easier for them to do so.) The likelihood that either of these countries would use nuclear weapons against the United States seems very low, but the United States and its allies would nonetheless have serious concerns about the increase in nuclear-weapons dangers and arms-race potential in and around South Asia that such developments would portend.
  • Plausibly larger than the direct effects of testing by nuclear-weapon states and non-parties to the NPT in the absence of a CTBT is the potential indirect effect of such testing in the form of a breakdown of the NPT regime, manifested in more widespread testing (by such countries as North Korea, Iraq, and Iran, for example), which could lead in turn to nuclear weapons acquisition by Japan, South Korea, and many others.

A future no-CTBT world, then, could be a more dangerous world than today's, for the United States and for others. In particular, the directions from which nuclear attack on the United States and its allies would have become conceivable...would have multiplied alarmingly.

In our second reference case of a CTBT scrupulously observed, nuclear threats to the United States could still evolve and grow, but the range of possibilities would be considerably constrained. Boosted fission weapons and thermonuclear weapons be confined to the few countries that already possess them and to those to which such weapons might be transferred, or to which designs might be communicated with sufficient precision that a trusting and competent recipient might be able to reproduce them. Other countries might have less stringent confidence requirements than does the United States, but, in general, they also are much more limited in the technology available for pursuing an exact reproduction; substitution of materials or techniques might bring uncertainty or even failure. Perhaps most importantly, in a world in which nuclear testing had been renounced and the NPT remained intact, nuclear proliferation would be opposed by a powerful political norm in which nuclear-weapon states and other parties to the NPT and CTBT would find their interests aligned. ...

States with extensive prior test experience are the ones most likely to be able to get away with any substantial degree of clandestine testing, and they are also the ones most able to benefit technically from clandestine tests under the severe constraints that the monitoring system will impose. But the only states in this category that are of possible security concern to the United States are Russia and China. As already noted, the threats those countries can pose to US interests with the types of nuclear weapons they have already tested are large. What they could achieve with the very limited nuclear testing they could plausibly conceal would not add much to this. ...

In relation to two of the key "comparison" questions posed at the beginning of this section...we therefore 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. ...

The worst-case scenario under a no-CTBT regime poses far bigger threats to US security - 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."

Note: on June 7, the US conducted its latest subcritical nuclear test - Oboe 9 ­- at the Nevada test site. Subcritical tests involve no self-sustaining nuclear reaction, and are not prohibited by the CTBT.

Related material on Acronym website:

Reports: Subcritical nuclear test successful after delays, Las Vegas Review-Journal, June 8; Technical issues related to the Comprehensive Nuclear Test Ban Treaty, Committee on Technical Issues Related to Ratification of the Comprehensive Nuclear Test Ban Treaty, US National Academy of Sciences, released July 31; Text - study cites value of a test ban treaty to US national security, Washington File, July 31; Council for a Livable World lauds test ban study, Council for a Livable World Press Release, July 31; Panel - nuke test ban would aid US, Associated Press, July 31; American nuclear scientists tell Bush to ratify test treaty, The Guardian, August 1.

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