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The Threshold Test Ban Treaty

Edward Ifft

As the likelihood increases that the Comprehensive Test Ban Treaty (CTBT) will get a new look in the Senate and perhaps advance closer to entry into force in the Obama administration,[1] it is useful to look back at its antecedents. There are three principal ones: the 1963 Limited Test Ban Treaty (LTBT), known in most of the world as the Partial Test Ban Treaty; the 1968 nuclear Nonproliferation Treaty (NPT); and the 1974 Threshold Test Ban Treaty (TTBT). The NPT put in place the promise to end nuclear testing forever, and the LTBT repeated this promise and prohibited nuclear explosions in all environments except underground. Meanwhile, the NPT's Article VI disarmament provision was widely understood to include a commitment to enact eventually a comprehensive ban on nuclear testing. Quite properly, constant attention is given to the NPT, and Arms Control Today discussed the LTBT and NPT negotiations recently.[2] The TTBT, however, is sometimes forgotten, which is unfortunate because it was an important step in the 60-year struggle to control nuclear explosions and it has interesting lessons to tell about the art and science of negotiations, as well as verification and compliance issues.

Origins of the TTBT

Beginning with Indian Prime Minister Jawaharlal Nehru's famous 1954 call for an end to all nuclear testing, there have been many starts and stops to political discussions and efforts of groups of experts to solve the political, military, and verification problems associated with a potential CTBT. President John Kennedy settled for the LTBT rather than a comprehensive treaty because of Soviet General Secretary Nikita Khrushchev's resistance to on-site inspections. Yet, the Soviet Union was pressing for a CTBT by 1974 and gaining public relations points while the United States resisted because of concerns about verification and stockpile reliability. In March 1974, at a meeting between Secretary of State Henry Kissinger and Foreign Minister Andrei Gromyko, it was agreed in principle to seek to ban nuclear explosions above some yet-to-be-determined yield. The issue was given even greater urgency in May when India exploded a nuclear device in the Rajasthan Desert, calling it a "peaceful nuclear explosion" (PNE).

A few days earlier, a U.S. delegation had flown by military plane to Moscow to begin what were described as "technical discussions" on a possible TTBT. Those of a conspiratorial nature may conclude that this term was used to keep the bureaucracy from getting too excited about what was about to happen. Moscow was chosen as the venue to get the U.S. team away from the distractions of Washington and to allow greater access to Soviet officials, some of whom would not have been allowed to travel outside the Soviet Union because of the sensitive nature of their work.

The chief U.S. negotiator was Walter Stoessel, ambassador to the Soviet Union. I functioned as his deputy, chairing the internal meetings and supervising the preparation of delegation statements and reports to Washington. Norm Terrell, who worked for Helmut Sonnenfeldt, counselor of the Department of State and close colleague of Kissinger, was the primary link to Kissinger through Sonnenfeldt. Nine other members of the U.S. team represented the other relevant agencies. They had a very high level of expertise, especially on the verification issues. Carl Romney, a legendary Air Force seismologist, took the lead on the crucial issue of seismic detection of underground explosions. An unusual and very useful feature of the group was that two eminent geophysicists were "public members" of the delegation, not representing any agency. They were Professor Lynn Sykes of Columbia University and Professor Gene Herron of Southern Methodist University. They added an independent scientific view, which sometimes served as a good counter to entrenched U.S. government views, which were not always correct, and provided added credibility with the scientific community.

The Soviet delegation was also impressive. It was led by Igor Morokhov, deputy minister of the so-called Ministry of Medium-Machine Building (Minsredmash), which ran the Soviet nuclear complex. His deputy was Ambassador Roland Timerbaev, an experienced and skillful negotiator from the Ministry of Foreign Affairs. General Aleksandr Osin, thought to be director of the Soviet nuclear test site in Kazakhstan, was the chief military representative. Another important figure was Aleksandr Zakharenkov, introduced as a professor at the Kurchatov Institute, but actually from Minsredmash.[3] The rest of the Soviet team consisted of experts on nuclear testing, PNEs, and verification.

The U.S. delegation had been told they would be in Moscow for "a few days" for "technical discussions." Some unfortunately believed this and packed their suitcases accordingly. In actuality, the negotiations continued for five intensive weeks. I wrote the initial plenary statement on the overnight flight to Moscow. We hit the ground running and held nine plenary meetings in the first 10 days. This was rather remarkable. In the initial Strategic Arms Limitation Talks (SALT) negotiations in 1969, Ambassador Gerard Smith had wanted to hold a formal plenary every day but quickly found it impossible to have more than three such meetings a week. This was due to the burden of getting U.S. statements prepared and agreed, along with the necessary extensive reporting and analysis for Washington.

Working conditions in Moscow for the U.S. delegation were cramped. We lived in the old Rossiya Hotel across Red Square from the Kremlin and had our offices in the U.S. embassy. This was the old building on Ulitsa Tchaikovskaya, the current embassy not having been constructed yet. Space was at such a premium that the delegation was allotted two small rooms with three desks and chairs, plus an acoustic conference room for classified work. The team consisted of 13 officers, plus two secretaries. The secretaries got two of the desks and chairs, which left one for the 13 other officers. I joked that the first person to show up in the morning could have the chair and the others would have to stand all day. In practice, the day consisted of one long working session in the conference room. The success of the whole operation seemed to validate the proposition that progress in negotiations is inversely proportional to the comfort of the negotiators. If this is indeed correct, it may explain why progress is so frustratingly slow in places such as Geneva, New York, and Vienna.

The atmosphere in the negotiations was very good from the start. Part of the sense of urgency was the knowledge that there was a summit meeting in July and it would be good to have a treaty ready to sign. Still, it was clear from the start that there were three fundamental issues that needed to be addressed:

1. What should be the yield threshold?

2. How could this threshold be successfully monitored?

3. How should PNEs be dealt with?

The Yield Question

Although this was the fundamental question, there was only so much one could say about it. Both sides were guarded, waiting for the other to state a preference. It became clear that the United States would come down somewhere between 100 kilotons and 200 kilotons, although some in the U.S. delegation preferred a higher number. The Soviets, who preferred a CTBT in any case, were guarded in their statements.[4] There was a limit to what could be tested at the Nevada Test Site-the resultant shaking of buildings in Las Vegas was becoming a problem. Multimegaton tests, such as the 1971 Cannikan test of the Spartan ABM warhead conducted at Amchitka in the Aleutian Islands, caused a public uproar and no longer seemed necessary in any case.

Stoessel normally did not attend internal delegation meetings but did attend an important one near the end of the negotiations in which he asked each person for his recommendation on the number. Not surprisingly, the State Department and Arms Control and Disarmament Agency (ACDA) representatives advocated a lower number, while those from the Department of Defense asked for a higher level. It is not known what Stoessel recommended to Washington, but it was wisely decided to leave the final number to the summit, which decided on 150 kilotons.

Although this level seemed high to some, it was a real constraint, both sides having conducted many tests above this level before the threshold took effect in 1976. For example, during 1972-1976, the Soviet Union conducted at least 24 tests with yields reported to be more than 150 kilotons, including three with yields between 1.5 megatons and 10 megatons.[5] During this same period, the United States conducted at least 13 tests with yields reported to be more than 150 kilotons. This includes the Cannikan test, whose yield was reported to be "less than five megatons."[6] In contrast to these very large explosions, the device dropped on Hiroshima had a yield of about 15 kilotons, while the Nagasaki bomb had a yield of about 21 kilotons.


Everyone knew that verification would be a big problem because even the testers themselves could not predict or measure yields precisely, but both countries set about to solving it in a scientific, nonpolitical way. The meetings were held in a facility near the famous Tretyakov Gallery. It had blackboards, and it was unusual and gratifying, after the extreme formality of earlier arms control negotiations, to be able to encourage scientists from either side to go spontaneously to the blackboard in the middle of a formal meeting to illustrate their points and argue their case. This informal, problem-solving approach should be applied more frequently in international negotiations. It was crucial to producing a complete treaty from scratch in only five weeks. The completed verification protocol involved a surprisingly detailed exchange of calibration information regarding the actual yields and locations of earlier tests in each geophysically distinct area of the test site, along with the relevant characteristics of these areas, such as depth of water table, seismic velocity, porosity of the rock, etc.

Peaceful Nuclear Explosions

A particularly devilish problem concerned so-called PNEs. The Soviets had a very active program that involved using nuclear explosives for excavations, putting out oil well fires, mining gold, deep seismic sounding, etc. One very ambitious and controversial project under consideration was the Kama-Pechora Canal, which was part of the plan to divert water from flowing north into the Arctic Ocean to instead go south into the Volga and thence to Central Asia. This would have involved excavating a canal with high-yield PNEs. Although the United States was losing interest in PNEs by 1974, it had conducted 27 such experiments under Operation Plowshare and had an interest in using them for exploiting oil shale and other possibilities. More radical U.S. ideas were to excavate a sea-level canal using PNEs across Nicaragua to replace the Panama Canal and the excavation of a harbor at Cape Thompson, Alaska.[7]

Another complication was the fact that Article V of the NPT promised that "potential benefits from any peaceful applications of nuclear explosions will be made available to non-nuclear-weapon States Party to the Treaty on a nondiscriminatory basis." There was in fact some interest in "PNE services," for example, excavating a canal to bring water from the Mediterranean to Egypt's Qatarra Depression or excavating chambers deep in hard rock for the permanent storage of high-level radioactive waste. Exasperated at hearing of some negative remarks in Washington by ACDA Director Fred Ikle regarding PNEs, Morokhov described him as "a man divorced from reality." As fate would have it, the tables were turned in 1977 when General Secretary Leonid Brezhnev declared a moratorium on Soviet PNEs. In all, the Soviet Union conducted 124 such explosions.[8]

In theory, it might have been possible to design a regime to distinguish between weapon tests and PNEs and even provide PNE services, perhaps through the International Atomic Energy Agency. In practice, the problem seemed just too difficult, and the United States showed no interest in solving it. It was clearly impossible to combine weapons tests and PNEs in such a way as to prevent any possible weapons benefit from PNEs. The high yields and diverse locations of PNEs would have undermined the efforts to constrain weapon tests. The solution to the seemingly intractable PNE problem was to punt it to a separate agreement. The idea came about late one night when Norm Terrell and I were discussing the issue with Timerbaev. We agreed that the best and perhaps only way to deal with the interrelationship between weapons and PNEs was to have two separate treaties. Although without instructions on the matter, with the summit rapidly approaching, we each agreed to recommend it to our respective governments.

Even though it was near midnight (it is still light then in Moscow in late June), Norm and I went straight to Spaso House to present the idea to Stoessel. He came downstairs in his bathrobe, we discussed it by his fireplace, and he agreed. Norm and I then went directly to the embassy and sent a cable to Washington. Washington and Moscow both readily approved the idea. Unlike the celebrated "Walk in the Woods" between negotiators Paul Nitze and Yuli Kvitsinsky in the Intermediate Nuclear Forces negotiations eight years later, which was rejected by both sides, this was an example of an ad referendum approach that succeeded.[9] The idea that negotiators must never try such approaches is simply not correct.

As the summit drew near, the bureaucracy in Washington suddenly realized that the "technical talks" were about to give birth to an actual treaty, which would be signed in a few days by the two presidents. Several "political-level" officials were rushed to Moscow to take charge, with the result that the already overcrowded work space now became truly chaotic. In reality, the new folks, through no fault of their own, never really caught up to the fast-moving train and had a minimal impact.

The PNE Treaty

President Richard Nixon and Brezhnev signed the completed TTBT on July 3, 1974. The sides then set about to negotiate the companion treaty to govern PNEs. I returned to Washington to work on the SALT II negotiations, and Robert Buchheim became Stoessel's deputy. Other key players were the State Department's Robert Martin and leading scientists from the Los Alamos and Lawrence Livermore National Laboratories. The details of the PNE Treaty, which was signed on May 28, 1976, need not be discussed here. The key problem was to allow large PNE projects without circumventing the 150-kiloton limit on weapons tests. It was solved by allowing group explosions up to 1,500 kilotons, no one of which could exceed 150 kilotons, and setting up an elaborate system of on-site inspections to make sure this limit was not exceeded. The fundamental problem of how to distinguish between weapons tests and PNEs was solved in a clever way. The sides had to declare their weapon test sites-the Semipalatinsk site in Kazakhstan and Novaya Zemlya for the Soviets and the Nevada Test Site for the United States. Any nuclear explosion within these sites would be considered a weapons test and governed by the TTBT. Any nuclear explosion elsewhere would be considered a PNE and governed by the PNE Treaty. Thus the impossible question of the intent of a nuclear explosion was neatly swept aside. The PNE Treaty was never really used because each side had lost interest in high-yield PNEs by that time. This is not the only example in which arms control delegations worked diligently to solve a difficult problem only to have it disappear shortly thereafter.

Verification Problems

Although the sides had given Gromyko and Kissinger exactly what they had asked for with the TTBT, problems soon arose. It had been well understood by the scientists that seismic means alone would carry a significant band of uncertainty when converted into a yield estimate. This conversion required assumptions regarding a host of geophysical factors: type of rock in which the explosion occurred, water content, the coupling to deeper layers of rock, and the transmission paths to the seismic sensors, many of which would be thousands of kilometers distant. This was the reason for the unprecedented exchange of data mentioned above, which would reduce these uncertainties. This remarkable degree of cooperation would only occur with the exchange of the instruments of ratification, which did not occur, due to political factors. In particular, charges that the Soviets were cheating began to appear, most commonly through leaks to conservative newspaper columnists. Although the White House had been warned about the band of uncertainty in estimating yield, they were unprepared for the storm of criticism regarding the alleged cheating; and the scientists could not refute these charges because, due to measurement uncertainties, the data could be used to support a yield estimate either above or below 150 kilotons.

Although many sensitive aspects of the Soviet nuclear program have been revealed since the collapse of the Soviet Union, to the author's knowledge, we still do not know whether the Soviets exceeded the agreed 150-kiloton limit, either accidentally or on purpose. Indeed, it is not clear how often the United States itself may have accidentally exceeded the limit. An interesting footnote to these damaging controversies is that there were differences between U.S. and British seismologists regarding the methodology for estimating the yields of Soviet tests. The British approach, which led to lower yield estimates, compensated more effectively for differences between the hard-rock geology of the Soviet Union's test site, which absorbed less of the blast, and the U.S. test site, which contained softer rock. These "yield wars" continued for years until it was finally accepted that the British estimates were more accurate.[10]

That a test might accidentally exceed the estimated yield was well known. The sides attempted to deal with this in advance with a rather remarkable understanding to the effect that "one or two slight, unintended breaches per year" of the yield ceiling would not be considered a violation, although they would be a cause for concern and a subject for consultations. This understanding was part of the public record submitted to the Senate for its advice and consent to ratification.[11]

In 1976, with ratification increasingly problematic, each side agreed to observe the 150-kiloton limit pending ratification. This state of affairs, which continued for 16 years, is interesting from several perspectives. Politically, it created the bizarre situation that the sides accepted the limit without the verification regime and data exchange we had painstakingly worked out and that everyone agreed was essential to monitor the constraint. From a legal perspective, it is an interesting variation of the concept of "provisional application," which is recognized in the Vienna Convention on the Law of Treaties.[12]

Solving the Verification Problem

By 1986, President Ronald Reagan, having canceled the trilateral CTBT negotiations initiated by President Jimmy Carter, was under pressure from General Secretary Mikhail Gorbachev's "Peace Offensive," which included a strong push for a CTBT. Reagan proposed an alternative: if Gorbachev would agree to work on solving the verification problems in the TTBT and the PNE Treaty first, the United States would then negotiate "on ways to implement a step-by-step program...limiting and ultimately ending nuclear testing." Gorbachev agreed, and new delegations were sent to Geneva to improve the two verification protocols. The U.S. team was skillfully led by Ambassador Paul Robinson. It included a notable cast of characters with great experience in the U.S. nuclear test program. It was obvious what part of the country some of them came from, adorned with boots, huge silver belt buckles, string ties, and turquoise-fashion statements not usually seen in diplomatic circles in Geneva.

Taking full advantage of Gorbachev's openness to verification and transparency, the teams devised a very intrusive and effective set of measures for both treaties, including a requirement to notify all test explosions in advance. The most innovative provision allowed the monitoring side to make a yield measurement of any explosion more than 50 kilotons using a satellite hole drilled near the actual emplacement hole. The CORRTEX[13] technology employed relies on the fact that, unlike most sound waves, whose velocity is independent of the source, in the extreme conditions of temperature and pressure close to a nuclear explosion, there is a correlation between the yield and the velocity of the shock wave. Measuring the crushing of a coaxial cable in the satellite hole measures the velocity of the shock wave as it moves toward the surface and hence the yield of the explosion. Each side was also allowed to use on-site inspection for tests of more than 35 kilotons, along with three seismic stations to gather data on specified tests. In the United States, these were in Oklahoma, South Dakota, and Washington state, providing a good angular perspective on the Nevada Test Site. Similar stations were established in the Soviet Union.

To test the CORRTEX method, each side agreed to allow the other to monitor a nuclear explosion at its test site. The Soviets did so at the Nevada Test Site on August 17, 1988, and the United States monitored a test at the Semipalatinsk site in Kazakhstan on September 14. These operations were logistically complicated, but both sides were satisfied with the results and gained greater confidence in their ability to verify the treaty. At the participants' 10-year reunion held in Las Vegas and the Nevada Test Site in 1998, it was clear that the experience of working closely with mysterious former enemies had been a positive, life-changing one for many on each side. Ratification proceeded smoothly, and the TTBT and the PNE Treaty, with their new verification protocols, entered into force on December 11, 1990. The Soviets were able to monitor two U.S. nuclear tests before testing was stopped in 1992 by President Bill Clinton.

The United States never got to do the same because Gorbachev instituted a moratorium on Soviet tests in 1991, which was later extended by Russian President Boris Yeltsin. Congress responded in 1992 with legislation imposing a U.S. test moratorium in 1993. The new Clinton administration decided to extend the U.S. moratorium and negotiate a CTBT in the Conference on Disarmament in Geneva, negotiations that were launched the next year.[14]

Lessons from the TTBT Negotiations

One important lesson from the experience is that the oft-repeated statement that U.S.-Russian arms control negotiations must unavoidably be long, painful, and "adversarial" is simply not true. Of course, any negotiation will be adversarial in the sense that the sides will begin with different positions and perspectives, otherwise there would be no need for a negotiation at all. The TTBT negotiations went from zero to a full-blown and fairly complicated treaty in only five weeks.[15] The secret is that the political leadership must establish clear goals at the outset: What is the problem we are trying to solve? Then the sides must send negotiators who have the authority and the courage to make difficult decisions. Such negotiations need not be painful and certainly can be conducted in an atmosphere of mutual respect, regardless of whether one approves of all aspects of the other side's system. Recognition of this fact could help in solving even the most difficult future international security problems.

Another important lesson is the need for effective verification in arms control agreements and prompt and effective responses to compliance problems.[16] Charges and denials of cheating, particularly when played out in public and allowed to fester, will severely damage any regime, no matter how well crafted. This remains true for U.S.-Russian relations in spite of the admirable openness and easy communication we now enjoy. It will be even more so in future agreements with more difficult partners, such as Iran or North Korea.

One of the major problems with the TTBT and the PNE Treaty is that they never had much of a constituency in the United States. Advocates of nuclear testing feared that they put us on a slippery slope that would lead to a CTBT. On the other hand, the arms control community was not enthusiastic because they felt the treaties were a diversion that would delay a CTBT. Some were also displeased that the outcome appeared to legitimize PNEs. History has proven both sides partly right and partly wrong. The treaties provided many useful lessons and innovative ideas, only some of which are discussed here, and were probably the best approach in the circumstances of their time.

As a signatory state for the CTBT, the United States is bound by the Vienna Convention not to undermine the object and purpose of that treaty. In addition, the TTBT and the PNE Treaty remain in force. Their provisions would come into play if nuclear testing were resumed by either side. It is problematic whether either side has the equipment and trained inspectors ready to carry out these provisions on short notice. It is the hope of essentially the entire world, including all close allies of the United States, that the day is not far off when the CTBT will enter into force and take the place of the TTBT and the PNE Treaty and we can close the books on these agreements, which are fascinating, but only stepping stones to the ultimate goal.

Edward Ifft is a retired Department of State official who served as the deputy head of negotiations on the Threshold Test Ban Treaty, deputy director of the On-Site Inspection Agency, and senior adviser to the Defense Threat Reduction Agency. He is an adjunct professor in the security studies program of the School of Foreign Service at Georgetown University. The views expressed in this paper are those of the author and do not necessarily reflect the policy of the U.S. government.


1. As a candidate, Senator Barack Obama (D-Ill.) stated, "As President, I will reach out to the Senate to secure the ratification of the CTBT at the earliest practical date and will then launch a diplomatic effort to bring onboard other states whose ratifications are required for the treaty to enter into force." "Arms Control Today 2008 Presidential Q&A: President-elect Barack Obama," Arms Control Today, December 2008, special section.

2. Daryl Kimball and Wade Boese, "Limited Test Ban Treaty Turns 40," Arms Control Today, October 2003, pp. 37-38; George Bunn and John Rhinelander, "The Nuclear Nonproliferation Treaty Then and Now," Arms Control Today, July/August 2008, pp. 56-60.

3. Roland Timerbaev, "On the Threshold Test Ban Treaties of 1974-76," Security Index, Vol. 13, No. 2 (82) (2006).

4. For a discussion of the internal Soviet deliberations, see ibid.

5. I. A. Andryushin et al., "50 Years of Peace-The History of Nuclear Testing in the USSR," DSWA-TR-96-87, October 1997.

6. U.S. Department of Energy, "United States Nuclear Tests," DOE/NV-209 Rev. 14, December 1994.

7. Comprehensive Test Ban Treaty Organization (CTBTO) Preparatory Commission, "Peaceful Nuclear Explosions,"

8. Timerbaev, "On the Threshold Test Ban Treaties of 1974-76." See Milo Nordyke, "The Soviet Program for Peaceful Nuclear Explosions," Science and Global Security, Vol. 7, No. 1 (1998), pp. 1-117.

9. Miles A. Pomper, "In Memoriam: Paul H. Nitze (1907-2004)," Arms Control Today, December 2004, pp. 44-45.

10. Peter Marshall and Alan Douglas, "Strategic Perspectives Through Oral History," in U.S.-UK Nuclear Cooperation After 50 Years, ed. Jenifer Mackby and Paul Cornish (2008), pp. 291-295.

11. U.S. Arms Control and Disarmament Agency, Arms Control and Disarmament Agreements (1996), p. 134.

12. Vienna Convention on the Law of Treaties, art. 25 (1969). For a discussion of the legal issues associated with possible provisional application of the Comprehensive Test Ban Treaty (CTBT), see Edward Ifft et al., "A New Look at the Comprehensive Nuclear-Test-Ban Treaty," Security Paper, No. 6 (2008).

13. CORRTEX stands for Continuous Reflectometry for Radius Versus Time Experiments. The Soviets had a similar system known as Methods of Impulse Sensing (MIS).

14. For further details on the history of negotiations on nuclear testing, see UN Institute for Disarmament Research, "In Pursuit of a Nuclear Test Ban Treaty," 1991. For a detailed chronology of events, see SAIC, "A Chronology of Comprehensive Test Ban Proposals, Negotiations and Debates," 1994. See also Jaap Ramaker et al., The Final Test, (Vienna: CTBTO, 2003).

15. Of course, one may object that the treaties required extensive "fixing" later, but this does not invalidate these conclusions.

16. "Effective verification" has become one of the key criteria for most arms control agreements. The concept clearly will be interpreted differently by different people in different contexts but has generally been defined as the ability to detect cheating that would be militarily or politically significant in time to take corrective action. For a good discussion of effective verification in the context of the CTBT, see David Hafemeister, "The Comprehensive Test Ban Treaty: Effectively Verifiable," Arms Control Today, October 2008, pp. 6-12.