The CTBT and scientific issues

Print edition : October 10, 1998

An analysis of whether India's apprehensions about the hegemony of the nuclear weapon states under the CTBT regime are no longer valid, as some claim, and whether the Indian nuclear establishment's claim that with Pokhran-II a "minimum credible nuclear deterrent" has been acquired is tenable.

FOLLOWING the nuclear weapons tests of May 1998, the Bharatiya Janata Party-led Government, as well as a substantial section of those who support its decision to declare India a nuclear weapon state, have swung in favour of India signing the Comprehensive Test Ban Treaty (CTBT). At the core of the argument for reversing India's earlier stand are two inter-related propositions. The first is that the only material Indian objection to signing the CTBT earlier was India's national security concerns. The second proposition is that with Pokhran-II India acquired a "minimum credible nuclear deterrent" with capabilities of weapons testing and development comparable, it is suggested somewhat obliquely, to that of the established nuclear weapons powers. Hence, it is argued, having broken the hegemony of the weapons powers, India can make its peace with the global nuclear order as a credible player.

The first proposition is demonstrably false (for an incisive discussion in this regard, see "Getting off the tiger" by N. Ram, Frontline, October 9). The record shows that India's main objections to the CTBT lay essentially in the fact that the treaty did little to advance disarmament in time-bound fashion, and in practice preserved the nuclear hegemony of the five nuclear powers while limiting so-called horizontal proliferation. The nuclear weapon powers, above all the United States, could maintain and improve their arsenals without any need for further explosive testing. The national security factor was important to the decision to stay out of the CTBT, but it was only one of a set of well-articulated objections. India's security, it was asserted officially, was best guaranteed by the elimination of nuclear weapons.

This analysis focusses on the second proposition, the claim that having acquired a "minimum credible nuclear deterrent", India as a self-proclaimed nuclear weapon state can take its due place in the discriminatory global nuclear order. In examining this proposition and its implications, two questions need to be addressed in detail from a technological standpoint. The first is whether India's original apprehensions about the continuing hegemony of the nuclear weapon states under the CTBT regime are still valid. The second is whether the claim to have acquired a "minimum credible nuclear deterrent" is tenable, whether Pokhran-II has enabled India to overcome the asymmetry between its capabilities and those of the five nuclear weapon states.

Testing nuclear weapons

We need to begin with some basic facts about nuclear weapons and testing that have been important to the CTBT debate. Among the different classes of nuclear weapons, there is a clear hierarchy with regard to the amount of explosive testing needed to develop weapons of each class. Highly enriched uranium weapons with a gun-barrel type assembly require the least amount of testing to develop. Fission weapons with plutonium typically require some testing while boosted fission weapons need more testing than ordinary fission weapons. There would be very little confidence in thermonuclear weapons developed without extensive explosive testing.

What can be learned, broadly speaking, from the different types of tests of nuclear weapons?

A monument at the site in Alamogordo, New Mexico, in the U.S., where the world's first nuclear device was exploded on July 16, 1945.-THE HINDU PHOTO LIBRARY

Explosive testing of nuclear weapons has played a major role in validating the design of nuclear weapons, obtaining data for their improvement and for fresh designs. The number of tests done by each nuclear weapon state for a new weapon design has varied, ranging from an average of six for the U.S. to a reported 22 in the case of France.

The basic reason for this is that while the fundamental physics laws that govern the behaviour of nuclear weapons are well-known, actual weapon performance involves a lot of detail that cannot be worked out completely from first principles. An important example is the detailed behaviour and properties of the fissile material mass in the extreme conditions of pressure and temperature that prevail as the nuclear chain reaction (which leads to the explosion) commences. These can be approximated and modelled partially by computer, but in general, such results have to be compared with experiments. Fewer tests could be used to validate nuclear weapons codes, but this would imply that the weapons design would have much less flexibility.

Explosive tests that allow the fission chain reaction to proceed without hindrance to the release of nuclear energy are prohibited under the CTBT; this is the so-called zero-yield barrier. The ban does not rule out what are known as Sub-Critical Experiments (SCEs). One form of SCEs is when high pressure can be put on a plutonium mass by hitting it with an aluminium plate driven by a high-explosive charge. Such 'equation of state' tests provide information on the state of the fissile material for some ranges of pressure and temperature. There is no danger of criticality (that is, a very low-yield nuclear explosion) in such a situation.

There are also other SCEs which can be done, particularly to study the effect of the high-explosive on the core of fissile material, a subject important to weapons design. Such experiments, when conducted with the fissile material replaced by inert material, are referred to as 'hydrotests'. The term SCEs generally refers to the actual use of fissile material, in which it has to be carefully ensured that over-criticality is not achieved. Over-criticality is a particular risk with plutonium weapons because plutonium behaves in a complex manner under extreme conditions. These require underground testing facilities.

In the case of boosted fission weapons or the boosted fission primary of a two-stage thermonuclear device, the mixing of the boost gas with the plutonium under the impact of the high-explosive is an important issue that is studied by sub-critical tests. There appears to be no real analogue of SCEs for the secondary fusion stage of thermonukes.

Another class of tests not permitted under the CTBT is the 'hydronuclear test'. Here, slight over-criticality is achieved, that is, there is the actual release of nuclear energy (typically of a few kg equivalent).

These tests have been important in understanding the behaviour of nuclear weapons to ensure their safety against accidental detonation. The U.S. standard here is referred to as 'one-point' safety, that is, ensuring that there is only a slight release of nuclear energy (the U.S. technical standard is, in fact, less than 2 kg yield) in case there is accidental detonation of the high-explosive at one point (by a rifle bullet, for instance). More than 40 such tests were conducted by the U.S. early in its programme (1958-1961). They are no longer deemed necessary. Such tests for safety became necessary once ready-to-use nuclear warheads for missiles and artillery shells were developed. The hydronuclear tests were vital to developing the codes required for understanding these issues.

Apart from some of the tests described above, there is a large class of non-explosive tests that are done on the various components that make up the nuclear weapon.

The U.S. and the CTBT

Before deciding to sign the CTBT, the U.S. studied in detail the requirements to preserve its arsenal in 'optimal' condition. A good part of these requirements is known in the public domain.

Given the fact that the U.S. has conducted more than 1,000 nuclear tests, it is the most advantageously placed among nuclear weapon states in being able to manage without any further explosive testing. In pursuit of this goal, it has developed the principle of 'Science Based Stockpile Stewardship' which is embodied in the U.S. Department of Energy's (DOE's) 'Stockpile Stewardship and Management Plan' (SSMP). The SSMP lays out in detail what is possible by the U.S. in this regard. In fact, with the declassification of excerpts from this document (in its February 1996 version) there emerges an interesting and fairly detailed picture of the U.S. nuclear weapons programme under the CTBT regime. These have been discussed in detail in an August 1997 report of the U.S.-based non-governmental organisation, the Natural Resources Defense Council. It is titled "End Run: The U.S. Government's plan for designing nuclear weapons and simulating nuclear explosions under the Comprehensive Test Ban Treaty" and written by Christopher E. Paine and Matthew G. Mckinzie.

From the SSMP, it is clear that the U.S. game plan goes well beyond mere 'custodianship' or maintaining the existing weapons in working order. The SSMP makes it clear that the plan "provides for continual enhancement of the technology infrastructure and core competencies to meet national security objectives." Further, incomplete areas of nuclear weapons science are to be addressed using non-nuclear experiments in new advanced facilities. "Data from experiments in these facilities, together with data from past nuclear tests, will be used to validate new or improved models in weapon simulation codes."

Such development is also expected to train a new generation of scientists and engineers in handling nuclear weapons. The DOE is not coy about the fact that it will maintain "a surge capability to rebuild a larger stockpile". Among other things, it is also clear that fissile material from some older weapons is to be used to produce newer weapons based on modifications of more recent designs. A chilling example is the proposed B61-11 (a thermonuclear weapon) that is based on the B61-7 and is to replace the B53 (an older thermonuke).

Farther on in the SSMP, the role of the new facilities in sub-critical testing and advanced computational tools is made very clear. "New, improved computer-based tools, which the Accelerated Strategic Computing Initiative (ASCI) is enabling, will be essential in establishing the appropriate links between past nuclear test data and new information obtained from advanced tools..." Clearly, the effectiveness of the new techniques depends critically on the extensive testing done earlier.

Some other aspects of testing revealed by the SSMP are relevant to the assessment of current Indian capabilities. The most reliable part of the stockpile stewardship programme appears to be related to the fission or boosted fission primaries of two-stage nuclear weapons. Here, attempts to develop a "from first principles" understanding of the details of nuclear weapons design are likely to meet with success in the short and medium term. The facilities that are proposed (or beginning to operate) and the pre-CTBT nuclear testing data will provide the basis for the validation of more refined theoretical approaches.

But a part of the programme is also devoted to intense fundamental research in various fields in the hope that this will lead to developments that will enable better predictability of other aspects of weapons design, particularly the second stage of thermonuclear weapons. These appear to require new experimental facilities using lasers and other means to create the conditions that approach the high energy densities relevant to the fusion stage of a thermonuclear weapon. Here past test data are crucial to checking whether the new understanding is correct. This assessment of the SSMP is in accord with the general perception that the development of new pure-fusion weapons is quite difficult under the CTBT. However, one cannot rule out breakthroughs in understanding the physics of these processes that would open fresh possibilities.

The report, "End Run", offers extensive information on how far the U.S. intends to continue with the development and refinement of its nuclear arsenal. What is striking about the SSMP is that, quite apart from the range of technological facilities that are to be developed (many of these are already known), much of its spirit and basic thrust appears quite contrary to the claimed objectives of the CTBT.

Other weapon states have programmes for stockpile stewardship that are either similar or scaled down, though clearly not all of them will have the same capabilities as the U.S.

In sum, the original objections raised by India to the non-restriction of nuclear weapons research and nuclear weapons development under the CTBT, and the extension of the "hegemonistic" technological lead of the nuclear weapon powers, particularly the U.S., remain amply valid.

Current Indian capabilities

In the background of the claim made by the BJP-led Government and the authorities of the Department of Atomic Energy and the Defence Research and Development Organisation that India has obtained the data it requires from the Pokhran-II tests, that no further tests are necessary from a scientific and technological angle, that India is now ready for the CTBT with a proven nuclear weapon programme in place, it is worth examining where the country's current nuclear weapons capabilities stand.

We may assume for the purpose of this discussion that official Indian claims about the yield and nature of their weapons are basically correct, even though this remains a matter of considerable controversy (see box). One may add to the earlier claims the more recent one that the primary of the thermonuclear device was a boosted fission device. Reliable sources also indicate that the DAE has acquired the ability to conduct 'equation-of-state' tests by hitting plutonium targets with high-explosive-driven projectiles, using X-ray probes to study the behaviour of plutonium under these conditions.

Going by the official information, most of the Pokhran devices were plutonium-based implosion weapons. There was a single test of a thermonuclear device in an unknown configuration. There were clearly no tests in the hydronuclear range of yield.

What can we infer from the available information about the nature of Indian nuclear weapon capabilities? First, that no reliable thermonuclear weapon can probably be built, particularly for deployment as missile warheads. Whatever the code used to design the one claimed to have been tested at Pokhran, it has most probably been insufficiently validated. Secondly, as with the thermonuclear weapon, probably no boosted fission weapon configuration has been sufficiently validated. Thirdly, the only reasonably validated device is that of an implosion-type plutonium device. It is not clear whether this has provided the flexibility to design tactical nuclear weapons. Fourthly, important questions remain about the reliability and safety - to the user - of such weapons, given that no hydronuclear tests appear to have been conducted. It is also clear that the one-dimensional and two-dimensional codes developed do not adequately answer the demands of safety, which means three-dimensional codes. Questions of safety are clearly important for fully armed nuclear warheads, especially if they are deployed and mounted on delivery systems. No attention seems to have been paid to the issue of safety in the DAE authorities' comments on the weapons tests, although this has dominated the CTBT debate, especially in the U.S.

Regarding the ability to undertake SCEs, only one of the range of techniques that this term encompasses appears to be currently available. Even with the development of more advanced techniques of sub-critical testing, they will not be sufficient to validate designs of either thermonuclear weapons or boosted fission weapons given that only one test has taken place. It is also important to ensure that sub-critical tests with plutonium do not 'creep' up to the hydronuclear tests, which will be difficult to ensure without further testing. This point was made by a Frontline Special Correspondent (see "Matters of Technology", Frontline, July 17).

With regard to claims of computer simulation capabilities, the issue is not really the availability of super-computing power. The utility of super-computing capabilities lies in two aspects of weapons design in a non-explosive testing regime. One is the challenge of calculating the behaviour of nuclear weapons in detail from the basic physics principles with as little empirical input as possible. The second is the challenge of extending empirical codes to study more complicated weapons configurations, or even fresh ones, using the input from sub-critical tests as well as tests with inert material. However, such exercises are not of much use when there is little data to validate the final computer codes developed.

Persistence with weaponisation under these conditions is highly unstable; it could eventually lead to a demand for fresh testing of a wide range of devices.

Dr. R. Chidambaram, Chairman, Atomic Energy Commission, and Secretary, Department of Atomic Energy, and A.P.J. Abdul Kalam, Scientific Adviser to the Defence Minister, at a press conference in New Delhi on May 17.-AJIT KUMAR/AP

In short, current capabilities amount to only moderately reliable, safety-untested weapons of the plutonium-based implosion type without too much leeway in the choice of delivery systems. Quite apart from the problems associated with the design of a suitable command and control system, such capability hardly merits the description of a "minimum credible nuclear deterrent".

Thus the claims made by authorities of the DAE and the DRDO at the press conference of May 17, 1998, that the five nuclear explosions provided "critical data for the validation of our capability in the design of nuclear weapons of different yields for different applications and different delivery systems" and that they "have significantly enhanced our capability in computer simulation of new designs and taken us to the stage of sub-critical experiments in the future, if considered necessary" appear seriously exaggerated.

This position has recently been reiterated by the DAE and the DRDO together with the additional claim of tactical nuclear weapons capability (made officially for the first time). The added explanation that fewer tests are needed today than they were several years ago to attain a credible nuclear deterrent does not appear to be borne out by the information available on the nuclear weapons programmes of other countries. As for the current official Indian claims to thermonuclear power status, they are unlikely to carry any weight in international expert circles.

Clearly, India's nuclear weapons capabilities are sufficient to threaten Pakistan, spurring it on to develop more highly enriched uranium weapons, probably with a gun-type assembly. Likewise, Pakistan's nuclear weapons capabilities seem sufficient to threaten India, rendering unstable its "minimum credible nuclear deterrent". Unless both nuclear weapons programmes are rolled back and a commitment is made not to deploy nuclear weapons, regional tensions will intensify notwithstanding the political talking process that has been agreed upon between the governments of the two countries. Given the nuclear weapon postures adopted on both sides, the regional security situation must be regarded as highly unstable. The point that the real issue is not the CTBT but nuclear weaponisation in South Asia must be grasped as the key to getting out of the current dangerous mess.

To inflate the scientific and technological outcome of Pokhran-II to justify signing the CTBT appears to be a cynical attempt to retain a posture of weaponisation at any cost - by discarding the basic principles that underlay India's peace-oriented nuclear policy.

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