JonAsad
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Pakistan did survive the sanctions, we always did, till 9/11, but that was a totaly different situation.
and we are not Doomed.. Keep Dreaming.
17 Days That Shook The World : Pokhran 1998
- Thread starter Chanakyaa
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Pakistan did survive the sanctions, we always did, till 9/11, but that was a totaly different situation.
and we are not Doomed.. Keep Dreaming.
notorious_eagle
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But Pakistan was never interested in acquiring a nuclear weapon, the project to acquire a nuclear bomb began after the Indians conducted their test in 1974. Ayub Khan froze Pakistan's civil nuclear program when he was the President, Dr. Munir Khan made a case in a meeting with him about the necessity of having a nuclear bomb but Ayub Khan clearly rejected the proposal. Pakistan was compelled to acquire a nuclear weapon after it was clear the Indians were going to acquire one. It would have been a suicide for Pakistan to let its arch enemy acquire a nuclear weapon and live under constant threat of annihilation. In my opinion, acquiring a nuclear weapon was a huge mistake that India's policy makers made. They just delivered the trump card to Pakistan, so now whenever India thinks of even punishing Pakistan they risk starting a nuclear war.
India should not have announced the nuclear tests too early. Before announcing, The Government should have gone ahead and secretly tested at-least 50 tests up-to the yield of 2000 KT to make the weapons as reliable and credible. Now India's nuclear deterrence is only on paper, no major power takes India seriously. Tiny 50 KT weapon is not going to threaten major powers. Keeping secrecy is not something in the gene of baniyas.
deckingraj
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Very good analysis...Now replace Pakistan with India and India with China you will understand why India went for nukes...
Now let me explain how your bolded part is very wrong...Look at India's potential adversaries...
China : They are bigger than us(though we are catching big time) and since both are nuclear capable there is little chance we will get into a war..
Pakistan : There is no appetite for teaching Pakistan a lesson...We are happy with status quo and nukes will ensure that status quo will not change unless and until India wants...I am not even talking about conventional edge that we enjoy viz-a-viz Pak...
So now please tell me how bad this decision was for India???
RPK
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The May 1998 Pokhran Tests -Scientific Aspects
THE MAY 1998 POKHRAN TESTS: Scientific Aspects
by R. Chidambaram
(Written on October 16, 2000 as an invited article in the book to be published. “Technology & Security: India’s Long Term Interests” , edited by Dr. Brahma Chellaney.)
INTRODUCTION
Tests of Advanced Weapon Designs
India conducted five nuclear tests1 on May 11 and 13, 1998 at the Pokhran range in Rajasthan Desert. The details are given in Table 1. The first three detonations took place simultaneously at 15=45 hrs. IST on May 11. These included a 45 kt thermonuclear device, a 15 kt fission device and a 0.2 kt sub-kiloton (i.e. less than one kiloton) device. The two nuclear devices detonated simultaneously on May 13 were also in the sub-kiloton range - 0.5 kt and 0.3 kt. The necessity for the simultaneous detonation of the thermonuclear device and the fission device on May 11 was due to the fact that the two shafts where these detonations were carried out were separated by only 1 kilometer . There was a concern that otherwise the shock wave from a detonation in one shaft may damage the second shaft and the equipment located therein. The inclusion of a sub-kiloton device in the third shaft a few kilometers away in the May 11 tests and the simultaneous detonation of the two sub-kiloton devices on May 13 were dictated by the advantages of convenience and speed.
These successful tests of advanced and robust weapon designs -- “perfect” tests as Dr. Anil Kakodkar calls these because the dozen new weapon-related ideas and sub-systems we tried out in these tests (including the sub-kiloton tests), some repeatedly in more than one test, all worked perfectly -- were very carefully planned and carried out in close synergistic cooperation with the Defence Research and Development Organisation. Each test was carried out in a different shaft and these ground locations after the tests are shown in Figure 1.
After the successful tests, Prime Minister Atal Bihari Vajpayee declared that India was now a Nuclear Weapon State. Having achieved the objectives of the tests, the Prime Minister also declared a moratorium on further testing on May 13.
Optimisation of Shaft Depths for containment of Radioactivity
The depth of each shaft was so chosen that, after the tests, there was no radioactive contamination at the test sites. To establish this, extensive radioactivity monitoring (both ground based and air-borne) surveys were carried out before and immediately after the events and at periodic intervals thereafter. Again, as in the May 1974 test, reconnoitering health physics teams went up to surface ground zeros soon after the detonations and, after necessary checks, pronounced the areas as radiation free, for recovery of instruments for experiments fielded nearby.
When Prime Minister Vajpayee stood at one of the craters a few days after the 1998 tests, it was demonstrated to him using sensitive measuring equipment that the radioactivity level at the place he was standing was at the background level and no higher than at the place where he had his breakfast a few kilometers away.
The physical-mechanical processes associated with the propagation of the stress field set up in a geological medium by a sudden release of the explosive energy of a nuclear device - like vaporisation, melting, crushing, fracture and motion of the rock - are dependent on the chemical composition of the rocks and their physical and mechanical properties like density, porosity, water content, strength, etc. In a detailed computation of the phenomenology of the May 1974 test2, we have shown that the Pokhran rocks have a lower cratering efficiency, compared to the buckboard Mesa basalt rock at the Nevada Test Site in U.S.A., and this is related to the relatively lower fraction of the total energy of the nuclear explosive that is converted into the kinetic energy of the region above the shot-point. The computer simulation code also explains the containment of radioactivity in this event. Similar detailed computer simulation calculations were carried out for each of the five shafts of the May 1998 tests in order to ensure containment of radioactivity.
Self-Reliance in the Nuclear Weapons Development Programme
These tests were the culmination of a committed team effort and backed by development of the necessary knowhow and expertise over decades. Nuclear weapons development requires expertise in a range of disciplines including explosive ballistics, shock wave physics, condensed matter physics, materials science, nuclear and neutron physics, radiation hydrodynamics, radiation - matter interaction physics, advanced electronics engineering backed by production, fabrication and processing technologies over a wide range. It requires complex computer simulation software development to enable accurate prediction of weapon yields. In each one of these areas, we have some of the world’s leading experts. In the field of shockwave physics, for example, we are one of the leading groups in the world in the area of equation of state at high pressures3.
It is universally recognized that India’s nuclear weapons development programme is based on self-reliance. An example is the Venn Diagram (See Figure 2) showing the historical sharing of nuclear weapons knowledge among countries in the article by Paine & McKinzie4 on U.S. Science-Based Stockpile Stewardship Program, which shows the self-reliant unique nature of the Indian weapons programme. Sharing of knowledge is expressed by intersection of circles. The number in the brackets after each country is the number of tests carried out by it. The number 4 against India is related to the authors’ axiom that any two tests carried out simultaneously within one kilometer would be counted as one. Thus the May 11 tests count as two because the 15 kt fission device and the 45 kt thermonuclear device were tested simultaneously a kilometre apart while the sub-kiloton device was tested further away. The two sub-kiloton devices tested simultaneously on May 13 are again counted as one test. With inclusion of the May 1974 test, the total number is 4.
Figure 2: Historical sharing of Nuclear Weapons knowledge indicating the self-reliance of the Indian Nuclear Weapons development program (from Paine & McKinzie, Reference 4)
Nuclear Weaponization
The 15 kt fission nuclear weapon had evolved from the PNE device tested in 1974, with substantial changes that were needed to make it smaller in size and weight from the point of view of weaponization. It was gratifying that it functioned perfectly in all aspects certifying the quality and robustness of the design. The two-stage thermonuclear device, with a fusion-boosted fission trigger as the first stage and with the features needed for integration with delivery vehicles, was tested at the controlled yield of 45 kt and had the purpose of developing nuclear weapons with yields upto around 200 kilotons. The sub-kiloton devices tested again had all the features needed for integration with delivery vehicles and were tested from the point of view of developing low-yield weapons and of validating new weapon-related ideas and sub-systems. Thus the carefully-planned series of tests carried out in May 1998 gave us the capability to build nuclear weapons from low-yields upto around 200 kilotons. A great deal of further scientific and technical development work has taken place since then.
Generation Genealogy - a Misleading Terminology
The expression “fourth generation weapons” has been used by some analysts recently in media reports. This is not universally accepted terminology and it is misleading since it could give the incorrect impression that “third” and “fourth” generation weapons (the latter have not been developed or tested by any Nuclear Weapon State) are superior to, say, “second” generation weapons. It may, therefore, be useful to quote from the recent report of Gsponer and Hurni 5 :
“First Generation Nuclear Weapons are all uranium or plutonium atomic bombs……Second Generation Nuclear Weapons are fusion-boosted fission explosives and two-stage thermonuclear devices. Third Generation Nuclear Weapons are “tailored or enhanced” effects warheads. Like many tactical nuclear weapons, these devices have never found any truly convincing military use. A typical example is the so-called neutron bomb (enhanced radiation weapon), which has not proved to be an effective anti-tank weapon…..Fourth Generation Nuclear Weapons are new fission or fusion explosives which could have yields in the range of 1 to 100 tons equivalent of TNT, i.e. in the gap which today separates conventional weapons from nuclear weapons…. Physical processes which could be used to make such low-yield nuclear explosives or compact non-fission triggers for large scale thermonulcear explosives (include)…Super heavy elements, nuclear isomers and super lasers.”
Searching for superheavy elements, studying nuclear isomers or developing more and more powerful and compact lasers are exciting areas of research. What is being suggested above is that, in future, new weapon designs based on such research could possibly be developed. It is, however, clear that the weapons in the arsenals of Nuclear Weapons States (including India) in the foreseeable future will mostly consist of the so-called first and second generation weapons.
The May 1974 PNE Experiment
Peaceful Nuclear Explosions
The International Atomic Energy Agency held a series of panel meetings in Vienna during 1970-75 to discuss the many potential industrial and engineering uses of Peaceful Nuclear Explosions (PNEs). In the first of these meetings, we expressed6 our interest in their use for mining of copper, by crushing the ore underground by a nuclear explosion, followed by in-situ leaching of the ore and pumping of the mineral-rich liquid to the surface. In a report to the panel meeting in 1975, Chidambaram and Ramanna7 said that, as a step towards studying fracturing effects in rocks, ground motion, containment of radioactivity and the problems involved in post-shot access of the shot-point environment, a PNE experiment had been carried out underground in Pokhran on 18 May, 1974 and gave details of the test. Incidentally, U.S.A. which was very enthusiastic about PNEs - ‘swords into plowshares’ approach - abandoned this program soon after our May 1974 PNE experiment !
Physics of PNE Devices and of Nuclear Weapons
The physics that goes into the design of a PNE device and of a nuclear weapon are obviously similar, but the detonation methodologies are quite different as could be the packaging. It is known that many special kinds of weapon designs were tested by U.S.A. in the name of PNE devices. For example, for stimulation of hydrocarbon reservoirs, you may need pure fission devices to avoid tritium contamination of gas/oil and high-yield fission devices were, therefore, developed. For earth-moving applications, you may need thermonuclear explosives with a low yield fission trigger to avoid excessive ground contamination with radioactive fission products and their characteristics8 , in the low total-yield range, appear no different from those required of the tactical (battlefield) weapon called the enhanced radiation weapon or the neutron bomb.
The yield of the May 1974 PNE Experiment
The common physics makes a PNE relevant for weapon design and, therefore, the success of the May 1974 test was important for us. Even at that time, our diagnostic capabilities were good. The announced yield7,9 of the May 1974 test was 12-13 kt. This yield reproduced many post-shot experimental results9 like measured cavity radius, surface velocity, and the extent of rock fracturing using rock mechanics phenomenology calculations2 . This yield value of 12-13 kt for the May 1974 test is also accepted internationally 10,11. The IRIS consortium of the U.S.A., in its report to the U.S. Senate12 (Van der Vink, Simpson, Hennet, Park and Wallace) gives the May 1974 test yield as 10-15 kt.
Seismic and other Data on May 11, 1998 Tests
Nature of Seismic Magnitudes
Detailed analysis of the seismic data on the 11 May 1998 tests have been published1,13,14 by us and a seismic overview is under publication15 and it fully confirms the total announced yield for these tests.
Most of the global analysis of seismic data on underground nuclear explosions is based on two seismic “magnitudes” mb and Ms - the so-called body-wave magnitude and surface-wave magnitude respectively. The former is calculated from measurements of compressional seismic waves (P waves) in the body of the Earth and the latter from measurements of surface seismic waves (Rayleigh waves). The first step is to analyse the waveforms to obtain the seismic magnitude and to discriminate between an earthquake and a nuclear explosion, which is done on the basis of the source characteristics. In the case of an earthquake, the seismic source is two bodies sliding against each other deep inside the earth and, in the case of a nuclear explosion, it is a point source relatively close to the surface of the earth.
International Analysis of the 11 May 1998 Seismic Data
Surprisingly, indicative of the need for careful analysis, the Prototype International Data Centre for verifying the compliance of CTBT first announced our May 11 nuclear explosion seismic event as “an earthquake at a depth of 56 km on the India-Pakistan border” ! But this was later corrected16 as “explosions - with a combined yield - - - consistent with the announced yield (by India)”. An article in New Scientist (U.K.) of 13 June, 1998 was more explicit. “Roger Clark, a seismologist at the University of Leeds, found that when data from 125 stations - closer to the number required by the treaty (CTBT monitoring network) - are taken into account, the estimate is nearer to 60 (kilotons).”
Prof. Jack Evernden, a world renowned U.S. seismologist, has always maintained17 that, for correct estimation of yields, one should “account properly for geological and seismological differences between test sites”; this was in the context of what he called the “incorrect (U.S.) claims of Soviet cheating on the (1976 Threshold Test Ban) treaty limit of 150 kilotons.” He had also warned about the use of indiscriminate “magnitude bias” while analysing mb (body wave magnitude) teleseismic data. The underestimation18 of our May 11 total yield by one group in the U.S. can be traced to the use of such an invalid “bias”. Jack Evernden prefers the use of surface wave magnitudes to body wave magnitudes and his analysis of the 11 May 1998 seismic data is consistent with ours19.
Analysis by Indian Seismologists
Strong Lg and Rayleigh waves (period 3.5 - 7 sec) were observed from the 11 May tests at several sensitive in-country stations of the Indian Meteorological Department (IMD) and of the Department of Atomic Energy. These have been analysed by Falguni Roy et al 20. Figure 3 shows the broad-band seismogram recorded by IMD at Bhopal and compares it with the seismogram recorded at Nilore in Pakistan, which is at almost the same distance from Pokhran as Bhopal. The difference in quality of the two sets of data can be seen; the large variations in the amplitudes of the Lg waves at Bhopal and Nilore may be attributed to the different geological and tectonic settings of these locations. The mb (Lg) estimates as obtained from three IMD stations, viz., Bhopal, Pune and Bilaspur and from the Gauribidanur array of BARC give an average of 5.47 ± 0.06.
All the available seismic data have been carefully analysed by Indian seismologists and the results have been published in detail in several publications13,14,20. The main conclusions are summarised below:
* A comparison of body wave magnitudes of the May 11, 1998, tests and of the May 18, 1974 test from 8 stations around the world (U.K., Canada, India, Finland, Ukraine, Norway, U.S.A. (two)) gives an average difference D mb of 0.5. The seismic signals recorded from the same channel (one among twenty channels) in the Gauribidanur array for the two events are given in Figure 4, as an example.
Figure 3: Comparison of the Seismograms of the May 11, 1998 Tests at Bhopal and at Nilore in Pakistan. (a) Broad band seismogram as recorded at Bhopal (BHPL), India. (b) seismogram generated at Nilore (NIL), Pakistan. High attenuation of Lg waves on NIL record in comparison to BHPL record is conspicuously seen.
Figure 4: Comparison of (a) POK1 and (b) POK2 seismograms. The data is from the same channel of Gauribidanur array.
* The estimated mb values at any recording station are susceptible to geological and seismological uncertainties at the test site and at the recording site. But these get cancelled out when taking the difference in mb’s for two underground explosions at the same site and for the same recording station. So this value of D mb of 0.5 is reliable and gives a ratio of yields of 4.46. As explained in section 2, the yield of the May 1974 test was 12-13 kt. So this method gives the total yield of the 11 May, 1998 tests as between 54 and 58 kt.
* From the surface wave magnitude obtained from an analysis of regional Rayleigh waves, a total yield of 49-52 kt is obtained for the 11 May, 1998 tests.
* The average mb (Lg) magnitude obtained from the data from the IMD stations and the Gauribidanur array is 5.47. A comparison of Lg waves from the latter for the 11 May, 1998 tests and the May 1974 test gave a yield ratio of 4.83 between these events. So this method gives the total yield of the 11 May, 1998 tests as between 58 and 63 kt.
Thus the yield estimates of the 11 May, 1998 tests from the teleseismic and regional seismic data are fully consistent with the yields announced immediately after the tests for the fission device and the thermonuclear device and given in Table 1.
Confirmatory Evidence
We have other confirmatory evidence from close-in measurements carried out on the day of the tests. For example, comparison of the acceleration data with the available global data from a similar geophysical environment gives a total yield value of 58 kt (see Figure 5 of reference 13).
The bore-hole gamma radiation logging and radiochemical measurements on the rock samples extracted from the sites give the yield for the fission device 21 as 13+3 kt and for the thermonuclear device 22 as 50 + 10 kt.
The Sub-Kiloton Tests
Design of a Sub-kiloton Test
In a sense, the design of a sub-kiloton (i.e. less than one kiloton) test with an accurately predicted yield is more difficult than the test of a standard fission device. In the latter, you take a configuration of fissile material, plutonium or high-enriched uranium, from a sub-critical state (value less than one for the neutron multiplication factor k - defined as the ratio of the neutron population in the assembly in one generation to the neutron population in the previous generation) to maximum possible super-criticality - by imploding a mass and thereby increasing its density or by bringing masses together or by doing a bit of this and a bit of that - before the chain reaction is initiated. If there is an error in the design, the device might still work but with a lower yield. But, in a sub-kiloton test, you go from sub-criticality to marginal super-criticality, and if there is an error in the design, you may not cross the criticality line and the chain reaction may not be initiated at all.
Match between Design Yields and Achieved Yields
What was satisfying about the sub-kiloton tests was that there was good match between design yields and achieved yields. This has been confirmed23 by the results of gamma radiation logging measurements in boreholes at the sites of sub-kiloton tests as well as by radioactivity measurements on the samples extracted from the sites. Figure 5, taken from this report gives part of a typical gamma spectrum of a sample collected from the 0.3 kt device test site. The spectra clearly show the presence of fission products such as 137Cs, 95Zr and 95Nb, which are signatures of the success of a test.
Figure 5: Gamma spectrum of a typical sample from the test site of 0.3 KT device (from reference 22)
Achieving a match between design yields and achieved yields for each of the three sub-kiloton tests - 0.5 kt, 0.3 kt and 0.2 kt - has confirmed, among other things, the equation of state of Plutonium used by us.
Computer Simulation Capability
In the early days of nuclear weapon design, the physics knowledge - both in terms of experimental data and in terms of theoretical techniques - in areas like nuclear cross-sections, equation of state, radiation hydrodynamics, etc. was weak. There would be considerable discrepancy between the predicted yield of a test and the yield actually achieved and one or more parameters in the computer calculation package would be adjusted to achieve a match. But by 1998, when we refined our computer calculations for the designs we tested, physics knowledge had advanced tremendously in every field, whether it was the use of density functional methods for calculating the total energy of a material at any given pressure and temperature or the nuclear cross-sections for any isotope, with the result that such “fudging” was not necessary. If you get all your physics right, there is no basic difference between computer design calculations for a nuclear weapon and computer simulation calculations; after all, the same physics has to go into both. The computer design program developed by us was validated in parts through our own laboratory hydrodynamic experiments as well as by a few international benchmark data sets -- on marginal supercriticality experiments like GODIVA-II24, inertial confinement fusion-related experiments on radiation driven shock waves at various radiation temperatures25, etc. -- which are available in the literature. That is why there was such a good match between design and achieved yields in all our tests, including that of the thermonuclear device. In a large complex system like a nuclear weapon, the performance of an integrated test nowadays is the culmination of a large number of precise laboratory tests of subsystems and validation of individual parts of the computer simulation package through benchmark experimental data.
Significance of the Sub-kiloton Tests Down to 0.2 kt
It is worth recalling that originally the U.S wanted to set the zero yield limit of CTBT at less than the device’s chemical explosive yield. A nuclear explosion in which a chain reaction has been initiated leaves a valuable fission product signature indicating success even at the lowest yield. That is why the U.S was later willing to take the above limit down to a few kilograms of TNT - equivalent. France and China demurred saying that they would prefer to set the threshold at 0.2 kt, which presumably was the precision limit of their calculation capability. The successful match between the design and achieved yields in the sub-kiloton tests down to 0.2 kt suggests that we have similar computer simulation capability. Incidentally, the finally agreed limit is true zero yield and the k = 1 line is the Lakshman Rekha of CTBT !
Threshold of Seismic Detection
This brings us to the question of seismic detectability of low yield tests. It is well known that the International Seismic Network, which is part of the International Monitoring System (IMS) for verifying CTBT compliance, has a threshold detection limit of 1 kt in hard rock 26, provided the test is non-evasive. The purpose of the IMS is detection and not yield calculation; the IMS relies on inspection visits for post-detection verification where considered necessary. Apart from concerns that have been expressed about evasion through decoupling by carrying out the test in an underground cavity, the nature of the emplacement medium is also important. The threshold limit for seismic detection is much higher in, say, a sand medium than in hard rock; the Pokhran geological medium comes somewhere in between. It is not surprising that the IMS did not detect our sub-kiloton tests of May 13, 1998.
The Thermonuclear Device
The Two-Stage Device
The thermonuclear device tested on May 11 was a two-stage device of advanced design, which had a fusion-boosted fission trigger as the first stage and a fusion secondary stage which was compressed by radiation implosion and ignited. For reasons of proliferation sensitivity, we have not given the details of the materials used in the device or their quantities. Also, our nuclear weapon designers, like nuclear weapon designers all over the world, have not given the fusion component of the total yield for our thermonuclear test.
Thermonuclear Devices can be of different Types
Thermonuclear devices can be designed in many ways. In devices designed specially for PNE excavation applications, the fission trigger yield is minimised and the same is done in the low yield (in the region of one kiloton) battlefield weapon called the Enhanced Radiation Weapon or, in popular parlance, the neutron bomb. In a conventional themonuclear weapon like the W-87 of USA, there is a high enriched Uranium ring around the fusion secondary, in which further fissions are caused by the 14 MeV fusion neutrons. In fact, it appears that the yield from the tertiary fission stage can be varied between 0 and 175 kt as the total yield varies between 300 and 475 kt for this weapon which is said to be phased out under the START negotiations and replaced by the W-88. The latter has a fixed yield of 475 kt and is perhaps the most favoured weapon in the US arsenal; it may be recalled that this weapon was in the news recently in the context of alleged spying in a U.S weapons laboratory. There are other thermonuclear weapons in the U.S. stockpile where the warhead yield is reported to be widely variable, while the dimensions and the weight are said to be the same. Engineering wise, this is desirable.
Controlled Thermonuclear Yield
We tested our thermonuclear device at a controlled yield of 45 kt because of the proximity of the Khetolai village at about 5 km, to ensure that the houses in this village will suffer negligible damage. All the design specifications of this device were validated by the test. Thermonuclear weapons of various yields upto around 200 kt can be confidently designed on the basis of this test.
The post - shot radioactivity measurements22 on samples extracted from the thermonuclear test site have confirmed that the fusion secondary gave the design yield. The radioactivity generated from an underground thermonuclear explosion, apart from unburnt fissile material and tritium, consists essentially of two parts:
* fission products from the fission trigger and from the fission component in the fusion secondary stage, if present;
* neutron-induced radioactivity in the surrounding rock mass and construction materials; here one can look specifically for the neutron activation products of high energy neutrons like Sodium - 22 and Manganese - 54. which are produced much more in fusion reactions compared to fission reactions.
Figure 6, taken from reference 22, gives the gamma ray peaks due to fission and activation products. From a study of this radioactivity and an estimate of the cavity radius, confirmed by drilling operations at positions away from Ground Zero, the total yield as well as the break-up of the fission and fusion yields could be calculated. A comparison of the ratios of various activation products to fission products for the 15 kt device and for the 45 kt thermonuclear device also shows that these ratios are in agreement with the expected fusion yield in the thermonuclear device. The total yield comes out
Figure 6: Gamma ray spectrum of a typical rock sample from the thermonuclear test site (from reference 21)
as 50 ± 10 kt for the thermonuclear device, consistent with the design yield and with the seismic estimate of the total yield.
It needs re-emphasis that the testing of the thermonuclear weapon design at the controlled yield of 45 kt was necessitated by the proximity of Khetolai village . As mentioned earlier, we have not given the fusion-fission breakup and, since we have not given the composition of the materials used nor their quantitites, for reasons of proliferation sensitivity as mentioned earlier, no one outside the design team has data to calculate this fission-fusion yield breakup or any other significant parameter related to fusion burn.
Number of tests
The US carried out more than a thousand nuclear tests and developed perhaps 70 - 80 types of devices, though it has retired many of them and is now believed to have only less than a dozen types of weapons in its current stockpile. Furthermore, just like in other technology areas, the needed number of tests came down with increase in available computing power. Figure 7 gives the testing frequency (the number of tests done per year) of U.S over the years till they stopped nuclear explosive testing in 1992. The growth in computing power over the years is plotted in the same figure. There is a clear inverse correlation.
In fact, in the early years of nuclear weapons development, it was faster and cheaper to test out a new idea by actual nuclear explosive detonation than to carry out a computer calculation, and the lower computing power available was also compounded by inadequate physics knowledge. The situation is very different now.
Conclusion
The May 1998 tests were fully successful in terms of achieving their scientific objectives:
* Certification of the fission nuclear weapon of 15 kt yield, evolved from the PNE device tested in 1974, with substantial changes that were needed to make it smaller in size and weight from the point of view of weaponisation. It was gratifying that it functioned perfectly in all aspects , certifying the quality and robustness of the design.
* Testing a two-stage thermonuclear device with a fusion-boosted fission trigger as the first stage and with the features needed for integration with delivery vehicles at the controlled yield of 45 kt with the purpose of developing nuclear weapon systems with yields upto around 200 kilotons.
Figure 7: Plot of U.S. Testing Frequency and of Growth of Computing Power over the years
* Testing sub-kiloton devices, with all the features needed for integration with delivery vehicles, from the point of view of developing low-yield weapons and of validating new weapon-related ideas and subsystems.
* Establishing the computer simulation capability to predict the yields of nuclear weapons--fission, boosted fission and two-stage thermonuclear - of designs related to the designs of the devices tested by us.
* Thus the carefully-planned series of tests carried out in May 1998 gave us the capability to design confidently and build nuclear weapons from low yields upto around 200 kilotons. A great deal of further scientific and technical development work has taken place since then.
Acknowledgement
I am grateful to Dr. Anil Kakodkar and Dr. S.K. Sikka for their critical reviewing of the manuscript and for their valuable comments. I am also thankful to a number of other colleagues for useful suggestions.
References
1. “Some Preliminary Results of May 11-13, 1998, Nuclear Detonations at Pokhran”, by S.K. Sikka and Anil Kakodkar, BARC Newsletter, No. 172, May 1998.
2. “Phenomenology of the Pokhran PNE Experiment” by R. Chidambaram, S.K. Sikka and Satish C. Gupta, Pramana, 1985, Vol. 24, pp. 245-58.
3. “Equation of State at High Pressures” by S.K. Sikka, B.K. Godwal and R. Chidambaram, Edited by R.A. Graham, Lee Davison and M. Shahinpoor, Springer - Verlag, pp 1-35, April 1998 (Invited Review Article)
4. “Does the U.S. Science - Based Stockpile Stewardship Program Pose a Proliferation Threat ?” by Christopher E. Paine and Matthew G. McKinzie, Science and Global Security, 1998, Vol. 7, pp. 151 - 193.
5 “Fourth Generation Weapons” by Andre’ Gsponer and J-P Hurni, September 1999, Independent Scientific Research Institute (ISRI), Technical Report 1 (Sixth Edition), Switzerland.
6. “Statements on National Programmes: India”, by R. Chidambaram, Peaceful Nuclear Explosions - Phenomenology and Status Report, 1970 (Proc. Panel), IAEA, Vienna, p.9.
7. “Some studies on India’s Peaceful Nuclear Explosion Experiment” by R. Chidambaram and R. Ramanna, Proceedings Peaceful Nuclear Explosions IV, IAEA, Vienna, 1975, pp. 421-36.
8. “Production of Radioactivity in Peaceful Nuclear Explosions” by R.A. Siddons, Proceedings Peaceful Nuclear Explosions III, IAEA, Vienna, 1972, pp. 353-367.
9. “Some Seismic Results of Rajasthan Explosion of 18 May 1974” by G.J. Nair, AG 224, Procurement Executive, Mod, U.K.
10. “Magnitude corrections for attenuation in the upper mantle” by P.D. Marshall, D.L. Springer and H.C. Rodean, Geophys. Jnl. Royal Astron. Soc., 1979, 57, 609-638.
11. “Estimating the yield of underground nuclear explosions” by T.C. Bache, Bull. Seism. Soc. Amer., 1982, 72, 5131.
12. “Nuclear Testing and Non-proliferation” by Gregory E. Van der Vink, D.W. Simpson, C.B. Hennet, J. Park and Terry Wallace, IRIS Consortium Report, Arlington, U.S.A., February 1994.
13. “Update on the yield of May 11-13 Nuclear Detonations at Pokhran” by S.K. Sikka, Falguni Roy, G.J. Nair, V.G. Kolvankar and Anil Kakodkar, BARC Newsletter, No. 178, November 1998.
14. “Indian Explosions of 11 May, 1998: An analysis of global seismic body wave magnitude estimates” by S.K.Sikka, Falguni Roy and G.J. Nair, Current Science, 10 Sept., 1998, 75(5), pp. 486-491.
15. “The Recent Indian Nuclear Tests - A Seismic Overview” by S.K. Sikka, G.J. Nair, Falguni Roy, Anil Kakodkar and R. Chidambaram, 2000, Current Science (under publication).
16. “The Indian and Pakistani Tests : Did Verification Fail?” by Trevor Findlay and Roger Clark, Trust & Verify, Issue 80, May 1998, Verification Technology Information Centre, London.
17. “Yields of U.S. and Soviet nuclear tests” by Jack F. Evernden and Gerald E. Marsh, Physics Today, August 1987, pp. 37-44.
18. “Monitoring Nuclear tests” B. Barker et.al., Science, 1998, 281, 1967, See also H.T. Wallace, Seismol. Res. Lett., 1998, 36(5), 386.
19. Jack E. Evernden, Letter to the Editor in Physics Today, December 1998; see also his article “Estimation of yields of underground explosions with emphasis on recent Indian and Pakistani explosions” in Physics and Society (U.S), October 1998, page 10.
20. “Indian Explosions of 11 May, 1998 : Analysis of regional Lg and Rayleigh Waves”, by Falguni Roy, G.J. Nair, T.K. Basu, S.K. Sikka, Anil Kakodkar, S.N. Bhattacharya and V.S. Ramamurthy, 25 December 1999, Current Science, 77(12), pp. 1669-1673.
21. S.B. Manohar et al. (Unpublished)
22. “Post-shot radioactivity measurements on samples extracted from thermonuclear test site” by S.B. Manohar, B.S. Tomar, S.S. Rattan, V.K. Shukla, V.V. Kulkarni and Anil Kakodkar, BARC Newsletter, No.186, July 1999.
23 “Fission signatures of Tests on Sub-kiloton Devices” by R.B. Attarde, V.K. Shukla, D.A.R. Babu, V.V. Kulkarni and Anil Kakodkar, BARC Newsletter, No. 188, September 1999.
24. `Applications of Godiva II Neutron Pulses’, by T.F. Wimett and J.D. Orndoff, Proc. Second U.N. Conf. On Peaceful Uses of Atomic Energy, 1958, 10, 449-460.
25. `High Temperatures in Internal Confinement Fusion Radiation Cavities Heated with 0.35 Micrometre Light’ by Robert L. Kauffman, L.J. Suter, C.B. Darrow, J.D. Kilkenny, H.N. Kornblum, D.S. Montgomery, D.W. Phillion, M.D. Rosen, A.R. Theissen, R.J. Wallace and F. Ze, Phys. Rev. Lett., 1994, 73, 2320-2323.
26. “The Comprehensive Test Ban Treaty” by Jeremiah D.Sullivan, Physics Today, March 1998, pp. 24-29; See also Position Statement on “The Capability to Monitor Compliance with CTBT”, by T.C. Wallace, J. Park, G.E. Vander Vink et al,Proceedings Seismological Soc. Amer. Meeting (with Amer. Geophys. Union), 6 October 1999.
(The author is Former Chairman, Atomic Energy Commission, India)
THE MAY 1998 POKHRAN TESTS: Scientific Aspects
by R. Chidambaram
(Written on October 16, 2000 as an invited article in the book to be published. “Technology & Security: India’s Long Term Interests” , edited by Dr. Brahma Chellaney.)
INTRODUCTION
Tests of Advanced Weapon Designs
India conducted five nuclear tests1 on May 11 and 13, 1998 at the Pokhran range in Rajasthan Desert. The details are given in Table 1. The first three detonations took place simultaneously at 15=45 hrs. IST on May 11. These included a 45 kt thermonuclear device, a 15 kt fission device and a 0.2 kt sub-kiloton (i.e. less than one kiloton) device. The two nuclear devices detonated simultaneously on May 13 were also in the sub-kiloton range - 0.5 kt and 0.3 kt. The necessity for the simultaneous detonation of the thermonuclear device and the fission device on May 11 was due to the fact that the two shafts where these detonations were carried out were separated by only 1 kilometer . There was a concern that otherwise the shock wave from a detonation in one shaft may damage the second shaft and the equipment located therein. The inclusion of a sub-kiloton device in the third shaft a few kilometers away in the May 11 tests and the simultaneous detonation of the two sub-kiloton devices on May 13 were dictated by the advantages of convenience and speed.
These successful tests of advanced and robust weapon designs -- “perfect” tests as Dr. Anil Kakodkar calls these because the dozen new weapon-related ideas and sub-systems we tried out in these tests (including the sub-kiloton tests), some repeatedly in more than one test, all worked perfectly -- were very carefully planned and carried out in close synergistic cooperation with the Defence Research and Development Organisation. Each test was carried out in a different shaft and these ground locations after the tests are shown in Figure 1.
After the successful tests, Prime Minister Atal Bihari Vajpayee declared that India was now a Nuclear Weapon State. Having achieved the objectives of the tests, the Prime Minister also declared a moratorium on further testing on May 13.
Optimisation of Shaft Depths for containment of Radioactivity
The depth of each shaft was so chosen that, after the tests, there was no radioactive contamination at the test sites. To establish this, extensive radioactivity monitoring (both ground based and air-borne) surveys were carried out before and immediately after the events and at periodic intervals thereafter. Again, as in the May 1974 test, reconnoitering health physics teams went up to surface ground zeros soon after the detonations and, after necessary checks, pronounced the areas as radiation free, for recovery of instruments for experiments fielded nearby.
When Prime Minister Vajpayee stood at one of the craters a few days after the 1998 tests, it was demonstrated to him using sensitive measuring equipment that the radioactivity level at the place he was standing was at the background level and no higher than at the place where he had his breakfast a few kilometers away.
The physical-mechanical processes associated with the propagation of the stress field set up in a geological medium by a sudden release of the explosive energy of a nuclear device - like vaporisation, melting, crushing, fracture and motion of the rock - are dependent on the chemical composition of the rocks and their physical and mechanical properties like density, porosity, water content, strength, etc. In a detailed computation of the phenomenology of the May 1974 test2, we have shown that the Pokhran rocks have a lower cratering efficiency, compared to the buckboard Mesa basalt rock at the Nevada Test Site in U.S.A., and this is related to the relatively lower fraction of the total energy of the nuclear explosive that is converted into the kinetic energy of the region above the shot-point. The computer simulation code also explains the containment of radioactivity in this event. Similar detailed computer simulation calculations were carried out for each of the five shafts of the May 1998 tests in order to ensure containment of radioactivity.
Self-Reliance in the Nuclear Weapons Development Programme
These tests were the culmination of a committed team effort and backed by development of the necessary knowhow and expertise over decades. Nuclear weapons development requires expertise in a range of disciplines including explosive ballistics, shock wave physics, condensed matter physics, materials science, nuclear and neutron physics, radiation hydrodynamics, radiation - matter interaction physics, advanced electronics engineering backed by production, fabrication and processing technologies over a wide range. It requires complex computer simulation software development to enable accurate prediction of weapon yields. In each one of these areas, we have some of the world’s leading experts. In the field of shockwave physics, for example, we are one of the leading groups in the world in the area of equation of state at high pressures3.
It is universally recognized that India’s nuclear weapons development programme is based on self-reliance. An example is the Venn Diagram (See Figure 2) showing the historical sharing of nuclear weapons knowledge among countries in the article by Paine & McKinzie4 on U.S. Science-Based Stockpile Stewardship Program, which shows the self-reliant unique nature of the Indian weapons programme. Sharing of knowledge is expressed by intersection of circles. The number in the brackets after each country is the number of tests carried out by it. The number 4 against India is related to the authors’ axiom that any two tests carried out simultaneously within one kilometer would be counted as one. Thus the May 11 tests count as two because the 15 kt fission device and the 45 kt thermonuclear device were tested simultaneously a kilometre apart while the sub-kiloton device was tested further away. The two sub-kiloton devices tested simultaneously on May 13 are again counted as one test. With inclusion of the May 1974 test, the total number is 4.
Figure 2: Historical sharing of Nuclear Weapons knowledge indicating the self-reliance of the Indian Nuclear Weapons development program (from Paine & McKinzie, Reference 4)
Nuclear Weaponization
The 15 kt fission nuclear weapon had evolved from the PNE device tested in 1974, with substantial changes that were needed to make it smaller in size and weight from the point of view of weaponization. It was gratifying that it functioned perfectly in all aspects certifying the quality and robustness of the design. The two-stage thermonuclear device, with a fusion-boosted fission trigger as the first stage and with the features needed for integration with delivery vehicles, was tested at the controlled yield of 45 kt and had the purpose of developing nuclear weapons with yields upto around 200 kilotons. The sub-kiloton devices tested again had all the features needed for integration with delivery vehicles and were tested from the point of view of developing low-yield weapons and of validating new weapon-related ideas and sub-systems. Thus the carefully-planned series of tests carried out in May 1998 gave us the capability to build nuclear weapons from low-yields upto around 200 kilotons. A great deal of further scientific and technical development work has taken place since then.
Generation Genealogy - a Misleading Terminology
The expression “fourth generation weapons” has been used by some analysts recently in media reports. This is not universally accepted terminology and it is misleading since it could give the incorrect impression that “third” and “fourth” generation weapons (the latter have not been developed or tested by any Nuclear Weapon State) are superior to, say, “second” generation weapons. It may, therefore, be useful to quote from the recent report of Gsponer and Hurni 5 :
“First Generation Nuclear Weapons are all uranium or plutonium atomic bombs……Second Generation Nuclear Weapons are fusion-boosted fission explosives and two-stage thermonuclear devices. Third Generation Nuclear Weapons are “tailored or enhanced” effects warheads. Like many tactical nuclear weapons, these devices have never found any truly convincing military use. A typical example is the so-called neutron bomb (enhanced radiation weapon), which has not proved to be an effective anti-tank weapon…..Fourth Generation Nuclear Weapons are new fission or fusion explosives which could have yields in the range of 1 to 100 tons equivalent of TNT, i.e. in the gap which today separates conventional weapons from nuclear weapons…. Physical processes which could be used to make such low-yield nuclear explosives or compact non-fission triggers for large scale thermonulcear explosives (include)…Super heavy elements, nuclear isomers and super lasers.”
Searching for superheavy elements, studying nuclear isomers or developing more and more powerful and compact lasers are exciting areas of research. What is being suggested above is that, in future, new weapon designs based on such research could possibly be developed. It is, however, clear that the weapons in the arsenals of Nuclear Weapons States (including India) in the foreseeable future will mostly consist of the so-called first and second generation weapons.
The May 1974 PNE Experiment
Peaceful Nuclear Explosions
The International Atomic Energy Agency held a series of panel meetings in Vienna during 1970-75 to discuss the many potential industrial and engineering uses of Peaceful Nuclear Explosions (PNEs). In the first of these meetings, we expressed6 our interest in their use for mining of copper, by crushing the ore underground by a nuclear explosion, followed by in-situ leaching of the ore and pumping of the mineral-rich liquid to the surface. In a report to the panel meeting in 1975, Chidambaram and Ramanna7 said that, as a step towards studying fracturing effects in rocks, ground motion, containment of radioactivity and the problems involved in post-shot access of the shot-point environment, a PNE experiment had been carried out underground in Pokhran on 18 May, 1974 and gave details of the test. Incidentally, U.S.A. which was very enthusiastic about PNEs - ‘swords into plowshares’ approach - abandoned this program soon after our May 1974 PNE experiment !
Physics of PNE Devices and of Nuclear Weapons
The physics that goes into the design of a PNE device and of a nuclear weapon are obviously similar, but the detonation methodologies are quite different as could be the packaging. It is known that many special kinds of weapon designs were tested by U.S.A. in the name of PNE devices. For example, for stimulation of hydrocarbon reservoirs, you may need pure fission devices to avoid tritium contamination of gas/oil and high-yield fission devices were, therefore, developed. For earth-moving applications, you may need thermonuclear explosives with a low yield fission trigger to avoid excessive ground contamination with radioactive fission products and their characteristics8 , in the low total-yield range, appear no different from those required of the tactical (battlefield) weapon called the enhanced radiation weapon or the neutron bomb.
The yield of the May 1974 PNE Experiment
The common physics makes a PNE relevant for weapon design and, therefore, the success of the May 1974 test was important for us. Even at that time, our diagnostic capabilities were good. The announced yield7,9 of the May 1974 test was 12-13 kt. This yield reproduced many post-shot experimental results9 like measured cavity radius, surface velocity, and the extent of rock fracturing using rock mechanics phenomenology calculations2 . This yield value of 12-13 kt for the May 1974 test is also accepted internationally 10,11. The IRIS consortium of the U.S.A., in its report to the U.S. Senate12 (Van der Vink, Simpson, Hennet, Park and Wallace) gives the May 1974 test yield as 10-15 kt.
Seismic and other Data on May 11, 1998 Tests
Nature of Seismic Magnitudes
Detailed analysis of the seismic data on the 11 May 1998 tests have been published1,13,14 by us and a seismic overview is under publication15 and it fully confirms the total announced yield for these tests.
Most of the global analysis of seismic data on underground nuclear explosions is based on two seismic “magnitudes” mb and Ms - the so-called body-wave magnitude and surface-wave magnitude respectively. The former is calculated from measurements of compressional seismic waves (P waves) in the body of the Earth and the latter from measurements of surface seismic waves (Rayleigh waves). The first step is to analyse the waveforms to obtain the seismic magnitude and to discriminate between an earthquake and a nuclear explosion, which is done on the basis of the source characteristics. In the case of an earthquake, the seismic source is two bodies sliding against each other deep inside the earth and, in the case of a nuclear explosion, it is a point source relatively close to the surface of the earth.
International Analysis of the 11 May 1998 Seismic Data
Surprisingly, indicative of the need for careful analysis, the Prototype International Data Centre for verifying the compliance of CTBT first announced our May 11 nuclear explosion seismic event as “an earthquake at a depth of 56 km on the India-Pakistan border” ! But this was later corrected16 as “explosions - with a combined yield - - - consistent with the announced yield (by India)”. An article in New Scientist (U.K.) of 13 June, 1998 was more explicit. “Roger Clark, a seismologist at the University of Leeds, found that when data from 125 stations - closer to the number required by the treaty (CTBT monitoring network) - are taken into account, the estimate is nearer to 60 (kilotons).”
Prof. Jack Evernden, a world renowned U.S. seismologist, has always maintained17 that, for correct estimation of yields, one should “account properly for geological and seismological differences between test sites”; this was in the context of what he called the “incorrect (U.S.) claims of Soviet cheating on the (1976 Threshold Test Ban) treaty limit of 150 kilotons.” He had also warned about the use of indiscriminate “magnitude bias” while analysing mb (body wave magnitude) teleseismic data. The underestimation18 of our May 11 total yield by one group in the U.S. can be traced to the use of such an invalid “bias”. Jack Evernden prefers the use of surface wave magnitudes to body wave magnitudes and his analysis of the 11 May 1998 seismic data is consistent with ours19.
Analysis by Indian Seismologists
Strong Lg and Rayleigh waves (period 3.5 - 7 sec) were observed from the 11 May tests at several sensitive in-country stations of the Indian Meteorological Department (IMD) and of the Department of Atomic Energy. These have been analysed by Falguni Roy et al 20. Figure 3 shows the broad-band seismogram recorded by IMD at Bhopal and compares it with the seismogram recorded at Nilore in Pakistan, which is at almost the same distance from Pokhran as Bhopal. The difference in quality of the two sets of data can be seen; the large variations in the amplitudes of the Lg waves at Bhopal and Nilore may be attributed to the different geological and tectonic settings of these locations. The mb (Lg) estimates as obtained from three IMD stations, viz., Bhopal, Pune and Bilaspur and from the Gauribidanur array of BARC give an average of 5.47 ± 0.06.
All the available seismic data have been carefully analysed by Indian seismologists and the results have been published in detail in several publications13,14,20. The main conclusions are summarised below:
* A comparison of body wave magnitudes of the May 11, 1998, tests and of the May 18, 1974 test from 8 stations around the world (U.K., Canada, India, Finland, Ukraine, Norway, U.S.A. (two)) gives an average difference D mb of 0.5. The seismic signals recorded from the same channel (one among twenty channels) in the Gauribidanur array for the two events are given in Figure 4, as an example.
Figure 3: Comparison of the Seismograms of the May 11, 1998 Tests at Bhopal and at Nilore in Pakistan. (a) Broad band seismogram as recorded at Bhopal (BHPL), India. (b) seismogram generated at Nilore (NIL), Pakistan. High attenuation of Lg waves on NIL record in comparison to BHPL record is conspicuously seen.
Figure 4: Comparison of (a) POK1 and (b) POK2 seismograms. The data is from the same channel of Gauribidanur array.
* The estimated mb values at any recording station are susceptible to geological and seismological uncertainties at the test site and at the recording site. But these get cancelled out when taking the difference in mb’s for two underground explosions at the same site and for the same recording station. So this value of D mb of 0.5 is reliable and gives a ratio of yields of 4.46. As explained in section 2, the yield of the May 1974 test was 12-13 kt. So this method gives the total yield of the 11 May, 1998 tests as between 54 and 58 kt.
* From the surface wave magnitude obtained from an analysis of regional Rayleigh waves, a total yield of 49-52 kt is obtained for the 11 May, 1998 tests.
* The average mb (Lg) magnitude obtained from the data from the IMD stations and the Gauribidanur array is 5.47. A comparison of Lg waves from the latter for the 11 May, 1998 tests and the May 1974 test gave a yield ratio of 4.83 between these events. So this method gives the total yield of the 11 May, 1998 tests as between 58 and 63 kt.
Thus the yield estimates of the 11 May, 1998 tests from the teleseismic and regional seismic data are fully consistent with the yields announced immediately after the tests for the fission device and the thermonuclear device and given in Table 1.
Confirmatory Evidence
We have other confirmatory evidence from close-in measurements carried out on the day of the tests. For example, comparison of the acceleration data with the available global data from a similar geophysical environment gives a total yield value of 58 kt (see Figure 5 of reference 13).
The bore-hole gamma radiation logging and radiochemical measurements on the rock samples extracted from the sites give the yield for the fission device 21 as 13+3 kt and for the thermonuclear device 22 as 50 + 10 kt.
The Sub-Kiloton Tests
Design of a Sub-kiloton Test
In a sense, the design of a sub-kiloton (i.e. less than one kiloton) test with an accurately predicted yield is more difficult than the test of a standard fission device. In the latter, you take a configuration of fissile material, plutonium or high-enriched uranium, from a sub-critical state (value less than one for the neutron multiplication factor k - defined as the ratio of the neutron population in the assembly in one generation to the neutron population in the previous generation) to maximum possible super-criticality - by imploding a mass and thereby increasing its density or by bringing masses together or by doing a bit of this and a bit of that - before the chain reaction is initiated. If there is an error in the design, the device might still work but with a lower yield. But, in a sub-kiloton test, you go from sub-criticality to marginal super-criticality, and if there is an error in the design, you may not cross the criticality line and the chain reaction may not be initiated at all.
Match between Design Yields and Achieved Yields
What was satisfying about the sub-kiloton tests was that there was good match between design yields and achieved yields. This has been confirmed23 by the results of gamma radiation logging measurements in boreholes at the sites of sub-kiloton tests as well as by radioactivity measurements on the samples extracted from the sites. Figure 5, taken from this report gives part of a typical gamma spectrum of a sample collected from the 0.3 kt device test site. The spectra clearly show the presence of fission products such as 137Cs, 95Zr and 95Nb, which are signatures of the success of a test.
Figure 5: Gamma spectrum of a typical sample from the test site of 0.3 KT device (from reference 22)
Achieving a match between design yields and achieved yields for each of the three sub-kiloton tests - 0.5 kt, 0.3 kt and 0.2 kt - has confirmed, among other things, the equation of state of Plutonium used by us.
Computer Simulation Capability
In the early days of nuclear weapon design, the physics knowledge - both in terms of experimental data and in terms of theoretical techniques - in areas like nuclear cross-sections, equation of state, radiation hydrodynamics, etc. was weak. There would be considerable discrepancy between the predicted yield of a test and the yield actually achieved and one or more parameters in the computer calculation package would be adjusted to achieve a match. But by 1998, when we refined our computer calculations for the designs we tested, physics knowledge had advanced tremendously in every field, whether it was the use of density functional methods for calculating the total energy of a material at any given pressure and temperature or the nuclear cross-sections for any isotope, with the result that such “fudging” was not necessary. If you get all your physics right, there is no basic difference between computer design calculations for a nuclear weapon and computer simulation calculations; after all, the same physics has to go into both. The computer design program developed by us was validated in parts through our own laboratory hydrodynamic experiments as well as by a few international benchmark data sets -- on marginal supercriticality experiments like GODIVA-II24, inertial confinement fusion-related experiments on radiation driven shock waves at various radiation temperatures25, etc. -- which are available in the literature. That is why there was such a good match between design and achieved yields in all our tests, including that of the thermonuclear device. In a large complex system like a nuclear weapon, the performance of an integrated test nowadays is the culmination of a large number of precise laboratory tests of subsystems and validation of individual parts of the computer simulation package through benchmark experimental data.
Significance of the Sub-kiloton Tests Down to 0.2 kt
It is worth recalling that originally the U.S wanted to set the zero yield limit of CTBT at less than the device’s chemical explosive yield. A nuclear explosion in which a chain reaction has been initiated leaves a valuable fission product signature indicating success even at the lowest yield. That is why the U.S was later willing to take the above limit down to a few kilograms of TNT - equivalent. France and China demurred saying that they would prefer to set the threshold at 0.2 kt, which presumably was the precision limit of their calculation capability. The successful match between the design and achieved yields in the sub-kiloton tests down to 0.2 kt suggests that we have similar computer simulation capability. Incidentally, the finally agreed limit is true zero yield and the k = 1 line is the Lakshman Rekha of CTBT !
Threshold of Seismic Detection
This brings us to the question of seismic detectability of low yield tests. It is well known that the International Seismic Network, which is part of the International Monitoring System (IMS) for verifying CTBT compliance, has a threshold detection limit of 1 kt in hard rock 26, provided the test is non-evasive. The purpose of the IMS is detection and not yield calculation; the IMS relies on inspection visits for post-detection verification where considered necessary. Apart from concerns that have been expressed about evasion through decoupling by carrying out the test in an underground cavity, the nature of the emplacement medium is also important. The threshold limit for seismic detection is much higher in, say, a sand medium than in hard rock; the Pokhran geological medium comes somewhere in between. It is not surprising that the IMS did not detect our sub-kiloton tests of May 13, 1998.
The Thermonuclear Device
The Two-Stage Device
The thermonuclear device tested on May 11 was a two-stage device of advanced design, which had a fusion-boosted fission trigger as the first stage and a fusion secondary stage which was compressed by radiation implosion and ignited. For reasons of proliferation sensitivity, we have not given the details of the materials used in the device or their quantities. Also, our nuclear weapon designers, like nuclear weapon designers all over the world, have not given the fusion component of the total yield for our thermonuclear test.
Thermonuclear Devices can be of different Types
Thermonuclear devices can be designed in many ways. In devices designed specially for PNE excavation applications, the fission trigger yield is minimised and the same is done in the low yield (in the region of one kiloton) battlefield weapon called the Enhanced Radiation Weapon or, in popular parlance, the neutron bomb. In a conventional themonuclear weapon like the W-87 of USA, there is a high enriched Uranium ring around the fusion secondary, in which further fissions are caused by the 14 MeV fusion neutrons. In fact, it appears that the yield from the tertiary fission stage can be varied between 0 and 175 kt as the total yield varies between 300 and 475 kt for this weapon which is said to be phased out under the START negotiations and replaced by the W-88. The latter has a fixed yield of 475 kt and is perhaps the most favoured weapon in the US arsenal; it may be recalled that this weapon was in the news recently in the context of alleged spying in a U.S weapons laboratory. There are other thermonuclear weapons in the U.S. stockpile where the warhead yield is reported to be widely variable, while the dimensions and the weight are said to be the same. Engineering wise, this is desirable.
Controlled Thermonuclear Yield
We tested our thermonuclear device at a controlled yield of 45 kt because of the proximity of the Khetolai village at about 5 km, to ensure that the houses in this village will suffer negligible damage. All the design specifications of this device were validated by the test. Thermonuclear weapons of various yields upto around 200 kt can be confidently designed on the basis of this test.
The post - shot radioactivity measurements22 on samples extracted from the thermonuclear test site have confirmed that the fusion secondary gave the design yield. The radioactivity generated from an underground thermonuclear explosion, apart from unburnt fissile material and tritium, consists essentially of two parts:
* fission products from the fission trigger and from the fission component in the fusion secondary stage, if present;
* neutron-induced radioactivity in the surrounding rock mass and construction materials; here one can look specifically for the neutron activation products of high energy neutrons like Sodium - 22 and Manganese - 54. which are produced much more in fusion reactions compared to fission reactions.
Figure 6, taken from reference 22, gives the gamma ray peaks due to fission and activation products. From a study of this radioactivity and an estimate of the cavity radius, confirmed by drilling operations at positions away from Ground Zero, the total yield as well as the break-up of the fission and fusion yields could be calculated. A comparison of the ratios of various activation products to fission products for the 15 kt device and for the 45 kt thermonuclear device also shows that these ratios are in agreement with the expected fusion yield in the thermonuclear device. The total yield comes out
Figure 6: Gamma ray spectrum of a typical rock sample from the thermonuclear test site (from reference 21)
as 50 ± 10 kt for the thermonuclear device, consistent with the design yield and with the seismic estimate of the total yield.
It needs re-emphasis that the testing of the thermonuclear weapon design at the controlled yield of 45 kt was necessitated by the proximity of Khetolai village . As mentioned earlier, we have not given the fusion-fission breakup and, since we have not given the composition of the materials used nor their quantitites, for reasons of proliferation sensitivity as mentioned earlier, no one outside the design team has data to calculate this fission-fusion yield breakup or any other significant parameter related to fusion burn.
Number of tests
The US carried out more than a thousand nuclear tests and developed perhaps 70 - 80 types of devices, though it has retired many of them and is now believed to have only less than a dozen types of weapons in its current stockpile. Furthermore, just like in other technology areas, the needed number of tests came down with increase in available computing power. Figure 7 gives the testing frequency (the number of tests done per year) of U.S over the years till they stopped nuclear explosive testing in 1992. The growth in computing power over the years is plotted in the same figure. There is a clear inverse correlation.
In fact, in the early years of nuclear weapons development, it was faster and cheaper to test out a new idea by actual nuclear explosive detonation than to carry out a computer calculation, and the lower computing power available was also compounded by inadequate physics knowledge. The situation is very different now.
Conclusion
The May 1998 tests were fully successful in terms of achieving their scientific objectives:
* Certification of the fission nuclear weapon of 15 kt yield, evolved from the PNE device tested in 1974, with substantial changes that were needed to make it smaller in size and weight from the point of view of weaponisation. It was gratifying that it functioned perfectly in all aspects , certifying the quality and robustness of the design.
* Testing a two-stage thermonuclear device with a fusion-boosted fission trigger as the first stage and with the features needed for integration with delivery vehicles at the controlled yield of 45 kt with the purpose of developing nuclear weapon systems with yields upto around 200 kilotons.
Figure 7: Plot of U.S. Testing Frequency and of Growth of Computing Power over the years
* Testing sub-kiloton devices, with all the features needed for integration with delivery vehicles, from the point of view of developing low-yield weapons and of validating new weapon-related ideas and subsystems.
* Establishing the computer simulation capability to predict the yields of nuclear weapons--fission, boosted fission and two-stage thermonuclear - of designs related to the designs of the devices tested by us.
* Thus the carefully-planned series of tests carried out in May 1998 gave us the capability to design confidently and build nuclear weapons from low yields upto around 200 kilotons. A great deal of further scientific and technical development work has taken place since then.
Acknowledgement
I am grateful to Dr. Anil Kakodkar and Dr. S.K. Sikka for their critical reviewing of the manuscript and for their valuable comments. I am also thankful to a number of other colleagues for useful suggestions.
References
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12. “Nuclear Testing and Non-proliferation” by Gregory E. Van der Vink, D.W. Simpson, C.B. Hennet, J. Park and Terry Wallace, IRIS Consortium Report, Arlington, U.S.A., February 1994.
13. “Update on the yield of May 11-13 Nuclear Detonations at Pokhran” by S.K. Sikka, Falguni Roy, G.J. Nair, V.G. Kolvankar and Anil Kakodkar, BARC Newsletter, No. 178, November 1998.
14. “Indian Explosions of 11 May, 1998: An analysis of global seismic body wave magnitude estimates” by S.K.Sikka, Falguni Roy and G.J. Nair, Current Science, 10 Sept., 1998, 75(5), pp. 486-491.
15. “The Recent Indian Nuclear Tests - A Seismic Overview” by S.K. Sikka, G.J. Nair, Falguni Roy, Anil Kakodkar and R. Chidambaram, 2000, Current Science (under publication).
16. “The Indian and Pakistani Tests : Did Verification Fail?” by Trevor Findlay and Roger Clark, Trust & Verify, Issue 80, May 1998, Verification Technology Information Centre, London.
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18. “Monitoring Nuclear tests” B. Barker et.al., Science, 1998, 281, 1967, See also H.T. Wallace, Seismol. Res. Lett., 1998, 36(5), 386.
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20. “Indian Explosions of 11 May, 1998 : Analysis of regional Lg and Rayleigh Waves”, by Falguni Roy, G.J. Nair, T.K. Basu, S.K. Sikka, Anil Kakodkar, S.N. Bhattacharya and V.S. Ramamurthy, 25 December 1999, Current Science, 77(12), pp. 1669-1673.
21. S.B. Manohar et al. (Unpublished)
22. “Post-shot radioactivity measurements on samples extracted from thermonuclear test site” by S.B. Manohar, B.S. Tomar, S.S. Rattan, V.K. Shukla, V.V. Kulkarni and Anil Kakodkar, BARC Newsletter, No.186, July 1999.
23 “Fission signatures of Tests on Sub-kiloton Devices” by R.B. Attarde, V.K. Shukla, D.A.R. Babu, V.V. Kulkarni and Anil Kakodkar, BARC Newsletter, No. 188, September 1999.
24. `Applications of Godiva II Neutron Pulses’, by T.F. Wimett and J.D. Orndoff, Proc. Second U.N. Conf. On Peaceful Uses of Atomic Energy, 1958, 10, 449-460.
25. `High Temperatures in Internal Confinement Fusion Radiation Cavities Heated with 0.35 Micrometre Light’ by Robert L. Kauffman, L.J. Suter, C.B. Darrow, J.D. Kilkenny, H.N. Kornblum, D.S. Montgomery, D.W. Phillion, M.D. Rosen, A.R. Theissen, R.J. Wallace and F. Ze, Phys. Rev. Lett., 1994, 73, 2320-2323.
26. “The Comprehensive Test Ban Treaty” by Jeremiah D.Sullivan, Physics Today, March 1998, pp. 24-29; See also Position Statement on “The Capability to Monitor Compliance with CTBT”, by T.C. Wallace, J. Park, G.E. Vander Vink et al,Proceedings Seismological Soc. Amer. Meeting (with Amer. Geophys. Union), 6 October 1999.
(The author is Former Chairman, Atomic Energy Commission, India)
