Press Statement by Dr. Anil Kakodkar and Dr. R. Chidambaram on Pokhran-II tests
India conducted five nuclear tests of advanced weapon designs on 11 and 13 May 1998 at the Pokhran range in Rajasthan Desert. The first three detonations took place simultaneously at 15:45 h. IST on 11 May. These included a 45 kt thermonuclear device, a 15 kt fission device and a 0.2 kt sub-kiloton (i.e. less than 1 kiloton) device. The two nuclear devices detonated simultaneously on 13 May were also in the sub-kiloton range – 0.5 and 0.3 kt.
The Atomic Energy Commission(AEC), in its press release of 15 September 2009, has stated that, in the meeting of the AEC held on May21, 1998, the Commission had been briefed about the technical details of the tests. In the meetings of the Commission held on March 26, 1999 and November 18, 1999, the results of the radiochemical analysis of bore-hole samples, reconfirming the estimated yields, were presented. The AEC, in the press release of 15th September, 2009, noted that the yield of the thermonuclear test was further confirmed through comparison of ground motion and displacement simulation with actual observations in the field. The AEC’s statement concluded that “the AEC has thus no reason to doubt the yield of the thermonuclear test carried out on May 11, 1998”.
Shaft depths for containment of radioactivity
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 – such as vaporisation, melting, crushing, fracture and motion of the rock – are dependent on the chemical composition of the rocks and their physical and mechanical properties such as density, porosity, water content, equation of state, strength, etc. 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 the development of the necessary know-how 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 shock wave physics, for example, we are one of the leading groups in the world in the area of equation of state at high pressures.
Nuclear weaponisation
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 weaponisation. 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 up to around 200 kt. 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. It was gratifying that all the devices functioned perfectly in all aspects certifying the quality and robustness of the designs. Thus the carefully-planned series of tests carried out in May 1998 gave us the capability to build nuclear weapons from low yields up to around 200 kt. A great deal of further scientific and technical development work has taken place since then.
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 yield of the May 1974 test announced by Chidambaram and Ramanna in a meeting on PNEs in the International Atomic Energy in Vienna in 1975 of the May 1974 test was 12–13 kt, which is also accepted internationally.
Seismic and other data on May 11, 1998 tests
Nature of seismic magnitudes
BARC scientists have published detailed analysis of the seismic data on the 11 May 1998 tests 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).
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 11 May nuclear explosion seismic event as ‘an earthquake at a depth of 56 km on the India–Pakistan border’! But this was later corrected to ‘explosions with a combined yield – consistent with the announced yield (by India)’
Professor Jack Evernden, a world renowned US seismologist, has always maintained 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 underestimation of our May 11 total yield by one group in the USA 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 ours.
Analysis by Indian seismologists
Strong Lg and Rayleigh waves (period 3.5–7 s) 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 BARC scientists.
The main conclusions are summarised below:
* A comparison of body wave magnitudes of the 11 May 1998 tests and of the 18 May 1974 test from thirteen stations around the world gives an average difference, ∆mb, of 0.45 between them.
* The estimated mb values at any recoding 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 values for two underground explosions at the same site and for the same recording station. So this value of ∆mb of 0.45 is reliable and gives a ratio of yields of 4.46. As explained earlier, 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 1988 tests.
* The average mb (Lg) magnitude obtained from the data from the IMD stations and the Gauribidanur array and the ARC stations is 5.47. A comparison of Lg waves 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.
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 (Sikka et al., 1998a).
The bore-hole gamma radiation logging and radiochemical measurements on the rock samples extracted from the sites by BARC scientists give the yield for the fission device (unpublished data) as (13 ± 3) kt and for the thermonuclear device as (50 ± 10) kt.
The Thermonuclear device
The two-stage device
The thermonuclear device tested on 11 May 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.
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 would suffer negligible damage. All the design specifications of this device were validated by the test. Thermonuclear weapons of various yields up to around 200 kt can be confidently designed on the basis of this test.
The post-shot radioactivity measurements 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, such as sodium-22 and manganese-54, which are produced much more in fusion reactions than in fission reactions.
Comparison of the radioactivity of samples extracted from the test sites of thermonuclear and pure fission devices showed a much higher activity of 22Na and 54Mn in the former. This unambiguously confirmed the occurrence of the expected fusion reaction in the thermonuclear test. 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 as (50 ± 10) kt for the thermonuclear device, consistent with the design yield and with the seismic estimate of the total yield.
As mentioned earlier, we have not given the fusion–fission break-up and, since we have not given the composition of the materials used nor their quantities, for reasons of proliferation sensitivity as mentioned earlier, no one outside the design team has data to calculate this fission–fusion yield break-up or any other significant parameter related to fusion burn. The full containment of radioactivity for a yield significantly more than 45 kt (the design yield and achieved yield) would also not have been a surety ; that was another reason for limiting the controlled yield of this device. Release of radioactivity occurs if the fractures reaching the ground surface get connected to the cavity of hot radioactive gases produced by the nuclear explosion.
CONCLUSION
* The May 1998 tests were fully successful in terms of achieving their scientific objectives and the capability to build fission and thermonuclear weapons with yields upto 200 kt.
* 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 has now been established.
* A great deal of further scientific and technical development work has taken place since May, 1998.
We have published as much data as is possible without releasing proliferation-sensitive information.
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AM(DD)/SB