The argument that the Pokhran-II thermonuclear (TN) test of May 11, 1998 had either a lower than the officially stated yield or failed altogether has now formed the basis for demanding a new round of tests.
Reiterating their arguments made in a recent article ( The Hindu, September 17) at a press conference on September 21, K. Santhanam, former official of the Defence Research and Development Organisation (DRDO) and a member of the Pokhran-II core team, and Ashok Parthasarathi, former Special Assistant on S&T to Prime Minister Indira Gandhi, called for lifting the country’s unilateral moratorium and launching “a comprehensive, well-focused series of TN bomb tests until such bombs are perfected.” It must be emphasised that they have not questioned the country’s capability to build an arsenal with deliverable fission weapons.
The arguments for a fresh round of TN tests towards weaponisation arise from two perspectives: (a) an unsuccessful Pokhran-II test and (b) improving the weapon design even if the Pokhran-II test was successful. While the available evidence — the technical information published by the Department of Atomic Energy (DAE) — does not show that the Pokhran-II was unsuccessful, there are compelling arguments against the need for resuming testing even if it was so.
DAE employed different techniques to estimate the test yields. The yield values obtained from the other five nuclear explosive tests are stated to be consistent with the original estimate of 60 kt for the May 11 tests — a 45 kt thermonuclear device and a 15 kt fission device that were exploded simultaneously. Of these, the post-shot radiochemical test, an on-site method, is considered the most accurate. Its results were published by S. B. Manohar et al ( BARC Newsletter, July 1999).
A TN weapon has a primary fission trigger and a fusion secondary. The radiochemical technique for a TN test looks for signatures of certain radioisotopes, like sodium-22 and manganese-54, produced as a result of nuclear fusion in material samples from the shaft. Dr. Santhanam has argued that, since these could arise in fission as well, detection of these without stating the absolute values cannot be evidence of fusion having occurred.
Fusion produces large quantities of high-energy 14 MeV neutrons whereas fission neutrons have an average energy of only about 2 MeV. But in fission too there would be a small number of high-energy neutrons. However, high-energy neutrons produce these radioisotopes with greater probability than low-energy neutrons. Therefore, while one may see these products during fission as well, fusion would produce these in copious amounts. According to the Pokhran-II measurements, the fission weapon too had produced manganese-54. But the amount in the fusion device is significantly higher and does provide an idea of the comparative yields. DAE scientists have also published the significantly higher gamma-ray activity from fusion products ( BARC Newsletter, July 1999). The absolute values and the scale have been withheld for obvious sensitivity reasons, but the qualitative difference in the levels is evident.
Further, from the post-explosion morphology of the test site, Dr. Santhanam has contended that, had a TN test occurred, “the shaft [and the A-frame sitting astride its mouth] would have been totally destroyed.”
This assertion is clearly not correct because qualitatively speaking, it is possible to conduct a 40+ kt TN explosion and still have the site morphology as seen with no cratering.
The crater morphology depends on the depth of burial, the surrounding geology, and the manner of emplacement of the device. In an underground explosion, the confining effect of the material overburden causes the energy to be directed downwards. This has a negative influence on cratering. At the same time, the confining strata are blown upwards by the expanding gas. As the depth increases, the mass of the overburden increases and, therefore, the confining effect increases. As a result a larger and larger fraction of the material thrown upwards falls back and the crater size would begin to decline as less and less material now gets ejected. So one can understand why, for example, the cratering effect of a 1 kt explosion at 20 m may be similar to a 125 kt explosion at 100 m. In fact, at some low enough value, there would be upheaval within but no material would be thrown out and there would be practically no crater.
The surface features caused by explosions at different depths and yields (under similar surrounding geology) can actually be quantified by scaling them to a standard yield, for example 1 kt. Such a scaled empirical relation does show zero cratering for some depth values, beyond which what is known as a subsidence crater results. From this relation, cratering effects for different depths and different yields can be calculated. DAE scientists have worked out such a relationship for Pokhran. Through simulations, yield and depth for the TN device were chosen so that there would be minimum cratering and there would be complete containment of radioactivity ( BARC Newsletter November 1998). Indeed, in the Pokhran-II TN test, there were only large fractures on the surface but no crater or venting of radioactivity.
Based on the Pokhran-II tests, the National Security Advisory Board (NSAB) assessed the Credible Minimum Deterrent (CMD) capabilities and drafted the Indian Nuclear Doctrine (IND) in August 1999. In fact, according to R. Chidambaram, former chairman of the Atomic Energy Commission (AEC), going by the Pokhran-II tests, a TN capability up to 200 kt and a “full capability to provide technological back-up to the Indian nuclear programme” existed. The IND, which had government and military acceptance, is premised on the following: a CMD, a ‘no first use’ policy, and nuclear retaliation against a first strike that will inflict unacceptable damage. Further, in January 2003, the National Democratic Alliance government stated: “The Cabinet Committee on Security reviewed the existing command and control structures, the state of readiness, the targeting for a retaliatory attack and operating procedures for various stages of alert and launch…[and] expressed satisfaction with the overall preparedness.”
Dr. Santhanam argued that, without high-yield (150-350 kt) TN weapons, an arsenal of lower yield (around 25 kt) fission warheads did not amount to a CMD for distances of 3500 km and beyond. In a recent article ( The Hindu, September 21), a former DRDO chief, V. S. Arunachalam, and a veteran strategic affairs analyst, K. Subrahmanyam, asked: “Can it be argued that only a 150 kiloton weapon will deter another warhead of a similar yield? Deterrence is not about the damage one causes to the adversary. It is about what the aggressive side will consider as unacceptable.” That is, given the extent of damage a 25 kt fission weapon can cause, an arsenal built up from lower yield fission weapons alone suffices for deterrence. Further, countering the argument of Dr. Santhanam and Parthasarathi that not only the warhead yield but the distance also was a factor, they stated: “It is not infra-dig for a 3500 km range missile to carry a 25 kt warhead. Cost-effectiveness calculations have no meaning since the nuclear war itself has no meaning.”
P.K. Iyengar, a former AEC Chairman, has argued that since TN devices are “compact, light, use less sensitive material, and offer better safety features,” they are better for weaponisation and deployment. “It is only prudent,” he says in his yet-to-be-released book, “that one continuously tests these and upgrades the technology so that it will be foolproof. Declaring a moratorium on testing immediately after the May 1998 test is unfortunate because we cannot test the veracity of the yield and also cannot improve the mechanical design of the device.”
In this context and from the perspective of only a second strike capability, the observations a Chief of Army Staff, General Krishnaswamy Sundarji, made in a 1996 private communication I have had access are pertinent: “I would not accept the proposition that continued open-ended testing would be required to keep our deterrence…For, in minimum deterrence, our weapons are going to attack enemy cities and…not…enemy weapons. Relative superiority of the nuclear weapons of the two sides should not, therefore, make any difference to deterrence…”
From the published data, there is no evidence that the 1998 thermonuclear test was unsuccessful and, in any case, since nuclear war has no meaning, an arsenal built up from lower yield fission weapons alone suffices for ‘credible minimum deterrence.’