Yields called into question

Print edition : October 10, 1998

THE controversy over whether the yields from the Indian nuclear tests of May 11 and May 13 were really as high as the Department of Atomic Energy (DAE) claimed remains unresolved.

In a paper published in the latest issue of the international journal Seismological Research Letters, University of Arizona geophysicist Terry C. Wallace has estimated the total yield of the May 11 tests to be 10-15 kilotons, against the official Indian claims of about 60 kilotons. The yields of the May 13 tests are estimated to be no more than 100-150 tons, considerably less than Indian claims of 0.8 kilotons. Wallace used seismic data collected all over the world to make his estimate. Crucial to the second estimate is the fact that the May 13 tests were not detected by seismic stations outside India, which puts an upper limit on their possible yield.

Wallace's paper also estimates the yield of the May 28 tests conducted by Pakistan to be 9-12 kilotons, much less than official claims of 40-45 kilotons, and those of the May 30 tests to be 4-6 kilotons. Wallace's analysis, accessible on the Internet at https://www.geo.arizona.edu/, has been judged to be reasonable by other experts quoted in the American media.

The DAE has buttressed its official estimates with a scientific paper in the September 10 issue of the Indian science journal Current Science. The analysis, by three scientists from the Bhabha Atomic Research Centre (S.K. Sikka, Falguni Roy and G.J. Nair), relies on a theoretical argument that the interference between the seismic waves from the two main explosions of May 11 would have led to a lowered estimate of the seismic signal strength at stations which were in the eastern and western directions. The stations in the northern and southern directions, they argue, are the only ones whose data should be taken into account. Taking this set of data alone into account, they obtain a higher average for the seismic magnitude and hence the yield, compared to those obtained by foreign seismologists. Interestingly, Wallace's paper explicitly claims that a study of the strength of the seismic signals from all over the world shows no such dependence on the direction.

An important input in both these analyses is the yield from the first nuclear test at Pokhran in May 1974. Wallace estimates it at less than 5 kilotons, while the DAE analysis uses the official estimate of 13 kilotons. A discreet debate has continued over the years among Indian and foreign experts over the yield from Pokhran-I; this has now surfaced.

There are high political stakes involved in the outcome of this debate. For the Indian atomic energy establishment it is important to establish that its claims are correct. If estimates such as those of Wallace are true, it demolishes official Indian claims to have established any capability of a credible nuclear deterrent. For other countries, particularly those that are signatories to the Comprehensive Test Ban Treaty (CTBT), it is essential to prove that the seismic monitoring system, projected as the scientific safeguard that guarantees against secret violations of the treaty, is as good as it is claimed to be.

In fact, 18 other scientists have joined Wallace in authoring a paper in the September 25 issue of the international journal Science, which claims that the seismic monitoring system did its job as expected in the case of the Indian and Pakistani tests. They express confidence in the capabilities of the full system, which will be soon in place, to meet the needs of the CTBT regime.

Clearly, this debate is far from over.

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