The government on Thursday strongly refuted claims that the 1998 test of a thermonuclear device had been a failure, with Principal Scientific Adviser R. Chidambaram telling The Hindu that those questioning the tests yield had an obligation to back up their charge with scientific evidence.

He was responding to the recent statement by a former defence scientist, K. Santhanam, that “the yield in the thermonuclear device test was much lower than what was claimed.” Mr. Santhanam, who cited only unspecified “seismic measurements and expert opinion from world over,” went on to say that this was the reason India should not sign the Comprehensive Test Ban Treaty (CTBT).

The stated success of the second generation nuclear device tested on May 11, 1998, was questioned at the time by a number of Western seismologists who said the seismic signatures detected by them were at variance with the claimed yield of 45 kilotons. Although the controversy subsided somewhat once scientists from the Bhabha Atomic Research Centre — which designed the weapon — published their scientific evidence, it is likely to be reignited once again since Mr. Santhanam represented the Defence Research and Development Organisation (DRDO) team at the Pokhran-II tests and is the first member of that group to echo the arguments of those who say the thermonuclear device failed to work properly.

“If Mr. Santhanam has any scientific data to back up what he has claimed, I am sure BARC scientists would be more than happy to debate it,” said Dr. Chidambaram. “Without that, this kind of statement means nothing.”

In a 2000 article, The May 1998 Pokhran tests: Scientific aspects, republished in 2008 with some updated details, in a French journal, ‘Atoms for Peace,’ Dr. Chidambaram has argued that western seismologists who under-estimated the Pokhran yields did so because they did not take into account the geological structure at the Indian testing range. They also failed to appreciate that India’s weapons designers purposely went for lower yields because the shots had to be fired in existing shafts which could not be dug any deeper for fear of detection. Higher yields, then, would have caused damage to nearby villages and also led to the possible venting of radioactivity.

Dr. Chidambaram wrote that the thermonuclear device tested 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.” He said the argument that the secondary stage failed to perform is belied by post-shot radioactivity measurements on samples extracted from the test site which showed significant activity of sodium-22 and manganese-54, both by-products of a fusion reaction rather than pure fission. “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.” Based on this, he said, BARC scientists worked out a total yield of 50 +/- 10 kt for the thermonuclear device, which was consistent with both the design yield and seismic estimates.

As for the sub-kiloton tests of 0.3 and 0.2 kt of 13 May 1998, which the International Monitoring System for verifying CTBT compliance failed altogether to detect, he said “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” and so it was not surprising these two tests did not show up on the IMS.

“Let someone refute what we have written, then we can look at it,” said Dr. Chidambaram, adding that he was yet to see a published critique of BARC’s scientific assessment by any laboratory-based scientist abroad.

A former senior official of the erstwhile Vajpayee government confirmed to The  Hindu that there had been differences of opinion between BARC and DRDO scientists after the May 1998 tests, with the latter asserting that some of the weapons tests had not been successful. The internal debate was complicated by the fact that the DRDO experts, including Mr. Santhanam, were not privy to the actual weapon designs, which are highly classified. But the issue was resolved after a high-level meeting chaired by Brajesh Mishra, who was National Security Advisor at the time, in which the BARC experts established that DRDO had underestimated the true yields due to faulty seismic instrumentation. And the radioactivity analysis provided the clincher.

Since 1998, whatever his private reservations might have been, Mr. Santhanam appears to have stuck closely to the official line in his public pronouncements.

On the fifth anniversary of Pokhran-II, for example, he said in an article in Outlook that “the asymmetry with respect to China stands largely removed” thanks to the 1998 tests. Since China was a proven thermonuclear power at the time and India was not, it is hard to reconcile this optimistic assertion with the scientist’s current claim that the thermonuclear device India tested was “a fizzle.”

Similarly, in June 2007, Mr. Santhanam declared on CNN-IBN on a programme about the Indo-U.S. nuclear deal in which this correspondent was also a participant: “After May 1998, there was a clear declaration from India that we don’t have to conduct any more nuclear tests. India should not have any problem legalising this position. But this is subject to the condition that if the international security condition changes, then we should be allowed to test."