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India, which surprised the world last week by carrying out underground nuclear tests, has been secretly working on the design of a whole range of nuclear weapons for the past 25 years.

The weapons programme had in fact remained under military control all along, something India managed to keep under wraps, in the same way that it was able to hide preparations for the latest tests — which included explosion of a 45-kilotonne hydrogen bomb at Pokran in Rajasthan — from US spy satellites.

The revelations emerged on 17 May during a joint press conference by Rajagopalan Chidambaram, the Department of Atomic Energy (DAE) secretary, and A. P. J. Abdul Kalam, chief of the Defence Research Development Organization (DRDO), at which they showed a video and gave some details of the underground tests — code-named Shakti-98 — carried out on 11 and 13 May (see over).

“Based on 25 years of research and development, we have designed and developed various kinds of nuclear explosives — fission, boosted fission, thermonuclear and low-yield,” said Chidambaram. All except the boosted fission bomb were tested last week.

Domestically produced enriched uranium-235 is believed to have been used in some of these bombs. But Chidambaram said that this information was classified. “All I can say is that the fissile material used was totally indigenous.”

Kalam revealed that the devices tested were the result of a “decades-old” partnership between DRDO and DAE. “When nuclear technology and defence technology met, they got transformed to nuclear-weapons technology.”

DRDO, he said, had been involved in the design and manufacture of detonators, electronic trigger systems and chemical explosives, besides making contributions “in aerodynamics, arming, fusing and safety interlocks”.

According to Kalam, those tested last week were not experimental devices, but deployable, computer-designed and certified bombs. The yield of the H-bomb — set off at a depth of 300 metres — was deliberately designed to be low so as to avoid damage to the closest village, 5 km away from ground zero.

“The tests marked the culmination of militarization which started years ago,” said Kalam. He added that the nuclear warheads now available can be put into the domestically produced Prithvi surface-to-surface missile and the intermediate-range Agni missile, which is ready for mass production and whose newer longer-range version is at an “advanced stage of development at DRDO”.

This is the first admission that India's nuclear weapons programme not only continued under different governments after the ‘peaceful nuclear explosion’ on 18 May 1974, but was actually militarized long before the Bharatiya Janata Party came to power two months ago.