In a disclosure with potential to further accelerate a nuclear arms race with Pakistan, Indian scientists confirmed Sunday that the largest of five underground nuclear tests they conducted last week involved a hydrogen bomb, a device with potentially enormous power that is known in American military circles as a "city-buster."
When the first round of three tests was announced last Monday, India described the biggest of the weapons tested as a "thermonuclear device," a term that set off a debate among weapons experts around the world about the kind of weapon involved.The scientists, appearing at a news conference, gave technical details of a bomb code-named Shakti-1, after a Hindi word for power commonly used when referring to the most potent of the Hindu gods.
The scientists said that the bomb was a two-stage device involving a so-called fission trigger and a second stage that gave the bomb its main explosive force, a design they said was popularly known as a hydrogen bomb.
According to figures released by the scientists, the bomb had an explosive force equivalent to 43,000 tons of TNT. This would be small by comparison with the most destructive nuclear weapons built by the United States, Russia and other established nuclear powers, which have tested hydrogen bombs with explosive power equivalent to several million tons of TNT.
But the Indian scientists said their weapon was deliberately kept small to avoid damage to several populated villages close to the test site in India's northwestern desert.
Rajagopal Chidambaram, chairman of India's Atomic Energy Commission, said the design of the bomb gave India the capability to increase its explosive power virtually at will. "In fact, we could have got much higher yields than we got," he said. "We were limited by possible seismic damage to the villages".
Another scientist, A.P.J. Abdul Kalam, regarded as the "father" of the Indian bomb, said of the possibility of building bigger bombs, "If there is a demand, we will do it."
"Thermonuclear device," American weapons specialists said Sunday, literally refers to a device that burns hydrogen fuel and thus has more punch than an atom bomb, the kind of weapon the United States dropped on Japan in 1945.
But hydrogen bombs come in two varieties. In the first, the American specialists said, scientists fill the core of a small atom bomb with hydrogen fuel, a trick that can boost its power 10 times or more.
But making a true hydrogen bomb hundreds or even thousands of times more powerful requires a more complex method. In this second variety, the secret is to harness the radiation from an exploding atomic bomb and use that to compress and heat a packet of hydrogen fuel that is located not internally but nearby.
It was this type of weapon that the Indian scientists described at their news conference Sunday. Chidambaram and Kalam repeatedly emphasized that the weapon they detonated involved a "fission trigger," meaning a small atom bomb that set off the main blast and not the other type of thermonuclear weapon, which they described as a "boosted fission" device. They said they had developed designs for this type of weapon in laboratories but did not test one last week.
In theory, the amount of hydrogen fuel ignited by the two-stage method has no limit. America's first hydrogen bomb, exploded in 1952, was about 700 times more forceful than the atomic bomb dropped on Hiroshima.
The Pacific isle of Elugelab, one mile in diameter, where it had been located, simply vanished. But the practical considerations of putting the bomb atop a missile can quickly lead to difficulties, and it was not clear from Sunday's statements by the Indian scientists that these have been resolved.
Harold Agnew, former director of the Los Alamos National Laboratory, the birthplace of the bomb in New Mexico, said the Indian claim of having made a true hydrogen bomb was quite believable.
"It's not a giant step if you have smart people and understand the basics," he said in a telephone interview. "They could probably make bigger ones," he added, referring to true hydrogen bombs. "If you can do a little one, you can do big ones."
The news conference in New Delhi turned into another occasion for Indians to demonstrate their soaring pride at the nuclear tests. The scientists were applauded by Indian reporters, asked for their autographs and generally treated much like early American astronauts returning from a space mission. In their remarks, they repeatedly emphasized that they had developed the versions of the nuclear weapons tested last week on their own, without any foreign help.
But their claim that they had detonated a hydrogen bomb appeared likely to increase tensions with Pakistan, where government officials have spent the days since the first Indian blasts weighing whether to respond with a nuclear test of their own and risk being punished with the same economic sanctions that the United States and some other nations have imposed on India, or hold off in the hope of isolating India.
For several years, Western intelligence experts have known that Pakistan had the capability to build an atomic bomb, but American nuclear experts have said it is far from clear that they could detonate a hydrogen bomb.
As India released the first technical details of its tests, Pakistani Foreign Minister Gohar Ayub Khan was quoted Sunday as having said that it was not a question of whether Pakistan would stage a nuclear test but when. It was unclear, however, if he was speaking on behalf of his government.
The technical details given by the Indian scientists also appeared to clear up uncertainty that developed among Western geologists and weapons experts over the number of weapons tested by the Indians and their explosive power.
The Indians said that the four other weapons they tested, also code-named Shakti, had an explosive force equivalent to 12,000 tons and 200 tons of TNT in the case of the two weapons tested with Shakti-1 on May 11 and about 600 tons and 200 tons for those tested on May 13.
Immediately after the first tests, American seismologists said they had detected only one blast and that it was smaller than Indian estimates. But India said the simultaneous triggering of the two devices likely caused "interference."