Big claims, small evidence

BIG claims require big evidence to back them up. That is a rule of thumb in science and, for that matter, in many fields.

On May 17, India made a big claim. It said, finally, that one of the nuclear devices it exploded in a series of tests on May 11 and 13 beneath its northwestern desert was a true hydrogen bomb, in theory allowing it to make arms of virtually unlimited destructiveness.

Now the emerging debate among experts is whether India has the evidence to back up this claim and, if so, what the implications may be for weapons, politics and the world. For Pakistan and China, and stability in the tense region, the H-bomb issue is crucial. But already scientists and weapon experts say that India's announcements are rich in mysteries, and that the evidence it has so far presented is insufficient to clear them up.

"The whole thing sounds odd," Herbert York, a former nuclear bomb designer and director of Pentagon research, said of India's H-bomb claim. "It's not odd enough to make me say it's not true. But it's still a very strange story."

Vipin Gupta, an arms-control expert at Sandia National Laboratories, one of the United States' centres for research on nuclear weapons, urged caution in evaluating India's nuclear claims, noting that the nation in the past appeared to have overstated the power of its first atom blast. "You have to have a healthy degree of scepticism," Gupta said. "We've heard so many conflicting accounts."

For nuclear-weapons experts, the current situation is fast becoming a detective story as they sort through the various clues and possibilities to make sense of the H-bomb enigma.

Shiv Mukherjee, press attache at the Indian embassy in Washington, said that he could make no comment on the analyses. "The experts will have to have their expert say," he said.

In interviews, analysts noted that a major reason for caution is the cloud of uncertainty around India's first detonation, in 1974. Indian experts had then claimed that it had a force equal to 15,000 tons of high explosive, or about the same as the atomic bomb dropped in 1945 on Hiroshima. But over the years, press reports and expert opinion reduced the estimate to as low as 2,000 tons.

"They definitely hyped it the first time around," said George Perkovich, director of the Secure World Programme at the W. Alton Jones Foundation, in Charlottesville, Virginia, and author of India's Nuclear Bomb, to be published next year by the University of California Press.

Twenty-four years after that first blast, India stunned the world by announcing that it had conducted a series of nuclear explosions, three on May 11 and two on May 13, shattering a global moratorium on such detonations. Indian officials at first gave few details. The tests on May 11 were said to be "a fission device, a low-yield device and a thermonuclear device," meaning that it burned hydrogen fuel and thus had more punch than an atom bomb. The tests on May 13 were said to be "subkiloton," meaning their force was less than 1,000 tons of high explosive.

In weaponry, "thermonuclear" is an ambiguous term. It can refer either to a small atom bomb that burns a bit of hydrogen fuel to raise its power as much as 10 times, or to a true hydrogen bomb that can be hundreds or even thousands of times as strong. In the first thermonuclear method, the core of a small atom bomb is filled with hydrogen fuel. In the second, the secret is to harness the radiation from an exploding atomic bomb and use that to compress and heat a packet of hydrogen fuel located nearby, igniting the fusion of hydrogen into helium.

Achieving this heating was the breakthrough made in 1951 by Stanislaw Ulam and Edward Teller, the fathers of the hydrogen bomb. Their solution was a jacket of complex materials that for a split second focussed the radiation from the exploding atom bomb onto the hydrogen fuel before the whole assembly disappeared in a titanic blast.

In theory, the amount of hydrogen fuel ignited by this two-stage method has no limit, making it potentially a weapon of terrifying force. But putting the bomb atop a missile leads to practical limitations.

The U.S.' first hydrogen bomb, a behemoth exploded in 1952, was about 700 times more forceful than the atomic bomb dropped on Hiroshima. In 1961, Russia exploded one that was about 4,000 times more forceful. In military circles, hydrogen bombs are known as city-busters.

In the past, nuclear powers have usually taken the first thermonuclear step before the second. Evidence available suggests that India did the same. The global network of seismometers that monitor the earth for repercussions from earthquakes and atomic blasts picked up only one faint rumble at the Indian test site, on May 11. Its power seemed equal to about 25,000 tons of high explosive, or about twice that of the Hiroshima bomb. That is quite small for a true hydrogen bomb, so most experts assumed that India's "thermonuclear device" was simply a boosted atom bomb.

On May 17, however, a team of Indian scientists came out of the shadows to say their creation was in fact a true hydrogen bomb. "We could have got much higher yields," Dr. Rajagopal Chidambaram, Chairman of India's Atomic Energy Commission, told reporters. "We were limited by possible seismic damage to the villages."

The scientists said that the blast's power was actually equal to 43,000 tons of high explosive - nearly twice as high as the world estimate but still falling well short of proving that the device was a true hydrogen bomb. In fact, its power suggested otherwise. The U.S' first boosted atom bomb, detonated in May 1951, had a power of 46,000 tons - virtually identical with the Indian test.

The Indian scientists' announcement on May 17 held other mysteries, experts say. A main one was the lack of any news about a boosted-atom test, the first logical step on the hydrogen-bomb road and a crucial development if a hydrogen bomb is to be easily deliverable. Regular atom bombs are considered too heavy for the triggering job, especially if the H-bomb is to ride atop a missile.

Some weapons experts said that the two tests on May 13 may have been failed boosting tests. An Indian news release said that the blasts had a power of between 200 tons and 600 tons of high explosive, or quite small.

"Maybe they tried and failed," said Ray Kidder, a former bomb designer at the Lawrence Livermore National Laboratory, in California. York said that very small blasts made little or no sense. "It's a funny thing to do at this stage" of India's atomic evolution, he said.

The experts, however, agreed that the Indian claims, if true, could add up to the prospect of the developing nation creating a usable - if heavy - hydrogen bomb of devastating force. "Anybody here would say you need to do some more testing to get it weaponised," said York. But he added that the goal moved much closer to reality if the hydrogen bomb were to be delivered not by a missile, where every ounce is critical and experts constantly struggle to lighten payloads, but by truck or airplane.

Kidder agreed. "If what they claim is true," he said, "it is very, very likely that they could develop a hydrogen bomb of any size they want. The only caveat is that with their present technology, it might not be as light as those in the American arsenal."

Some experts say that the Indian claims should be taken at their face value, and that India probably did achieve a string of nuclear successes. "They're pretty smart guys," said Harold Agnew, former director of the Los Alamos National Laboratory, the birthplace of the bomb in New Mexico. "The tests will give them confidence" about how to make a wide variety of nuclear warheads, including an assortment of hydrogen bombs.

"If you can do a little one," he said, "you can do big ones."

New York Times Service