The controversial letter describing the making of a hydrogen bomb would be a useful tool to countries such as India, South Africa and Israel that have the will and the means to produce such a weapon.

At the same time, the letter, which had been classified by the Department of Energy because the government said it contained sensitive information, probably would not be much help to less developed countries in crossing the termonuclear threshhold.

That's the opinion of a handful of nuclear weapons scientists who have seen the 18-page letter on hydrogen weapons technology written by California programmer Charles Hansen and published last week by three newspapers.

The scientists said the letter contains clues that would help industrialized countries solve the complex technological problems involved in the making of a hydrogen bomb.

"It's a good road map to a hydrogen bomb," said Princeton University's Dr. Theodore B. Taylor, a one-time designer of nuclear weapons at the Los Alamos Scientific Laboratory in New Mexico. "If I said anything more than that about this letter, I'd be violating security."

The letter is not so thorough and rich in detail as to help terrorists or a rogue nation build a hydrogen bomb, the scientists said, but it is detailed enough to assist a country like India to move from atomic to hydrogen bombs. That is an enormous step, the scientists emphasize. The explosive force of H-bombs ranges from one megaton (one million tons of TNT equivalent) to 60 megatons.

"How to make atomic bombs has been declassified for years," Taylor said. "Not so with hydrogen bombs, the less people know about how they work, the better it is."

The letter from Hansen says that eight separate explosions, all within millionths of a second of each other, are necessary to detonate a hydrogen bomb.

At least two of the eight explosions are fission explosions, Hansen said, which are atomic explosions caused by the splitting of atoms in heavy metals like uranium and plutonium. Two more are fusion or hydrogen explosions, which occur when the atoms in light elements like hydrogen, deuterium and tritium fuse together to create energy that is the equivalent of the sun's output of light and heat.

Scientists emphasize that to produce a hydrogen bomb, a nation would need a large trained cadre of technologists. They would also need at least $1 billion to build power plants and factories to process the right kinds of uranium and light elements like tritium for a hydrogen bomb.

"It's a completely different technology in a hydrogen bomb," said Dr. Ralph Lapp, a Washington-based consulting physicist who worked on the atomic bomb. "You're talking about creating something to produce temperatures of 100 million degrees in a hydrogen explosion."

Scientists said that countries the size of Pakistan and Libya would be hard-pressed to come up with the resources to build a hydrogen bomb, even with the help of the blueprints for such a device. Besides the technical base to make the materials, a country would have to test its hydrogen weapon to make sure is would work.

An atomic bomb could be built and stored without testing, scientists said. This is not true of a hydrogen bomb that requires so many different things working at the same time. If mistakes were made in putting a hydrogen bomb together, the bomb would fizzle and produce nothing more than a conventional atomic explosion.

"We've had several duds in our test experience," Lapp said, "where the yield [the explosive force] was low, nowhere near what was expected."

What worries nuclear weapons scientists about the publication of the Hansen letter is that it contains the kinds of clues to hydrogen bomb design that a country like India would need to help it go from a country that has produced only an atomic weapon to the megaton-sized hydrogen bomb.

"I've read Hansen's letter several times and while he made some mistakes, there are things in there that are quite accurate," said the Massachusetts Institute of Technology's Dr. George Rathjens, who worked on the development of hydrogen bombs. "The letter gives enough important clues that it would be a big help to somebody like India."

Scientists said the Hansen letter would be of little help to a country that does not have a thriving industrial base, but would be very useful to a nation wanting to make a hydrogen bomb and willing to invest at least $1 billion.

If Israel or South Africa do not already have atomic weapons stockpiled, they would certainly be helped by the Hansen letter in figuring out the best road to take in developing a hydrogen bomb.