By Vernon Loeb
Washington Post Staff Writer
Friday, April 30, 1999; Page A16

Secret computer programs transferred by an espionage suspect at Los Alamos National Laboratory from a classified computer network to a vulnerable desktop machine are mathematical models, known aptly as "legacy codes," embodying 50 years of American nuclear know-how.

Their discovery last month during a search of Wen Ho Lee's office computer, shortly after the physicist was fired for other security violations, has sent shock waves through the weapons laboratory and the Department of Energy because the codes are in some ways even more valuable than blueprints, nuclear weapons experts say.

With the FBI investigating Lee for possibly leaking nuclear secrets to China and a storm gathering in Congress over the Clinton administration's handling of the case, knowledge that the codes were taken out of their secure storage computers raises the possibility that Chinese espionage may have scored a major coup.

"The legacy codes themselves embody generations of work by the most knowledgeable experts in the nuclear weapons community," said Matthew G. McKinzie, a physicist and nuclear weapons expert at the Natural Resources Defense Council (NRDC). "The codes represent the best physics and the best computational techniques, along with the best tried-and-true understanding of these weapons. The Chinese, in getting these codes, would be able to assimilate all that – and that's what makes this so serious."

A nuclear weapons code is a set of equations, up to a million lines long, that describe the physical processes that occur in a nuclear explosion. A code, thus, becomes the principal tool for nuclear weapons designers as they seek to meet the specifications for each new weapon ordered by the military.

Codes exist for weapons in the nation's nuclear arsenal. They have evolved over the years as weapons scientists refined their expertise through dozens of new warhead designs and more than 1,000 nuclear tests. The Chinese, by contrast, have only their 45 tests to go on.

U.S. officials have not specified how many legacy codes Lee transferred, other than to say that not all the codes were involved. But they have revealed that Lee also transferred input data, relating to the design parameters of individual warheads – data plugged into the codes to calculate things like a warhead's explosive yield.

Even taken together, the codes and the input data would not provide a foreign adversary with an actual blueprint of a warhead, in all its engineering detail. But by using a code and the input data, McKinzie said, it would be possible to reverse engineer a weapon and go even further by manipulating the design parameters fed into the code.

This, in some senses, makes having the code and the input data even more valuable than a blueprint, McKinzie said, since it gives scientists the ability to both replicate a design and understand theoretically how various warheads actually work.

D.B. Henderson, a weapons designer at Los Alamos, has called computer computation "the center of the [nuclear weapons] program, the nexus where everything is joined."

While scientists working on the Manhattan Project developed the atom bomb without computer codes – computers did not exist in 1945 – calculations necessary to produce the hydrogen bomb in 1952 could not have been performed without an early computer, McKinzie said.

Indeed, the development of increasingly sophisticated thermonuclear weapons necessitated increasingly sophisticated computers. "They're intertwined to this day," said Robert S. Norris, another NRDC nuclear weapons expert.

Los Alamos has developed the world's most powerful computer, Blue Mountain, capable of 1.6 trillion calculations per second, as a tool to enable scientists to develop a whole new generation of computer codes that will produce three-dimensional simulations of warheads as they explode.

Those three-dimensional simulations are critical to what is now the laboratory's main mission: Stockpile Stewardship, an annual process of certifying that the nation's nuclear warheads remain functional and safe. Since actual tests of the weapons are now banned under the Comprehensive Test Ban Treaty, the only way to check them is through computer simulation.

The legacy codes Lee transferred from Los Alamos's classified network to its unclassified system are an older generation of programs – still highly valuable for Stockpile Stewardship – that produce two-dimensional models of a nuclear detonation. Lee worked in Los Alamos's X-Division, where warhead design takes place, specializing in hydrodynamics – the way metals behave when they are reduced to liquids as a warhead detonates.

Henderson, the Los Alamos weapons designer, has described legacy codes as "a compendium of lots of stuff, mostly physics."

"I look at it as books on a shelf: textbooks on chemical kinetics, detonations, fluid mechanics, equations of state, radiation processes," he has written. "We collect all these things together with numerical analysis and some ad hoc rules – and that makes a code. It is a big collection, this whole library of stuff . . . ."