April 29, 1999

Unsecure Codes Are Recipes for A-Bombs, Experts Say

By WILLIAM J. BROAD

The secret computer codes that were downloaded into a nonsecure computer at the Los Alamos National Laboratory and that federal experts fear were given to China are the distillation of more than a half-century of research on how to perfect nuclear weapons, experts in and out of government said Wednesday.

Using the programs, which calculate step by step how a bomb explodes, they said, weapons designers could produce simulations of nuclear explosions realistic enough to check the feasibility of new designs before taking the costly step of testing arms in actual blasts.

At their most basic level, the codes are equations rooted in the laws of physics, which are taught in high school and college. But added layers of hard-won tricks and lore turn the codes into informational gems, spelling out, for instance, not only how nuclear weapons can be constructed, but also how they can be made smaller, lighter and more powerful.

"It's a stunning revelation," said Dr. Matthew McKinzie, a former Los Alamos researcher now at the Natural Resources Defense Council, a group in Washington that tracks nuclear arms. "It's the distillation of 50 years of work, over 1,000 nuclear tests and thousands upon thousands of man-hours."

In the days when the designers of nuclear bombs liked nothing better than unleashing the atom's power in huge explosions literally felt halfway around the globe, the scientists first conducted mathematical calculations to try to insure that their creations would not be costly duds and embarrassments.

Over 50 years, experts said, that work yielded 100 or so advanced computer programs, about as many as there are types of nuclear warheads in the U.S. arsenal.

Those secrets, known as the "legacy codes," consist of millions of lines of computer instructions that detail the physical principles at the heart of U.S. atomic warheads.

The secrets are now at the center of a political firestorm. Federal officials assert that a scientist suspected of spying for China improperly transferred large parts of the codes from a computer system at the Los Alamos, N.M., lab, potentially giving away the secrets of virtually every weapon in the U.S. arsenal.

Downloaded as well, federal and lab officials said, were data on the materials and shapes of parts that make up specific weapons designs. Someone who put all that data together could create virtual blueprints of the nuclear explosive parts of a weapon.

Still, the codes are highly complex, because they mimic how colossal forces of nature alter materials -- for instance, how temperatures hotter than the surface of the Sun and pressures greater than at the center of the Earth can transform dense metals at the heart of a bomb into hot fluidlike plasmas in a split second.

Over the decades, federal scientists said, the codes allowed computerized dry runs that checked the feasibility of designs before millions of dollars were spent to build and test a new weapon.

"They were used to decide whether a particular design was worth testing," said Dr. Randy Christensen, a bomb-code physicist at the Lawrence Livermore National Laboratory in California, one of three national labs that developed nuclear arms and now maintain them. In recent years, with the advent of the ban on nuclear testing, the codes have been used as a substitute for test explosions themselves.

A Los Alamos physicist who has worked extensively on bomb codes, Dale Henderson, said, "Computation is the only place where everything comes together."

With computerized go-aheads, hundreds of underground nuclear blasts rocked the test site in Nevada, a desolate expanse of desert bigger than Rhode Island.

In 1945, when nuclear weapons flashed to life, scientists struggled with side rules and crude computers to find the best way of slamming together the critical mass of bomb fuel -- uranium 235 or plutonium 239 -- to make the chain reaction that released huge bursts of energy.

High explosives working at room temperature were fine to ignite an atomic bomb, it turned out. But complexity soared in 1952 with the advent of hydrogen bombs, which use an exploding atomic bomb as a fiery sunlike torch to ignite deuterium and tritium, both heavy forms of hydrogen that make up the fuel of a hydrogen bomb.

Scientists say the computerized codes, developed with the world's most powerful computers, were critical to perfect the first hydrogen bombs and their numerous progeny, which today dominate the world's nuclear arsenals. The first hydrogen bomb was 700 times more powerful than the bomb that hit Hiroshima.

The codes start with raw physics, scientists said, like the equations that describe how X-rays that flash out of an atomic blast interact with the fuel of the hydrogen bomb. Experts say what goes into the modeling recipe is as important as what is left out and ignored.

A senior scientist at a nuclear-weapons lab who insisted on anonymity likened the general recipes to building codes adopted by communities to make sure that buildings are safe and sound. "They're all the rules you have to apply," he said.

Added to those general rules are the architect's details of how an individual house is to be built, height, shape, siding, materials and plumbing and electrical systems, producing a plan "so you end up with a house," the scientist said.

Federal and lab experts say such "input data" for specific weapon types was included among the information downloaded at Los Alamos. Such details include the shapes and sizes of bomb parts and extensive data on the physical characteristics of nuclear materials, which vary widely among the 100 or so designs that have been in the nuclear arsenal.

The various designs are used to give the arsenal greater variety in how the warheads could be deployed and launched and would vary depending on the type of intended target like deeply fortified bunkers, cites or missile sites.

For newer weapons, the codes include large numbers of ad hoc refinements and tricks gleaned from decades nuclear testing, scientists said. Such improvements are informal rather than derived from the laws of physics. Learned through trial and error, the advances are needed because the science is insufficiently precise to make computerized models of bomb-blast complexities in all their details.

"It's one way we've always compensated for not having perfect science or infinitely powerful computers," said Christensen of Lawrence Livermore.

A nuclear-weapons physicist at Livermore, Dr. Ray Kidder, said the combination of source code and input data was equivalent to a scientific blueprint, allowing a bomb to be reverse-engineered, analyzing the completed product. In theory, Kidder said, "you cannot only find out how it was done, but you can also improve the weapon by running the code and changing some of the parameters."

Parts of the design not spelled out in such computer data, Kidder added, include the gadgets and conventional explosives that help start the chain reaction, factors considered more or less common in bomb design for 50 years.

Copyright 1999 The New York Times Company