All nuclear weapons now in the American stockpile were developed with the aid of computer models validated by comparison with nuclear tests. However, those models required the use of parameters that were not well understood and often needed adjustment to make computation and test agree. Facing the possibility of a test ban, the Department of Energy initiated a Stockpile Stewardship Project to develop a predictive capability with validated, physics-based simulation tools at its core. This program is charged with maintaining the performance, reliability, and safety of U.S. nuclear weapons without nuclear testing. To meet the requirement for maintaining the enduring stockpile, the Department of Energy engaged the three national weapons laboratories in creating the Accelerated Strategic Computing Initiative (ASCI). ASCI advanced computational capabilities and three-dimensional models, combined with major experimental and testing facilities, should make it possible for the United States to maintain its present nuclear stockpile indefinitely. The authors believe that the ASCI computational capabilities also will enable nuclear weapon designers to draw on archived data from more than 1,000 nuclear tests to adapt proven designs to future mission requirements. Through extensive computer modeling and nonnuclear testing, new nuclear weapons could be designed and introduced into the stockpile, so long as the new weapons used design concepts similar to those proven in nuclear tests.
All U.S. nuclear weapons have been designed on computers. This was true even in 1944-1945, when "computer" was a job title for a small army of young women. Many were the wives of the scientists and technicians designing and building the nuclear weapons. Each of the "computer elements" operated a Marchant or Frieden mechanical calculating machine, entering results of one or two operations on an index card and passing it on. Sometimes the choice of which person to pass the data on to depended on the result of the computer's calculations. The computations required to ensure that a modern nuclear weapon functions as desired are, in general, far too complicated to be performed by a few people armed with nothing but old-fashioned mechanical calculators. As the American experience with nuclear weaponry evolved from the crude devices of 1945 to more sophisticated designs that were lighter and more powerful and used less uranium or plutonium, the need for fast electronic computers grew. Indeed, the nuclear weapons establishment was one of the driving forces behind the early development of computers.
A thermonuclear weapon includes layers of materials, such as uranium and/or plutonium, some kind of tamper or reflector material, a high explosive, boosting gas made up of tritium and deuterium, fusion fuel using lithium deuteride, and an outer casing (radiation case) that encloses the explosive components. One major problem in designing a device is ensuring that the shock waves from the high explosive properly cross the boundaries between layers and that the energy from the primary is properly absorbed by the secondary. Matching such boundary conditions is a difficult computational task that is essential to success. Other tasks for the design program include understanding how solid metal is made to flow like a liquid or compress like a gas (that is, solving the equation of state of the material).
Nuclear design codes may be one-, two-, or three-dimensional. That is, they can reduce computations to the minimum by treating each component separately and only examining what goes on along a single line through the primary or secondary (one-dimensional); they can examine a section through the objects (two-dimensional); or they can model the actual three-dimensional shapes of the primary, secondary, and radiation case. One-dimensional calculations are clearly too simplified for modern nuclear weapons; three-dimensional codes stressed the capabilities of the largest computers in the world only a little more than 10 years ago. …