For nearly a half century, confidence in the U.S. nuclear deterrent was a product of computation, experimental science, and weapons physics. The final judgments about the safety, performance, and reliability of the country’s nuclear stockpile were confirmed by nuclear test results. Because of this, computer models much simpler than those needed today could be used with the best available computers to help design, modernize, and maintain the stockpile. Now, without nuclear testing as the final arbiter of scientific judgment, weapons scientists must rely much more heavily on sophisticated computers to simulate the complex aging process of the weapons components and the weapons systems as a whole, and determine the impact on the nuclear weapons stockpile.
ASC Program Background
The predecessor to ASC, the Accelerated Strategic Computing Initiative (ASCI), was established in 1995 as an essential element of the SSP to provide nuclear weapons simulation and modeling capabilities. Prior to the start of the nuclear testing moratorium in October 1992, the nuclear weapons stockpile was maintained through (1) underground nuclear testing and surveillance activities and (2) “modernization” (i.e., development of new weapons systems). A consequence of the nuclear test ban is that the safety, performance, and reliability of U.S. nuclear weapons must be ensured by other means for systems far beyond the lifetimes originally envisioned when the weapons were designed. The National Nuclear Security Administration (NNSA) was established in 2000 to carry out the national security responsibilities of the Department of Energy, including maintenance of a safe, secure, and reliable stockpile of nuclear weapons and associated materials capabilities and technologies.
The NNSA’s Science-Based Stockpile Stewardship Program was established to develop new means of assessing the performance of nuclear weapon systems, predict their safety and reliability, and certify their functionality. The program must not only fulfill its responsibilities without nuclear testing, but must also address constraints on non-nuclear testing, the downsizing of production capability, and the cessation of developing new weapon systems to replace existing weapons. Further complicating matters, weapon components are exceeding their design lifetimes, and manufacturing issues and environmental concerns will force changes in fabrication processes and materials of weapon components.
How ASC Supports the Science-based Stockpile Stewardship Program
ASC serves the Science-Based Stockpile Stewardship Program (SBSS) in the following ways:
- Directed Stockpile Work (DSW) involves evaluation, maintenance, and refurbishment of the stockpile. The ultimate output of DSW comes through the annual certification process, which specifies that each weapon in the stockpile is usable or not. The computing power needed to meet various DSW priorities varies over a large range. A major component of DSW is the set of Life Extension Programs (LEPs), which deal with the discovery and/or prediction of aging problems in specific weapons, and the problems of refurbishing these weapons in view of the aging and other problems, through the design, testing, and installation of new components.
- Campaigns organize the science-based stewardship effort into various functional areas (primaries, secondaries) and science or engineering areas. Campaigns involve theory and experiment, simulations, and surveillance. Campaigns call for ASC-related milestones. These are extremely demanding on computer resources, and explore the limits of ASC capability.
- Defects in specific stockpile weapons may be found as a result of the standard surveillance process, in which a number of weapons are disassembled and inspected each year. If the defect is considered serious enough to affect the reliability of the weapon type, a Significant Finding Investigation (SFI) is opened. An SFI may involve leaks, loose parts or wires, and similar defects; such defects, to the extent that they are not simply random individual defects, may trigger the SFI. Computing resources may be very helpful in resolving any particular SFI.
- Baselining is one of the most computer-intensive activities in the whole stewardship program. It is an attempt to find a unifed and integrated view of a great many underground nuclear tests (UGTs) through simulations, with the goal of reducing (ultimately to zero) special phenomenologies which may differ among UGTs.
- ASCI supports safety requirements through simulations of drop tests and other abnormal environments. While current safety calculations go far beyond what was possible a decade ago, they are not the most challenging simulations called for in the ASC Program. In many cases, experiments are quite feasible and preferable to calculations.
- The Stockpile-to-Target Sequence (STS) requirements specify what environments a given weapon type will encounter throughout its life, culminating in actual use. These include temperature excursions and hostile environments, such as might be encountered through enemy nuclear defense missiles or fratricide. Some STS requirements are extremely stressing on quantifying the margins and uncertainties of the weapon, and these requirements lead to complex ASC simulations.
- ASC is used for simulations related to assembly and disassembly at the plants, and also for design of better tools and processes. The computing loads are not usually unduly demanding.
- Surety includes a variety of tasks, some of which involve use control of U.S. weapons. ASC computing resources are an important part of surety activities.
- Advanced concepts need to be studied for a number of reasons, including keeping designers intellectually sharp and up to date, to understand possible foreign advances in nuclear weapons design, as well as to respond to perceived new needs for the stockpile. It is certainly possible that the demands on ASCI resources would be high, but this does not appear to be the case now.
- Weapons science includes materials properties such as strength at pressures not attainable in the laboratory; behavior of high explosives; physics of radiation transport; and a host of others. In many cases, understanding of basic phenomena, whether through theory or experiment, is limited. ASC demands are often high, because of the complexity of the phenomena being modeled.