WITHIN the Department of Energy, the word "surveillance" has a meaning closely akin to the word from which it derives--"vigilance." For years, the DOE has had an ongoing surveillance program to verify the safety and reliability of U.S. nuclear weapons. Surveillance has always dealt with the possible effects that aging may have on weapon materials and components. The study of aging effects is even more important now that nuclear testing has ceased, no new weapons are being developed, and the existing arsenal is growing older. Current plans call for many of the weapon systems in the arsenal to be in the stockpile well beyond their design lifetimes, and scientists must be able to predict the behavior of these systems as they age.|
DOE's enhanced surveillance program is just one facet of science-based stockpile stewardship.1 Since the program began in 1995, it has been managed by DOE's Office of Defense Programs. But the work is actually being done by the seven DOE facilities that designed and fabricated the weapons in the first place--Livermore, Los Alamos, and Sandia national laboratories as well as the Y-12, Kansas City, Pantex, and Savannah River plants.
The objective of the enhanced surveillance program is to develop diagnostic tools and predictive models that will make it possible to analyze and predict the effects that aging may have on weapon materials, components, and systems. With this information, program participants will be able to determine if and when these possible effects will impact weapon reliability, safety, or performance and thus will be able to anticipate needs for weapon refurbishment. Because the DOE weapons complex has been reduced in numbers of plants and personnel, the lead time necessary to manufacture critical components must be as long as is practical. Enhanced surveillance is crucial to providing the longest lead time the DOE complex can afford to provide.
Specifically, the program's goals are to predict component and material failure mechanisms; predict the service lives of materials, components, and overall systems; determine the feasibility of monitoring critical components in place, in real time, nondestructively; and develop diagnostics for failure mechanisms when time to failure cannot be adequately predicted.
Surveillance of Thermonuclear Weapons
The seven participating facilities are working on 110 tasks in three focus areas: primaries, secondaries, and nonnuclear components. Livermore has only minor involvement with project work related to nonnuclear components, which is Sandia's specialty. However, the Laboratory is heavily involved in the first two areas because its specialty has always been the development of primaries and secondaries, where the fission and fusion processes occur in a thermonuclear weapon. For the work at Livermore, Jeffrey Kass and John Kolb are leading a multidisciplinary team that includes physicists, engineers, materials specialists, and technicians from several directorates.
For weapon primaries, the Livermore team is evaluating changes that occur over time to the pit's special nuclear materials and to various types of high explosives. For example, plutonium irradiates itself and, given enough time, may change shape ever so slightly. Other tasks involve developing sensors, imaging devices, and diagnostic techniques for nondestructive evaluation of a primary. The team is also developing methods for studying the dynamic properties of primaries through small-scale testing.
Similar work is under way for weapon secondaries, characterizing materials in detail and developing material aging models to predict material life. Livermore staff are also developing diagnostic technologies to verify material and system predictability.
The Livermore project contributes to the work of the Surveillance Information Group, which includes representatives from all the DOE laboratories and plants. The Surveillance Information Group has conducted pilot projects in support of the DOE-wide Nuclear Weapons Information Group,2 whose mission is to develop a secure, Web-based, electronic archive of old and new classified documents and other information on weapons design, production, and testing.
Livermore is leading a task to develop a technique called microextraction for nondestructive evaluation of the weapon primary. Microextraction is one of several technologies under development that will be used to determine how aging and the environment may affect the stability of a weapon's components.
Initial work with microextraction analyzed the primary's headspace gases. Studies show that primaries outgas at significant levels. To study these outgasses, Laboratory scientists exposed a microfiber coated with a solid-phase adsorbent to the weapon headspace gas to collect any chemical species. They then analyzed the microextraction fiber using gas chromatography and mass spectrometry. They have also developed methods to move the fiber as close to the weapon's purge valve as possible to permit essentially direct sampling of the weapon headspace and obtain more accurate data (See Figure 1 above).
The Livermore team then characterized the material standards associated with various weapon systems. It found that many of the compounds absorbed in some high explosives may be traced to the use of other materials. For example, significant levels of toluene arise from its use as a solvent in the synthesis of the high explosive TATB. Data analysis thus far demonstrates that the outgassing and absorption processes observed on the core samples would not have significant effects on other materials in the near term because the outgassed species are nonreactive. The next step, which is still under way, is to complete an initial survey of systems and associated materials developed at Livermore.
Livermore is also leading an effort to implement microextraction to assess the aging of organics in closed environments. Valuable baseline information on new and aged weapons components has been obtained at DOE's Savannah River and Kansas City plants, with Livermore providing guidance on the effort.
Another task that Livermore is leading addresses modeling of material aging in the nuclear explosive package (NEP) of thermonuclear weapons. The NEP is a closed environment that contains exceptionally pristine and dry materials. It is enclosed in a can that prevents the interaction of the materials in the NEP with the outer atmosphere.
Livermore's goal is to develop a comprehensive computer model of the chemistry of this closed environment. Models are being developed of the interaction between the materials and between the materials and the gases left in the NEP during assembly. The time it will take for significant interaction to occur is important for the question of when these components will need to be refurbished or remanufactured.
The team is developing models for the reaction of gases with materials and for the diffusion of gases through the NEP. The reaction of gases with metals is a complicated process. Frequently, a layer of oxide on the metal causes the reaction to occur nonuniformly. As shown in Figure 2, a two-dimensional model demonstrates the pitting that may occur during this reaction.
These reaction models must be incorporated into a larger model of the transport and reaction of gases in the system. The Livermore team has begun to do just that using TOPAZ, one of the computer codes developed at the Laboratory for calculating the mechanical properties of materials. The team has demonstrated that TOPAZ, which was designed to model thermal diffusion, can be adapted to calculate gas transport through the NEP system when the grid for TOPAZ is carefully developed. Detailed models of the transport paths in the NEP have already been produced.
Continuing work for this task includes creating advanced gas-solid reaction models and, more important, modifying the computer code to include these models.