Detecting and Predicting Plutonium Aging Are Crucial to Stockpile Stewardship

During the past decade we have seen unprecedented changes in the world's political climate. The end of the Cold War, the breakup of the former Soviet Union, strategic arms reduction treaties-all have contributed to a decrease in nuclear arms buildup. These changes notwithstanding, nuclear weapons technology continues to play a key role in reducing the global nuclear danger.

However, the configuration of the weapons complex is far from static. The size and number of nuclear weapons within the U.S. arsenal have been dramatically reduced, nuclear testing has been curtailed, the weapons in the stockpile are aging, and downsized fabrication facilities are being tightly integrated and focused as much on maintaining capability as on delivering small numbers of new components.

Within this new environment, the Department of Energy (DOE) has implemented the Science-Based Stockpile Stewardship Program, which relies on the use of methods other than nuclear testing to ensure the safety, security, and reliability of the stockpile. These methods include advanced diagnostic equipment, data from critical new experiments, enhanced computational power, and retaining the very best scientists and engineers at the nation's nuclear research facilities. Several new missions are growing in importance, including the analysis and surveillance of weapon systems and the associated measurement of the effects of aging on the weapons in the stockpile.

A qualitative representation of the connection between changes in atomic structure, microstructure, material properties, and component performance. The onset of potential aging effects in plutonium is included along the time axis. The goal is to identify the signatures of aging at the earliest possible time. This requirement has driven the program to atomic and nanoscale scientific investigations.

Stockpile Surveillance

The Stockpile Surveillance Program ensures that the stockpile is free of defects that may affect performance, safety, or reliability. The surveillance program has two elements: the Stockpile Evaluation Program and the Enhanced Surveillance Campaign.

The first element, stockpile evaluation, provides examinations and assessments of war-reserve stockpile weapons and components. The second element, enhanced surveillance, provides the means to strengthen the Stockpile Evaluation Program to meet the challenges of maintaining an aging stockpile in an era of no nuclear testing. Enhanced surveillance also provides lifetime assessments and predictions for Stockpile Life Extension Program (SLEP) planning.

The goal of the Enhanced Surveillance Campaign is to protect the health of the stockpile by screening weapons systems for manufacturing and aging defects to identify units that need to be refurbished. It also will be used to predict material and component aging rates as a basis for annual certification, refurbishment scope and timing, and nuclear weapon complex planning. Results of the work will be used to make improvements to the basic surveillance program.

Because nuclear weapons will be retained in the stockpile for lifetimes beyond our experience, the DOE needs to be able to determine when stockpile systems must be refurbished or reconditioned. If new refurbishment capability is needed, the DOE needs to know when these capabilities must be operational and what the required capacity should be, if the capacity for existing facilities is adequate, and when potential refurbishment for the various stockpile systems must be scheduled.

The DOE also needs to have a basis on which to characterize the functional reliability of aged components, which is part of the annual assessment process.

Plutonium Aging

Detecting and predicting age-induced changes in nuclear materials are perhaps the most challenging and technically engaging aspects of Science-Based Stockpile Stewardship. Indeed, "science-based" arises principally from this need, as opposed to a "production-based" alternative or the "test-based" strategy used in years past. Analysis, prediction, and mitigation of aging effects are key to ensuring long-term functionality.

However, changes in weapon performance as a result of aging represent the end in a series of events that began years or decades earlier. Changes occur first in the fundamental (or atomic-scale) properties of the materials within the weapon‹properties such as composition, crystal structure, and chemical potential. Changes are later found in the applied behavior of these materials‹behaviors such as density, compressibility, strength, and chemical reaction rates.

Only when the applied properties have sufficiently changed can we anticipate their impact on weapon performance. Therefore, the needs of the stewardship program have driven our studies toward nanoscale scientific investigations. The essence of this approach can be seen in the illustration on page 9, where changes at the atomic scale precede changes at the microscopic or macroscopic scale, which lead to changes in material properties, and ultimately, in device performance.

Analyzing, predicting, and mitigating aging effects in pits, specifically plutonium pits, are key to ensuring long-term safety and reliability in the primary stage of nuclear weapons. We are studying the many changes that result from aging, including engineering and physics performance characteristics such as equation of state, spall and ejecta formation, strength, density, geometry, corrosion resistance, and nuclear reactivity.

Our understanding of plutonium aging is complicated by the fact that plutonium displays some of the most complex physical and chemical properties of any element in the periodic table. Aging mechanisms that can cause changes in fundamental plutonium material and mechanical properties include the in-growth of decay products, uranium recoil damage and associated void formation, void swelling, changes in density, phase stability concerns, changes in surface chemistry, and a variety of environmental changes, including thermal cycling.

Developing advanced characterization tools to measure changes in these properties will expand our nuclear materials knowledge base and form the basis for computational models necessary for predictive assessment.

We have adopted a dual strategy of using data obtained from the oldest available pits and validated accelerated aging experiments using plutonium-238-spiked alloys to characterize the physics, engineering, and materials properties of plutonium.

Accelerated aging alloys can be qualified by comparing them with normal alloys and the oldest pits. Changes in key properties can be predicted by modeling anticipated aging effects, especially radiation damage within the plutonium lattice. Measurement of these key pit properties will be used to determine age-related changes and to validate models.

The Enhanced Surveillance Campaign and the needs of the Science-Based Stockpile Stewardship Program have supported the development of new science and technology, including resonant ultrasound measurements of the plutonium modulus, X-ray absorption spectroscopy and neutron-scattering measurements of plutonium structure, microstructure and surface characterization, positron annihilation spectroscopy, isochronal annealing studies of radiation damage, dynamic property measurements, and new theoretical models of radiation damage effects.

These tools will be useful in making stockpile life-extension decisions, determining when or if a Modern Pit Manufacturing Facility will be built, and for the weapon systems' annual certification to the president.

This article was contributed by David Clark and Joseph Martz, G. T. Seaborg Institute, NMT Division.

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