Plutonium Materials Science Supports Science-Based Stockpile Stewardship and Management

Cessation of nuclear testing and the end of the Cold War caused the DOE weapons design laboratories to rethink their policies and practices for maintaining expertise in nuclear weapons science and for ensuring the integrity and safety of the increasingly smaller nuclear arsenal. In particular, weapons were originally intended for a stockpile life of approximately 25 years, but current knowledge and data indicate that the weapon life cycle may be extended beyond 25 years before the degradation of materials affects their performance.

Extending the stockpile life of existing nuclear weapons requires knowledge of the effects of aging on plutonium. In the past it was possible to remove a unit from the stockpile and test it in an underground explosion to measure any aging effects directly. Thus, much less knowledge of the specific materials and their influence on weapon performance was required. Without the advantage of underground weapons testing, we must now study the material itself.

Within the weapon pit component, clearly, it is the plutonium that causes aging-induced changes. The other components consist of relatively inert and otherwise well-protected materials. The materials-science-related phenomena that are likely to create changes in the plutonium can be categorized into three major areas: decay, equation-of-state, and high chemical reactivity.

Plutonium is radioactive (the 239 isotope has a half-life of approximately 24,000 years) and decays by emitting alpha radiation. Internally trapped alpha particles evolve into helium atoms, which can accumulate to about a thousand atomic parts per million over a 20- to 30-year time frame. Helium is known to cause embrittlement and swelling of metals such as stainless steel. The formation of such helium bubbles could affect the structural stability of the plutonium.

Another event that accompanies the ingrowth of helium during alpha decay is the injection of a uranium atom. Besides the possible chemical effects of the uranium atom on phase stability and physical properties, the recoil part of the nuclear decay process occurs at a high energy level, during which, it is conjectured, the uranium atom displaces many plutonium atoms before coming to rest. The defects thus produced may result in altering the chemical and physical stability of the plutonium.

Another measurable radiolytic process involves the decay of residual ²⁴¹Pu, which results in the ingrowth of americium in the metal matrix. Americium may affect plutonium phase stability, but this phenomenon has not been studied conclusively.

Plutonium research efforts at Los Alamos have accomplished many noteworthy goals, such as determining many known plutonium binary phase diagrams, thorough study of the physical and mechanical properties of the stable metallurgical phases, and development of the "tailwind" and "trunk" alloys, now standards in the stockpile. In the middle to late 1980s, new techniques such as transmission electron microscopy and neutron scattering were applied to plutonium, resulting in detailed studies of the crystallography and structure of the dÆa martensitic phase transformation, the first observation of helium bubbles in aged plutonium (Figure 2), and the determination of Debye-Waller factors for both a- and d-phase alloys. In the last few years small efforts have resulted in new technique breakthroughs, such as the advent of ultraviolet photoelectron spectroscopy, the laser-induced miniflyer technique (shock compression), and the extended x-ray absorption fine structure technique, which hold even further hope for understanding and predicting the behavior of plutonium under storage conditions. Such studies are important for thorough stockpile stewardship.

Figure 2: Helium bubbles are shown in 20-year-old plutonium. Helium generation affects plutonium properties in weapon components.

Several well-defined requirements within the new Stockpile Stewardship and Management Program necessitate renewed plutonium materials science activity. These requirements include recertification of existing weapons, a replacement-level pit manufacturing capability, and enhanced surveillance. Plutonium materials science research at Los Alamos now requires renewed commitment and funding to support enhanced stockpile surveillance, recertification of weapons, and replacement manufacturing.

Figure 3. Computer simulation of plutonium casting. Such simulations are used before plutonium parts are cast to study the behavior of the material in the casting process and to avoid defects.

Specific plutonium research topics at Los Alamos that are underway or proposed are divided into four major areas: