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Aging Effects in Plutonium Pose an Interesting Problem

Aging effects in plutonium metal are of metallurgical and practical interest. The Dismantlement Program has provided a wide variety of plutonium samples from a large number of retired nuclear weapons, enabling the development of a comprehensive database of materials properties as a function of age. Data are providing a substantive statistical foundation for these properties as well.

The primary cause of aging in plutonium is radioactive decay, which alters composition and deposits energy into the lattice. Three major consequences of this decay are annealing, changes in phase, and in-growth of helium. Daughter species produced during radioactive decay alter the composition of the metal directly with age, in a known way. Composition may in turn affect phase, or lead to metastability. Impurities can affect mechanical properties by controlling the mobility and distribution of defects. Ingrowth of daughter species during radioactive decay may restabilize a metastable condition, as some of the daughters are known to serve the same function as gallium in stabilizing the delta phase.

Energy deposited in the metal lattice produces disordered regions, including dislocations and vacancies. The number and movement of dislocations govern the plastic properties of a metal. Impurities or lattice defects produced by radioactive decay can block the passage of a dislocation, making the crystal harder to deform. Vacancies in the crystal can migrate to form small loop-shaped dislocations that can take part in plastic deformation. Thus, radioactive decay can alter the mechanical properties of a metal through a complicated set of competing processes.

Age-induced changes are measured by a variety of techniques, as part of the Enhanced Surveillance Program and surveillance studies on plutonium metal of various ages. LIBS (laser-induced breakdown spectroscopy) gives a measure of annealing by indicating how far impurities have moved within the metal. Precision density and indentation testing (hardness measurement) may be sensitive enough to indicate the degree of annealing. Metallography reveals any large-scale structural perturbations resulting from annealing. Changes in phase are best revealed by x-ray diffraction measurements and by examination of the density of the material. Indentation testing may also indicate phase changes. Helium bubbles within the metal reduce thedensity, and any bubbles present are usually observable in metallographic images. Helium dispersed in the lattice can be measured only by provoking helium release using differential scanning calorimetry (DSC) or helium evolution analysis.

Figure 1. The primary cause of aging in plutonium is radioactive decay. This figure shows the beginning of a damage cascade, the region of the lattice disrupted by the decay of a radioactive atom within the lattice.

A damage cascade is the region of the lattice disrupted by the decay of a radioactive atom within the lattice. (See Figure 1.) In a damage cascade the daughter products from the decaying atom knock neighboring atoms out of position. These atoms in turn move other atoms within the lattice. The entire "collision phase" process produces a large region in which the lattice has been disrupted, producing dislocations, interstitial atoms, vacancies, and other lattice defects.

The kinetic energy in the lattice is dissipated as heat and rapidly conducted away, producing the "thermal spike phase," in which the lattice rearranges, and many of the disrupted atoms settle back into nearby equilibrium lattice positions, leaving a few disruptions. (See Figure 2.) The volume affected depends on the energy of the decay and can be both calculated and measured. In plutonium the affected volume is as large as several million atoms. The energy of the decay may be sufficient to induce local melting in the region of the lattice affected by a damage cascade. Low-energy cascades may produce structural damage and minimal melting. In plutonium, the cascade energy is high, and local melting reduces the concentration of primary defects caused by damage cascades.

Figure 2. Perfect delta plutonium (fcc) lattice before radioactive decay, and disruption of the lattice during the "collision" phase of the decay cascade. The lattice will relax back to near perfection during the"thermal spike" phase of the cascade.

Annealing occurs when radioactive decay deposits energy in the lattice. Local recrystallization drives the microstructure toward equilibrium, usually toward a more-ordered condition, raising the purity of the ordered region and localizing impurities, usually moving them to grain boundaries. Thus, annealing can cause local compositions to vary from the bulk.

Phase changes may arise from these local changes in alloy composition, if there is a nucleation site to start the process. Phase changes may be nucleated by damage cascades arising from radioactive decay or they may be nucleated by lattice strain. Plutonium may be stabilized in the delta phase by addition of approximately 1 wt % of gallium. Radiation-induced changes in the distribution of the gallium can result in regions of the lattice that are no longer stabilized.

Without nucleation to provide a kinetic driver promoting transformation, metastable delta plutonium can persist for long times at room temperature or above, since the atoms lack sufficient thermal energy to move en masse into the new equilibrium lattice positions in the monoclinic alpha-phase lattice. The transition itself is martensitic.

Damage cascades may promote transformation by simply rearranging the lattice, by increasing the presence of interstitial decay products in the metal matrix, or by producing dislocation loops. (See Figure 3.) Interstitial atoms and decay products incorporated into the metal matrix produce disordered or strained regions in the lattice, which can more easily reorder themselves into the lowest energy state, the new phase. Time scales can be rapid, such as recrystallization during the damage cascade, or slow, such as the progress of thermal annealing at the edge of an existing disordered region or at a frontier between two phases. It has also been demonstrated that during decay, the trapped alpha particle produces an effective local pressure in the lattice as high as 2 GPa, which is sufficient to drive a pressure-induced transformation to the alpha phase.

Figure 3. Energy deposited in the metal lattice produces disordered regions including dislocations and vacancies. Such damage can alter the mechanical properties of a metal through a complicated set of competing processes. This figure shows dislocation in the lattice as a result of a damage cascade.

Helium is produced in the lattice by alpha decay of the 239Pu, reaching a predictable concentration as a function of age. Very little helium is released by the metal, even though the solubility of the helium in the metal is expected to be low. Helium that is not dissolved is evidently trapped in the lattice, in configurations that have varying degrees of stability. Configurations of low stability may permit mobility of trapped helium and a potential for accumulation into small bubbles leading to void swelling, a phenomenon seen in other metals in radiation environments. Void swelling may reduce the density and produce softening of the metal.

The migration of the helium is affected by its location. Helium atoms from alpha decay may come to rest in lattice sites, as interstitials, in vacancies, in dislocation cores and voids, in other large distortions of the lattice, or at grain boundaries. Different locations of helium atoms will produce different lattice energies, different lattice strains, and different barrier energies to mobility of the helium. Calculating the mobility of helium assuming simple energy barriers between helium occupation sites gives a rate for helium agglomeration that is linear or decreasing in time. Void formation generally begins only after an induction time, indicating that location and barrier height are not the only factors governing the process.

Either migration of vacancies to form the voids or dislocation mobility in the metal may be the rate-determining phenomenon in this process, as seen in other metals. The induction time for void formation in plutonium near room temperature is evidently as long as 50 years. Thus, data from particularly old metal is valuable; such metal may be further into the induction time than the majority of such samples examined to date.

A proper model of the process evidently should include the interaction of the helium with dislocations, voids, impurities, and other structural perturbations as well as with the uniform structure of the bulk metal. Disruption or melting of the lattice during the decay cascade introduces strong perturbations into any description of the migration of the helium.

Since significant changes in density are thought to occur only during void formation and not during the accumulation of dispersed trapped helium in the metal, it is important to be able to correlate subtle changes in lattice structure with precise measurements of helium in the lattice, made by observing evolution as a function of temperature.

Experimental data include measurements of helium evolution as a function of temperature, and observation of expansion of the solid metal with increasing temperature. The temperature dependence of mobility may itself provide structural information. Past studies of this type have observed nonuniform evolution of helium with increasing temperature, possibly helium released from the various sites listed above.

Aging effects in plutonium pose a complicated problem involving many interrelated variables, and dependencies on quantities such as dislocation mobility that are relatively difficult to measure or to estimate. Careful statistical analysis of a large database of measurements on varying samples may enable us to make meaningful extrapolations both forward and backward in time, permit analysis of the relationships between properties, and indicate the relative importance of several physical effects that are being explored. A clear understanding of the aging mechanisms in plutonium metal will enable us to more confidently predict the safety and reliability of our nuclear deterrent well into the twenty-first century.

This article was contributed by Roberta N. Mulford and Wendel Brown (NMT-15)


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