Plutonium Crystal Phase Transitions

Plutonium is a complex and perplexing element. Plutonium is a unique element in exhibiting six different crystallographic phases at ambient pressure (it has a seventh phase under pressure). The densities of these vary from 16.00 to 19.86 g/cm3. Plutonium has six temperature-dependent solid phases -- more than any other element in the periodic table. Each phase possesses a different density and volume and has its own characteristics. Alloys are even more complex; you can have multiple phases present in a sample at any given time. Because plutonium is so complex, surrogate materials cannot give a complete picture of plutonium's characteristics.

Plutonium undergoes more phase transitions at ordinary pressures than any other element. As plutonium is heated it transforms through six different crystal structures before melting -- a [alpha], ß [beta], ? [gamma], d [delta], d' [delta prime], and e [epsilon]. Physical properties like density and thermal expansion vary significantly from phase to phase making it one of the more difficult metals to machine and work.

One of plutonium's unique physical properties is that the pure metal exhibits six solid-state phase transformations before reaching its liquid state, passing from alpha, beta, gamma, delta, delta-prime, to epsilon. Large volume expansions and contractions occur between the stable room-temperature alpha phase and the element's liquid state.

Another unusual feature is that unalloyed plutonium melts at a relatively low temperature, approximately 640°C, to yield a liquid of higher density than the solid from which it melts. In addition, the elastic properties of the delta face-centered cubic (fcc) phase of plutonium are highly directional (anisotropic). That is, the elasticity of the metal varies widely along different crystallographic directions by as much as a factor of six to seven.

Delta-phase plutonium is desirable for use in many weapons systems because it is tough and malleable. However, the delta phase isn't stable at room temperature unless the plutonium is alloyed with elements such as aluminum, gallium, or indium. Because of differences in diffusion rates for these alloying elements in the high-temperature phases of plutonium, they can be unevenly distributed in the plutonium. This inhomogeneous distribution of alloying elements is generally undesirable, because regions of the material that are low in alloy content will behave more like pure plutonium, while the regions high in solute content will be delta-stabilized.

The plutonium in the first American atomic bombs was stabilized in the low density delta phase (density 16.9) by alloying it with 3% gallium (by molar content, 0.8% by weight), but was otherwise of high purity. The advantages of using delta phase plutonium over using the high density alpha phase (density 19.2), which is stable in pure plutonium below 115 degrees C, are that the delta phase is malleable while the alpha phase is brittle, and that delta phase stabilization prevents the dramatic shrinkage during cooling that distorts cast or hot-worked pure plutonium. In addition stablization eliminates any possibility of phase transition expansion due to inadvertent overheating of the pit after manufacture, which would distort and ruin it for weapon's use.

It would seem that the lower density delta phase has offsetting disadvantages in a bomb, where high density translates into improved efficiency and reduced material requirements, but this turns out not to be so. Delta stabilized plutonium undergoes a phase transition to the alpha state at relatively low pressures (tens of kilobars, i.e. tens of thousands of atmospheres). The megabar pressures generated by the implosive shock wave cause the transition to occur, in addition to the normal effects of shock compression. Thus a greater density increase and larger reactivity insertion occurs with delta phase plutonium than would have been the case with the denser alpha phase.

Scientists conducted research on metallic plutonium and plutonium alloys throughout the 1950s, '60s, and early '70s, but materials research in this area slowed significantly in the following years. One area of materials research receiving renewed attention is the thermodynamics of phase transformations and self-irradiation in plutonium and its alloys.

One of the greatest concerns about plutonium and its alloys is phase stability because the large volume changes that accompany phase changes can compromise structural integrity. By 1999, more than 50 years after the Manhattan Project, other scientists from Russia and the United States still disagreed about the stability of the d -phase plutonium-gallium (Pu-Ga)alloys used in nuclear weapons. Typically,the face-centered-cubic (fcc) d -phase of plutonium, which is malleable and easily shaped,is retained down to ambient temperatures by the addition of gallium or aluminum. But do those d -phase alloys remain stable for decades or do they decompose into the denser, brittle a -phase and something else at ambient temperature? .In fact, the Russians estimated that even a pre-conditioned Pu-Al alloy at room temperature would take on the order of 11,000 years to decompose based on room-temperature data on self-diffusion in d -phase plutonium.

The Stockpile Stewardship Program created a renaissance in plutonium materials science at Los Alamos with its mandate to manufacture new pits as well as to understand in detail the effects of aging on older stockpile weapons. The availability of very high purity zone-refined plutonium prompted researchers to remeasure the onset temperatures and enthalpies for all the phase transitions in plutonium. The measurements resulted in slightly different values for temperature and enthalpy, and are considered by many researchers to be the most accurate values measured to date. A scan of a plutonium sample showed some unusual features of the phase transitions. When heated from room temperature to about 500 C, the phase transitions appeared normal and well defined. However, when the sample was cooled to room temperature, things changed drastically. In the scan, the delta-to-gamma cooling transition was no longer seen as a single distinct peak, but instead was seen as a set of small, irregular peaks that occurred over a broad temperature range. In addition, the gamma-beta transition was seen to be much broader than usual after cooling and suppressed by approximately 100 C below its heating onset. Los Alamos researchers are making detailed studies of this anomalous behavior.