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Improved Differential Scanning Calorimeters Enable Scientists to Make More Accurate Studies of Plutonium and Its Alloys

Researchers are using a technique called power-compensated differential scanning calorimetry to study phase transformations in plutonium. In this close-up, a capsule containing a plutonium specimen (the small white container held in the tweezers) is being loaded into one of two furnaces in a calorimeter. The furnaces are the recessed areas in the center of the photo. The plutonium specimen will be placed in the furnace on the left; the furnace on the right remains empty and is used as a reference.

The Stockpile Stewardship Program has 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.

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. The Lab's commitment to stockpile stewardship has mobilized a strong effort in the Nuclear Materials Technology (NMT) and Materials Science and Technology (MST) divisions to push the understanding of plutonium and its alloys to a new level.

One area of materials research receiving renewed attention is the thermodynamics of phase transformations and self-irradiation in plutonium and its alloys.

This area of study is particularly interesting to researchers because plutonium undergoes more phase transitions than any other element. It transforms through six different crystal structures (alpha, beta, gamma, delta, delta prime, and epsilon) at atmospheric pressure as it is heated from room temperature to its melting point.

Researchers are using a powerful technique, differential scanning calorimetry, or DSC, to further the understanding of the plutonium's phase transitions and study the transformation phenomena in detail.

The type of DSC primarily used here, "power-compensated" DSC, was developed in the mid-1960s to improve measurements of key thermodynamic properties of materials.

Researcher Dan Schwartz of Nuclear Materials Science (NMT-16) loads a capsule containing a plutonium specimen into the furnace of a differential scanning calorimeter. Once Schwartz loads the specimen, he will close the cover over the furnaces and begin the measurement. Everything is done in a ventilation hood to protect the operator from the plutonium specimens during the loading and unloading process. The equip-ment is located in the plutonium characterization laboratories in the Chemistry and Metallurgy Research (CMR) Building.

In recent years, DSC has undergone significant improvements in sensitivity and control, allowing researchers to revisit old areas of plutonium research with new levels of accuracy.

The improved instruments also are allowing scientists to explore new areas of research that previously were beyond the capability of older DSC instruments.

The state-of-the-art differential scanning calorimeter used today at the plutonium characterization laboratories in the Chemistry and Metallurgy Research (CMR) Building is composed of two independent furnaces. One furnace contains a specimen for analysis while the other is empty and used as a reference.

The furnaces are heated by separate elements that follow a programmed temperature vs. time profile.

A compensating circuit maintains the temperature difference between the specimen and reference furnaces as close to zero as possible, while the instrument measures the amount of power input that is required to keep the temperature difference to a minimum.

This power input is directly related to the heat required to change the temperature of the specimen-a quantity called the heat capacity of the specimen.

A simple integration of the power input over time yields another important thermodynamic quantity-enthalpy. When a material undergoes a phase transition (for example, when it melts or changes crystal structure), it either releases or absorbs heat.

In power-compensated DSC, this causes the instrument to increase power input to compensate, until the phase transition is complete. The DSC instrument in the CMR Building is capable of measuring with high purity standards the onset temperature for phase transitions to an accuracy of plus or minus 0.05 degrees Celsius, and heat capacity and enthalpy to plus or minus 2 percent. The instrument also is capable of making these measurements at temperatures ranging from minus 150 C to 730 C.

Recently, the availability of very high purity zone-refined plutonium, manufactured by Jason Lashley of Structure and Property Relations (MST-8), 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 DSC 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. 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.

DSC scans of these inhomogeneous materials show phase transition peaks expected for pure plutonium, and measurement of the heat released during these transitions can be used to calculate the volume fraction of material that has a low solute content.

This volume fraction is a quantitative measure of how inhomogeneous the plutonium alloy is, which is important to know when manufacturing with plutonium alloys.

DSC also may prove useful for observing homogenization in action. When an inhomogeneous plutonium alloy containing aluminum, gallium, or indium is heated, solute begins to diffuse and redistribute itself evenly throughout the material.

These graphs show heat flow as a function of temperature for pure plutonium. The top graph shows the heating curve; the bottom graph shows the cooling curve.

This solute redistribution process, called homogenization, releases heat that the DSC instrument can measure. By observing changes in the heat released during homogenization, researchers hope that the rate of homogenization can be quantified.

For a variety of reasons, however, this kind of measurement pushes the limits of sensitivity and stability for even the most advanced DSC instruments. Recent results suggest that accelerator is made, the best fuel type cannot be decided upon. Therefore, the research and development effort will consider the four fuel types in parallel, including nitride, metal alloy, oxide, and dispersion (ceramic particles in a metal matrix). NMT Division will focus its efforts on development of ceramic-based and dispersion fuels.

The Advanced Accelerator Application (AAA) Program-consisting of Accelerator Production of Tritium (APT) and Accelerator Transmutation of Waste (ATW)-is truly a national and international effort.

While the program director, Edward Arthur, is headquartered at Los Alamos, many of the project leaders are located at other institutions.

Los Alamos is working in close collaboration with Argonne National Laboratory (East and West) to determine a viable fuel form.

Experimental fuel pellets fabricated at Los Alamos and metal segments fabricated at Argonne are scheduled to be inserted into the Advanced Test Reactor (a thermal-spectrum research reactor at Argonne-West) late next summer.

Internationally, the U.S. program is interacting with the European and Japanese transmutation programs.

The near-term goal of the fuel development effort is the insertion of a final (or near-final) fuel type into the Phenix Facility in France sometime after 2004. Exposure to Phenix's fast neutron spectrum will help determine a given fuel type's ultimate performance.

Many challenges to fully implementing ATW remain. They range from political (addressing reprocessing of spent nuclear fuel) to social (public acceptance of nuclear technology) to technical (fabricating fuel from very radioactive minor actinides).

But the consequence of not pursuing ATW is significant because spent fuel buried in geological repositories will require safe containment for tens of thousands of years.

However, if ATW is implemented, the nuclear fuel cycle will be nearly complete, and nuclear power can retain its position-and potentially increase its appeal-as a means for power production for many years to come.

For more on spent nuclear fuel reprocessing, see "The Actinide Research Quarterly," 2nd and 3rd Quarter, 2000.

This article was contributed by Dan Schwartz (NMT-16) and Tom Zocco (NMT-5).


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