3rd quarter 2001 TheLos Actinide Alamos National Research Laboratory N u c l e a r M Quarterlya t e r i a l s R e s e a r c h a n d T e c h n o l o g y a U.S. Department of Energy Laboratory Improved Differential Scanning Calorimeters Enable Scientists to Make More-Accurate Studies In This Issue Researchers are using a of Plutonium and Its Alloys technique called power- compensated differential 4 scanning calorimetry to Investigating a Viable The Stockpile Stewardship Program has created a renaissance study phase transforma- Fuel Form for ATW in plutonium materials science at Los Alamos with its mandate to tions in plutonium. In this manufacture new pits as well as to understand in detail the effects close-up, a capsule of aging on older stockpile weapons. containing a plutonium Scientists conducted research on metallic plutonium and specimen (the small white container held in plutonium alloys throughout the 1950s, ’60s, and early ’70s, but the tweezers) is being materials research in this area slowed significantly in the following loaded into one of two years. The Lab’s commitment to stockpile stewardship has mobi- furnaces in a calorim- lized a strong effort in the Nuclear Materials Technology (NMT) eter. The furnaces are and Materials Science and Technology (MST) divisions to push the the recessed areas in the center of the photo. understanding of plutonium and its alloys to a new level. The plutonium specimen will be placed in the furnace on the left; the furnace on the right remains empty and is Focus on Students: used as a reference. 6 Students are Vital to the Lab 8 Nuclear Engineer Steven Alferink 10 Social Scientist Andrew Koehler 12 Chemist Susan Oldham 14 Newsmakers continued on page 2 Nuclear Materials Technology/Los Alamos National Laboratory 1 Actinide Research Quarterly Differential Scanning continued from page 1 research that previously were beyond the One area of materials research receiving capability of older DSC instruments. renewed attention is the thermodynamics of The state-of-the-art differential scanning phase transformations and self-irradiation in calorimeter used today at the plutonium char- plutonium and its alloys. acterization laboratories in the Chemistry and This article was This area of study is particularly interesting Metallurgy Research (CMR) Building is com- contributed by to researchers because plutonium undergoes posed of two independent furnaces. One fur- Dan Schwartz more phase transitions than any other element. nace contains a specimen for analysis while (NMT-16) and It transforms through six different crystal the other is empty and used as a reference. Tom Zocco (NMT-5). structures (alpha, beta, gamma, delta, delta The furnaces are heated by separate prime, and epsilon) at atmospheric pressure as elements that follow a programmed tempera- it is heated from room temperature to its melt- ture vs. time profile. ing point. A compensating circuit maintains the Researchers are using a powerful technique, temperature difference between the specimen differential scanning calorimetry, or DSC, to and reference furnaces as close to zero as pos- further the understanding of the plutonium’s sible, while the instrument measures the phase transitions and study the transformation amount of power input that is required to keep phenomena in detail. the temperature difference to a minimum. The type of DSC primarily used here, This power input is directly related to the “power-compensated” DSC, was developed heat required to change the temperature of the in the mid-1960s to improve measurements of specimen—a quantity called the heat capacity key thermodynamic properties of materials. of the specimen. In recent years, DSC has undergone A simple integration of the power input significant improvements in sensitivity and con- over time yields another important thermody- Researcher Dan trol, allowing researchers to revisit old areas of namic quantity—enthalpy. When a material Schwartz of Nuclear plutonium research with new levels of accuracy. undergoes a phase transition (for example, Materials Science The improved instruments also are when it melts or changes crystal structure), it (NMT-16) loads a allowing scientists to explore new areas of either releases or absorbs heat. capsule containing a In power-compen- plutonium specimen into the furnace of a sated DSC, this causes differential scanning the instrument to in- calorimeter. Once crease power input to Schwartz loads the compensate, until the specimen, he will phase transition is close the cover over complete. The DSC the furnaces and begin instrument in the the measurement. CMR Building is ca- Everything is done in pable of measuring a ventilation hood to with high purity stan- protect the operator dards the onset tem- from the plutonium specimens during the perature for phase loading and unloading transitions to an accu- process. The equip- racy of plus or minus ment is located in the 0.05 degrees Celsius, plutonium characteriza- and heat capacity and tion laboratories in the enthalpy to plus or Chemistry and minus 2 percent. Metallurgy Research The instrument (CMR) Building. also is capable of 2 Nuclear Materials Technology/Los Alamos National Laboratory 3rd quarter 2001 12.0 These graphs show making these measurements at temperatures δ' − ε ranging from minus 150 C to 730 C. 10.0 heat flow as a function Recently, the availability of very high α − β of temperature for purity zone-refined plutonium, manufactured 8.0 pure plutonium. The top graph shows the by Jason Lashley of Structure and Property 6.0 β − γ γ − δ heating curve; the Relations (MST-8), prompted researchers to 4.0 bottom graph shows δ − δ remeasure the onset temperatures and enthal- ' the cooling curve. pies for all the phase transitions in plutonium. 2.0 The measurements resulted in slightly 0.0 different values for temperature and enthalpy, and are considered by many researchers to be -2.0 the most accurate values measured to date. 2.0 A DSC scan of a plutonium sample showed δ' − δ some unusual features of the phase transi- 0.0 tions. When heated from room temperature to δ − γ γ − β about 500 C, the phase transitions appeared -2.0 1.00 normal and well defined. However, when the -4.0 sample was cooled to room temperature, 0.00 things changed drastically. -6.0 In the scan, the delta-to-gamma cooling β − α 200 250 300 transition was no longer seen as a single dis- -8.0 ε − δ' tinct peak, but instead was seen as a set of -10.0 small, irregular peaks that occurred over a 100 200 300 400 500 broad temperature range. In addition, the gamma-beta transition was Temperature (Celcius) seen to be much broader than usual after cool- ing and suppressed by approximately 100 C calculate the volume fraction of material that below its heating onset. Los Alamos research- has a low solute content. ers are making detailed studies of this anoma- This volume fraction is a quantitative lous behavior. measure of how inhomogeneous the pluto- Delta-phase plutonium is desirable for use nium alloy is, which is important to know in many weapons systems because it is tough when manufacturing with plutonium alloys. and malleable. However, the delta phase isn’t DSC also may prove useful for observing stable at room temperature unless the pluto- homogenization in action. When an inhomoge- nium is alloyed with elements such as alumi- neous plutonium alloy containing aluminum, num, gallium, or indium. gallium, or indium is heated, solute begins to Because of differences in diffusion rates for diffuse and redistribute itself evenly through- these alloying elements in the high-temperature out the material. phases of plutonium, they can be unevenly dis- This solute redistribution process, called tributed in the plutonium. This inhomogeneous homogenization, releases heat that the DSC in- distribution of alloying elements is generally strument can measure. By observing changes undesirable, because regions of the material in the heat released during homogenization, that are low in alloy content will behave more researchers hope that the rate of homogeniza- like pure plutonium, while the regions high in tion can be quantified. solute content will be delta-stabilized. For a variety of reasons, however, this kind DSC scans of these inhomogeneous materials of measurement pushes the limits of sensitivity show phase transition peaks expected for pure and stability for even the most advanced DSC plutonium, and measurement of the heat re- instruments. Recent results suggest that leased during these transitions can be used to continued on page 15 Nuclear Materials Technology/Los Alamos National Laboratory 3 Actinide Research Quarterly Fuel Form for ATW Researchers Investigate a Viable Fuel Form This article was for Accelerator Transmutation of Waste contributed by NMT Division Assists R&D Effort in Advanced Nuclear Fuels Robert W. Margevicius (NMT-15). People in the United States currently get Those levels of discharge would require the about 20 percent of their power from the ap- construction and commission of a repository on proximately 100 operating commercial nuclear the scale of Yucca Mountain every three or four reactors in this country. years somewhere around the globe. There are more than 400 commercial Nearly all health-related risks arising from nuclear reactors supplying energy long-term disposal of spent nuclear fuel are Spent around the world. Nuclear power is eco- attributable to about 1 percent of the fuel’s nuclear nomically competitive and has several content. This 1 percent primarily consists of fuel advantages over fossil-fuel-burning plutonium, neptunium, americium, and cu- plants, including no carbon monoxide rium (called transuranic elements), and long- and nitrous oxide emissions. lived isotopes of iodine and technetium However, among the disadvantages, created as products from the fission process the “waste product” from nuclear in power reactors.