Los Alamos National Laboratory Research Quarterly, Fall 2002
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map of the epithermal neutron flux of the earth from Mars

Mars Odyssey Finds Water

Los Alamos researchers have determined that Mars has enough water to sustain human exploratory missions. Since February, a neutron spectrometer, designed and built at Los Alamos and flown aboard NASA's Mars Odyssey, has been measuring the neutron flux emitted from the red planet in order to map the global distribution of near-surface hydrogen. The neutron spectrometer is one of three instruments combined in the Mars Odyssey Gamma-Ray Spectrometer, which is operated by the University of Arizona. All three instruments are being used to determine the elemental composition of the Martian surface.

"The surface soils of Mars are rich in hydrogen," said Bill Feldman, the Laboratory's principal investigator for the neutron spectrometer. Soil in the upper and lower latitudes around both Martian poles "contains from 35 to 100 percent of water ice buried beneath a shallow overburden of hydrogen-poor soil. Although scientists have known that water ice is stable close to the surface in these regions, our new measurements are the first to give the amount of near-surface water on Mars," Feldman commented. "We have anticipated these results for 17 years and are excited that all of our wishes and hard work have been fulfilled."

The Lab's neutron spectrometer has been measuring the flux of neutrons escaping the Martian surface that were generated as a result of cosmic rays striking the planet. The neutrons are emitted in three energy bands: fast, epithermal, and thermal. The energy of these escaping neutrons indicates the degree to which they have been produced and moderated in subsurface nuclear reactions, with both effects related to subsurface composition. Hydrogen is particularly effective in lowering the production and moderating the energy of neutrons, and the relative fluxes of fast, epithermal, and thermal neutrons are strong indicators of near-surface hydrogen.

Modeling that combined the measured neutron fluxes with measured gamma-ray fluxes points to two regions near the poles that are so highly enriched with hydrogen as to require the presence of large reservoirs of near-surface ice. (Gamma-ray emission at 2.2 million electronvolts, an indicator of the capture of thermal neutrons by hydrogen, was also measured.)

The Los Alamos neutron spectrometer will continue to measure neutrons that escape from the top meter of Martian soil for several more years. Scientists will use these data not only to determine the amount of water on Mars but also to map the basaltic lava cover, measure the seasonal variation of polar carbon dioxide frost (dry ice), and help interpret gamma-ray data to determine the quantity and composition of the planet's most abundant elements.
—Shelley Thompson

 

 

< Map of the epithermal neutron flux from Mars: low flux (blue) is indicative of a high hydrogen content.

 
 

 

hamburger-sized enriched plutonium ingotsResearchers Cast "Spiked" Plutonium Alloy

In May, Los Alamos researchers cast nine ingots of a "spiked" plutonium alloy and fabricated them into test samples at the Lab's Plutonium Facility (TA-55). The spiking, which enriched weapons-grade plutonium with plutonium-238, created an alloy that should age sixteen times faster than normal. As a result, within four years the researchers hope to have a material representative of sixty-year-old plutonium.

The much higher radioactive decay rate of plutonium-238 will accelerate the self-irradiation process in the spiked alloy and enhance the resulting radiation damage, such as the formation of helium bubbles and void swelling. Accelerating such aging effects will provide critical data for assessing how radiation damage affects the plutonium cores, or pits, of the nation's nuclear weapons. Detecting and predicting changes in the our aging stockpile are among the more challenging aspects of science-based stockpile stewardship.

"This is probably the most technically difficult project we have ever attempted, at least metallurgically, at TA-55," said J. David Olivas, technical lead on the project. The experiment required four years of preparation and included replicating the plutonium manufacturing process that used to be done at the Rocky Flats Plant near Denver, CO. Los Alamos researchers set up a one-of-a-kind, small-scale casting, rolling, and machining operation at TA-55. They also had to reproduce key process steps and produce a material that matched Rocky Flats specifications.

The enriched casting combined plutonium-239 metal that had been "imported" from the Rocky Flats Plant with plutonium-238 metal buttons made at Los Alamos. Fabricating the plutonium-238 buttons was itself a first at TA-55 and a challenge. During casting, every effort was made to duplicate the processing parameters, such as heating and cooling, that were previously used at Rocky Flats.

After casting, the nine spiked ingots were rolled into sheets from which test samples were fabricated. Working with the ingots led to two independent challenges. "First, we had to fabricate the enriched plutonium quickly, because once you make it, it starts aging," said Olivas. "To get time-zero data, we needed to make samples as quickly as possible, and then get them to the test station equally quickly." A twenty-three-day-old sample is about a year old in accelerated time.

The second challenge came from problems associated with the ingots' excess heat from the radioactive decay of plutonium-238. "The ingots started to oxidize within minutes of production, forcing us to conduct all our fabrication operations in a very pristine atmosphere," said Olivas. (Fabrication glove boxes were flushed with argon to minimize the presence of oxygen and moisture.) The extra heat also made taking measurements more difficult. "When we attempted to measure the density of one of the as-cast ingots using the Archimedes method, we boiled the immersion bath fluid," Olivas added.

Initial (time-zero) test results for the spiked plutonium are highly promising. Most of the diagnostic tests show that it behaves very similarly to weapons-grade plutonium in terms of its microstructure, lattice parameters, elastic constants, and dynamic and quasi-static mechanical properties. However, one test, density, is producing anomalous results. Although not yet explained, the anomalies are most likely associated with the presence of the extra plutonium-238.

It is important that the initial material properties of the spiked plutonium be similar to those of weapon plutonium because this is the premise of the study. Having similar results at the outset will allow the aging portion of the study to proceed—and the clock is now ticking.

This research is part of the Accelerated Aging of Plutonium Project, an experimental collaboration between Los Alamos and Lawrence Livermore National Laboratories. Scientists at the two labs are conducting parallel sample preparation work and will exchange both information and samples. Their work will be used to predict material and component aging rates as a basis for future decisions about replacing stockpile pits.
—Meredith Coonley

 

 

 

< Researchers cast nine hamburger-sized enriched plutonium ingots like the one shown here.

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