Los Alamos researchers celebrated a major success May 13 when they cast the very first "spiked" plutonium alloy, creating an accelerated-aging alloy that should age at a rate sixteen times faster than normal. As a result, in four years the researchers hope to have a material representative of sixty-year-old plutonium.
Researchers will measure the spiked material to look for any age-related changes in key physics, engineering, and materials properties. On the basis of these experiments, they'll determine if the nation's stockpile pits will last at least sixty years.
"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 and former Rocky Flats scientist.
The experiment required years of preparation and included replicating the Rocky Flats plutonium manufacturing process. To that end, Los Alamos researchers had to set up a capability that had never existed before: a one-of-a-kind small-scale casting, rolling, and machining operation at the Laboratory's Plutonium Facility (PF-4). The researchers also had to reproduce key process steps and produce a material that matched Rocky Flats specifications in a number of important properties.
And they did it on the first try.
The research endeavor, called the Accelerated Aging of Plutonium (AAP) Project, is an experimental collaboration between Los Alamos and Lawrence Livermore National Laboratories. Besides Olivas, the AAP team at Los Alamos consists of Franz Freibert, principal investigator; Richard Ronquillo, lead mechanical technician; Claudette Trujillo, materials accountability specialist; Chris Trujillo, mechanical technician; and David Dooley, graduate research associate. All are with the Nuclear Materials Science Group (NMT-16).
These Los Alamos scientists, working with others in NMT-16, the Structure/Property Relations Group (MST-8), and the Detonation Science and Technology Group (DX-1), will examine the spiked material with advanced characterization tools to measure aging-related changes in physical and chemical properties. Scientists at Livermore are conducting a parallel materials production and sample preparation activity. The two laboratories will exchange information and samples.
The photo at right shows the plutonium-238 metal button that was used as the starting material for the May 13 enriched casting. Plutonium-238 is normally only available in the form of plutonium oxide because its radioactive decay produces so much heat that the material must be present as a ceramic for it to be stable. The plutonium-238 button shown here was reduced to the metal form from oxide originally fabricated for space-program heat sources. Personnel in the 238Pu Science and Engineering Group (NMT-9), who followed a procedure developed at DP Site in the 1970s, performed the reduction. This was the first time that plutonium-238 metal had been fabricated at TA-55. This button weighs about 100 grams and is sitting at about 200 degrees Celsius.
The AAP activities support the Enhanced Surveillance Campaign goal to protect the health of the stockpile by examination of aged plutonium through the accelerated production of defects.
The information obtained from this research will be used to predict material and component aging rates as a basis for annual certification, refurbishment scope and timing, and nuclear weapon complex planning.
Chris Trujillo of the Nuclear Materials Science Group (NMT-16) machines a Kolsky test specimen. A non-water-based coolant is used to flood the specimen during machining. Flooding ensures that the sample is not overheated during the machining process, avoiding the introduction of nonaging-related artifacts into the plutonium sample.
Ultimately, this work will form the key basis for establishment of pit lifetimes. Results of the research will also be used to make improvements to the basic surveillance program (see ARQ 1st quarter, 2001).
The new casting and machining capability also will enable Nuclear Materials Technology (NMT) Division to expand its research and development efforts in the study of weapons-related actinides and other special isotopes and materials.
Researchers spiked the plutonium alloy cast May 13 with isotopic blends containing 7.5 percent plutonium-238. The greater alpha decay rate of the plutonium-238 isotope accelerates the self-irradiation process and enhances the self-irradiation damage as a function of time. Accelerating the self-irradiation aging effects in weapons alloys should provide critical data at extended effective lifetimes in the manifestation of aging effects on weapon design parameters and component function, according to the researchers.
The casting yielded nine enriched plutonium "cookies,"one of which is shown here. The thickness of this miniature ingot duplicates the thickness of the Rocky Flats ingots, giving researchers a good simulant for the next stage of processing: rolling the ingot into a sheet.
The detection and prediction of changes in an aging stockpile are perhaps the most challenging and technically engaging aspects of science-based stockpile stewardship. Originally, weapons systems were designed with the expectation that the nuclear components would provide a reasonable lifespan and that the systems would be modernized or replaced on a consistent basis. Weapons systems were not designed with the goal of long-term (fifty or sixty years) robustness.
Given the current constraints and conditions in the nuclear weapons stockpile, systems will require extended lifetimes for various components with only a modest remanufacture capability to replace excessively degraded units. Determining an appropriate response (recertification, refurbishment, or remanufacture) ultimately depends on an accurate assessment of individual component lifetimes.
During the aging process, the enriched samples will be stored in incubators like the one in the photo at left. The samples will be cleaned before being loaded in the incubator chamber, and then will be stored in a pristine atmosphere for up to four years. Four years of storage in the incubator in actual time is equivalent to sixty years of aging in accelerated time. The samples will be removed periodically from the incubators and tested to obtain aging information at intermediate times. The incubatochambers were designed and built at Lawrence Livermore National Laboratory.
The most acute challenge lies with testing and certification of plutonium components and the pits in which they reside. Previously, certification and recertification of the plutonium components relied heavily on full-scale nuclear tests‹an approach that is no longer viable.
Relevant properties of weapons-grade plutonium and its decay products that affect performance include equation of state, spall and ejecta, material strength, density, geometry, corrosion resistance, and nuclear reactivity. Among the time-dependent phenomena that could affect the properties of weapons-grade plutonium are radiation-induced void formation and swelling; ingrowth of decay products such as helium, americium, and uranium; and phase instability. These effects may well be synergistic, making it particularly difficult to assess the importance of any one phenomenon by itself.
Because of the lack of a suitable number and variety of aged plutonium samples, researchers sought to produce alpha-decay- induced damage at an accelerated rate. They accomplished this by adding relatively small amounts of the short-lived isotope plutonium-238, which has a half-life of 86.4 years, to weapons-grade plutonium-239. This technique simulates many years of aging in just a few years.
The AAP researchers assume that the damage created by alpha decay of plutonium-238 is comparable to the damage that occurs in the normal material and that sensitive, fundamental measurement techniques can be employed to characterize this damage. The spiked plutonium is being made into samples for many different experiments, and the data will give researchers a composite image of the aging process. A suite of advanced techniques is being used to characterize the enriched material and track subtle changes as the aging progresses. (See sidebar on page 7 for a description of the tests that are being used.) ³Most of these experimental techniques have been used in the past to study research-grade material, but this is the first time they will all be applied to the same material, and one that has such a well-known pedigree,² said Freibert. Testing diagnostic methods focus on assessing effects of extended aging on important design properties. Tests include density, dilatometry, elastic constants and bulk compressibility, conventional and high strain rate mechanical testing, x-ray diffraction, helium effusion, light and electron microscopy, and others.
A note on nomenclature
Using plutonium-238 to enrich weapons-grade plutonium resulted in the creation of a new material previously not used in the weapons complex. Addition of the plutonium-238 has led to the material being called by several names‹spiked, enriched, and doped‹all of which are acceptable.
Because this material is significantly different from other plutonium alloys, the researchers have chosen to name the material "Hatch." This is, of course, in honor of the southern New Mexico city that produces the regionally famous "hot" chile peppers.
Now, if they could just decide if Hatch qualifies as red or green.
These experimental capabilities will be applied to measure material properties sensitive to the reversible and irreversible thermodynamics of this plutonium alloy, as well as to provide information on the kinetics of aging-related processes. This information will provide a sound technical and scientific foundation for predicting weapon component lifetimes.
Researchers know from the analysis of nuclear reactor materials that the most obvious consequences of radiation damage are helium bubble formation and void swelling. The net effect is that the metal will swell and therefore reduce the density of the metal. Although there are other time-dependent phenomena that may affect the properties of weapons-grade plutonium, helium bubble formation and void swelling will unquestionably affect weapon performance.
Experiments using transmission electron microscopy (TEM) to image helium bubbles in weapons-grade metal have provided evidence supporting this theory (see ARQ 4th quarter, 2001). So far, the accumulation of aging effects and the overall impression on important weapons design properties are still quite speculative.
Fabrication "blitz" The Los Alamos researchers are now in a fabrication "blitz" so that they can get experimental data near time equals zero. "That kind of data was missing at Rocky Flats," said Olivas. "It's not that Rocky Flats was not interested in scientific data, it's just that they were more interested in making the product to specifications. Information that was not part of the specification typically was not collected."
Working with the enriched plutonium has led to two independent challenges. "First, we must fabricate the enriched plutonium quickly, because once you make it, it starts aging," said Olivas. "To get time-equals-zero data, we need 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 twelve months old in accelerated time.
Independent from the timing issue are the problems associated with the additional heat present in the samples from the presence of plutonium-238. Recall that plutonium-238 is normally used as a heat source. For example, the spiked material also appears to oxidize at an accelerated rate outside its storage environment.
"It started to oxidize within minutes of production, forcing us to conduct all our fabrication operations in very pristine atmosphere," said Olivas. The extra heat also makes taking measurements more difficult. "When we attempted to measure the density of one of the as-cast disks using the Archimedes method, we literally boiled the immersion bath fluid."
Personnel from the Nuclear Materials Technology (NMT) Division load extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) test specimens into special containers. The specimens were shipped to the Stanford Linear Accelerator for testing. The containers isolate the plutonium beneath a special window and facilitate the testing of plutonium-bearing materials in facilities where nuclear materials are not normally handled.
Tony Valdez of the Weapons Component Technology Group
(NMT-5) loads the casting furnace with the precursor materials for the enriched casting: plutonium-239 metal ³imported² from the Rocky Flats Plant and plutonium-238 metal buttons made at Los Alamos. For this casting, every effort was made to duplicate the processing parameters, such as heating and cooling, that were previously used at the Rocky Flats Plant to cast plutonium ingots. The metal is placed in a tantalum crucible, heated above the melting point of the plutonium, and then poured into the mold. All operations with molten plutonium are done in vacuum because of the extreme reactivity of the liquid plutonium.
The May 13 spiked casting comes one year after a "control" material of the same chemistry, but significantly lower plutonium-238 isotopic content, was cast and machined to validate the installed equipment and thermomechanical processing. A portion of the control batches received special processing to create samples with minute yet finite differences in physical and mechanical properties. These samples are now undergoing testing by a suite of diagnostics on a blind-sample basis to discern minute differences in physical and mechanical properties of plutonium alloys. Results will be used to quantify the sensitivity of the measurement methods.
Data from the spiked casting show that the density of the heat-treated spiked disks matches the density of the control casting
disks at the same point in the processing. This is good news, according to Freibert and Olivas, because it indicates that metallurgically, Los Alamos' processing for the spiked material was equivalent to that of the control casting.
In the photo below, an as-cast cookie sits in the immersion bath of the density measurement station. The decay from the plutonium-238 generates so much heat that it causes the immersion bath fluid, FC43, to boil. The boiling point of FC43 is 165 degrees Celsius. The bubbles are faintly visible as they rise above the surface of the cookie.
More important, the researchers say, it also suggests that there seems to be no significant physical difference in the spiked material that may be attributed to adding the plutonium-238.
Progress on the enriched casting is proceeding nicely, according to Olivas and Freibert. As of this writing, all metallurgical processing has been completed, samples are being fabricated, and time-equals-zero testing has begun. The researchers have completed time-equals-zero testing on most of the mechanical property tests (40-mm launcher tests at high strain rate, Kolsky tests at intermediate strain rate, and compression tests at quasi-static strain rate), resonant ultrasound spectroscopy (RUS) tests, and x-ray diffraction tests. In addition, they have shipped enriched plutonium to the Stanford Linear Accelerator Center for extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) tests.
-Franz Freibert, J. David Olivas, and Meredith S. Coonley
| Diagnostic test | Information obtained by the test | Analytical chemistry | Isotopics, d-phase plutonium stabilizers, and tramp elements (elements that are contaminated) | Density | Relative amounts of phases (a and d) plutonium, helium, and void swelling | Optical metallography | Grain size, relative amounts of phases (a,a¢,d) plutonium, Pu6Fe (an intermetallic compound that typically forms at grain boundaries), and inclusions | X-ray diffraction | Lattice parameters, relative amounts of phases (a, a¢, d) plutonium | Microprobe | Gallium segregation | Resonant ultrasound | Elastics constants, compressibility, and sound speeds 40-mm launcher | 40-mm launcher | Dynamic mechanical behavior (spall strength and phase transitions) | Kolsky bar apparatus | Intermediate strain rate mechanical behavior | Tensile/compression | Quasi-static strain rate mechanical behavior (elastic-plastic proper | Dilatometry | Thermal expansion, dÆa¢ plutonium Martensite transition, and density changes | Part evaluation cycle | Phase stability (e.g., Stockpile to Target Sequence [STS], which is the order of events involved in removing a weapon from storage and assembling, testing, transporting, and delivering it on target) | Thermoelectric transport | Transport properties and isochronal annealing (damage recovery) | Thermal gas desorption | Gas species determination and helium trap energetics | Calorimeters | Phase transformations, transformation enthalpies, and specific heat | Helium effusion | Energetics of helium diffusion | Positron annihilation | Location and distribution of atomic-scale defects | Other | Surface science and local structure techniques |
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