Los Alamos researchers are studying the behavior of oxide materials in sealed containers as part of a new Department of Energy (DOE) program aimed at safely storing large quantities of plutonium-bearing materials for up to 50 years. Experience with pure plutonium oxide and with impure materials has shown that gases generated by catalytic or radiolytic processes may accumulate over time. Of concern to researchers are the generation of hydrogen gas from adsorbed water, the generation of water in the vapor phase at elevated temperatures, and the generation of hydrochloric or chlorine gases from the radiolysis of chloride-containing salts that exist as impurities in the oxides. The combination of chloride-bearing gases and condensed water may corrode a storage container.
Researchers have designed these instrumented small-scale (left) and
large-scale shelf-life surveillance containers, which will be used to
monitor gases over oxide materials.
Materials destined for long-term storage include metals and oxides that are stabilized and packaged according to a standard that requires a set of nested, welded stainless-steel storage containers. The recently revised standard, DOE-STD-3013, allows for a wider range of plutonium content (containing at least 30 weight percent plutonium plus uranium) in the stored materials. The earlier standard specified storage for plutonium-bearing metals and oxides to contain at least 50 weight percent plutonium plus uranium. The revised standard also specifies that only materials that are represented in Los Alamos' Material, Identification, and Surveillance (MIS) Program may be packaged for long-term storage.
The Los Alamos Shelf-Life Surveillance Project will monitor gases over oxide materials in a limited number of full-scale 3013 inner containers and in many small-scale containers. Oxide materials representing inventories destined for long-term storage throughout the DOE complex will be monitored.
The MIS Program has evaluated 33 items from Hanford and Rocky Flats Environmental Technology Site. Residuals from these evaluated items make up the surveillance items to date. Future materials will come from the Los Alamos inventory and from other sites.
The project includes researchers from Actinide Chemistry Research and Development (NMT-11), Actinide Processing (NMT-2), Physical Chemistry and Applied Spectroscopy (C-PCS), and Energy and Process Engineering (ESA-EPE), and it involves two parallel studies.
A small-scale study will monitor many 10-gram samples for short time periods. This study continues the work initiated by Tom Allen of Pit Disassembly and Nuclear Fuels Technologies (NMT-15). A large-scale study will monitor, for longer periods, plutonium oxides prepared according to the 3013 standard, oxides exposed to high-humidity atmospheres, and oxides containing chloride salt impurities.
The small samples will allow researchers to compile a database of many materials prepared according to various site-specific packaging methods. The large samples will give the precise behavior of a limited number of full-scale samples. Comparison between the two sample types will determine the degree of confidence in small-sample experiments and fundamental measurements in predicting the long-term behavior of real materials.
Researchers have designed instrumented storage containers that mimic the inner storage can specified in the 3013 standard at both full- and small-scale capacities (2.34 liters and 0.0045 liter, respectively). The containers are designed to maintain the volume-to-material mass ratio while allowing the gas composition and pressure to be monitored over time.
The full-scale cans are instrumented with a Raman fiber-optic probe, a gas chromatography/mass spectrometer sampling port, an acoustic resonance chamber, two corrosion monitors, and pressure and temperature sensors. Data collection for the full-scale containers is automated to reduce worker radiation exposure.
This small-scale array has 45 sample containers located in the heating
block with the heated manifold. The nine containers in the left-hand row
have the lids in place, but do not have the gas sampling transfer lines
connected.
To minimize the effect of invasive gas sampling on the experiment, gas chromatography sampling will be limited. The majority of the gas compositional data for the large-scale study will be obtained noninvasively through Raman spectroscopy.
The small-scale containers are designed with a microliter gas-sampling capability, and pressure and temperature sensors. These small containers will be stored in a heated array to reproduce the increased temperatures arising from radioactive self-heating. The tubing between the small containers and the gas-analysis equipment will be heated to 100 degrees Celsius to avoid condensation of gases. Pressure and temperature data will monitored continuously and gas analysis will be conducted at periodic intervals.
An initial 45 samples are planned for the small-scale study, and will include experimental blanks and plutonium oxide samples from DOE sites. The samples are selected to address known existing uncertainties in gas generation and safety considerations associated with specific material types being considered for storage.
They also will contain samples of the actual materials going into the large-scale study. A percentage of the oxide materials in the small-scale study will be used for data validation of gas-generation models in development across the DOE complex.
In the large-scale study, 10 large-scale instrumented inner 3013 containers are being installed at Los Alamos' Plutonium Facility in Building PF-4 to study compositional changes in gases over plutonium oxide materials for an extended time. Nine of the containers will hold characterized oxide materials in varying forms obtained from NMT-2; the 10th container will be an experimental blank. A rack system to hold the large array has been designed, fabricated, and tested. Like the array for the small containers, it contains a heated gas manifold to minimize the effect of gas condensation and to ensure quantitative gas chromatography analysis.
Technical issues that will be explored through the large-scale experiments include the effect of specific surface area, water, salts, organics, fill gas, and mixed oxides on gas generation and equilibrium pressures.
Research from the Shelf-Life Surveillance Project will be used by the DOE to ensure that representative stabilized materials in approved containers are safe in long-term storage.
The data compiled at Los Alamos will provide guidance on what may or may not cause a container to pressurize, help researchers understand the changes in gas composition over time, and explain the effect of corrosive gas generation in the cans.
The study also will provide data that will be used to establish a DOE sitewide surveillance program for the extensive amount of material packaged according to the DOE standard.
Contributors to this project are: Tom Allen (NMT-15); Simone Balkey (NMT-2); John Berg, Max Martinez, Jim McFarlan, John Morris, Dennis Padilla, Karen Rau, Kirk Veirs, and Laura Worl (NMT-11); David Harradine (C-PCS); and Dallas Hill and Coyne Prenger (ESA-EPE). All photos by Mick Greenbank (NMT-16).
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