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Acoustic Resonance Spectroscopy (ARS) Shows Promise for Measuring Gas Composition and Pressure in Sealed Storage Containers

The interim storage of plutonium-containing materials has raised concerns about the generation of dangerous gas pressures and gas compositions in hermetically sealed containers. Pure plutonium oxide powders, if not properly prepared, may be able to generate pressures exceeding several hundred pounds per square inch of H2. Noninvasive characterization techniques would allow storage containers to be monitored for changes in gas composition and gas pressure and help to ensure the long-term safety of the materials stored under the Department of Energy's repackaging program.

We have been investigating the application of ARS to monitor changes in gas composition and pressure within sealed storage containers. The goal of our work is to design a robust, economical characterization system that can be used on containers destined for interim storage. In our experiments the speed of sound in the gas is measured utilizing a resonant gas cavity in the storage container. Standing waves in the gas are excited and detected using piezoelectric transducers mounted on the outside of the container. The frequencies of the standing waves depend upon the speed of sound in the gas‹and therefore the gas composition‹and upon the geometry of the resonant cavity. The signal intensities depend upon the gas pressure.

The resonant cavity in which the gas acoustic modes are excited must be accessible to ultrasonic transducers mounted on the outside of the container and be of a simple geometry for which the gas modes can be modeled with confidence. We have fashioned a cylindrical cavity within a storage container from a short section of thin-walled pipe fastened onto the bottom of a storage can with a circular plate glued on top of the pipe as a lid. The plate has a 50-mm filter fitted onto a small hole to let gas diffuse into the cavity. The storage container is filled with silica sand, surrogate for plutonium dioxide for these proof-of-principle experiments. The experimental geometry is shown in Figure 3.

Figure 3. Schematic of storage can with a cylindrical resonant cavity. Acoustic resonance spectra are acquired by driving a piezoelectric tranducer fastened to the bottom of the container with a variable frequency sine wave and picking up the response of the system using a second transducer.

The sand effectively dampens all container resonance modes except those from the area on the bottom of the can that is within the cavity. Transducers are glued to the bottom of the can. Data consist of the amplitude of an acoustic sine wave versus frequency.

Gas resonance modes are identified by comparing spectra acquired with the container under vacuum and filled with argon at a pressure of 40 psia. All observed gas resonances match with calculated resonances. Gas amplitudes are typically quite small, about one-tenth the amplitude of the continuous background and one-hundredth the amplitude of the strongest container mode observed.

The amplitude of a gas mode resonance is a function of the gas pressure, the gas composition, and whether there is a container mode with a frequency near that of the gas mode. The amplitude of a gas mode can be increased by orders of magnitude when the gas mode is resonant with a container mode. This is observed by gradually shifting a gas mode towards a container resonance by changing the gas composition. The integrated intensity of a gas resonance peak that is not appreciably affected by container resonances has been confirmed to increase linearly with gas pressure. Thus, the amplitudes‹more precisely, the integrated intensities‹of gas mode resonances can be used to obtain pressure information.

Information about gas composition is obtained from the velocity of sound. The observed gas resonance frequency, temperature, and dimensions of the cylindrical cavity are used to calculate the velocity of sound of the gas. An effective mass for the gas is calculated directly from the sound velocity. This effective molar mass is a combination of the mean molar mass, mean compressibility factor, and the mean heat capacity ratio; it is equal or close to the mean molar mass depending upon the molecular complexity of the constituents. Figure 4 shows the resonance frequency of a gas mode as a function of effective mass for the geometry shown in Figure 3. The applicability of the effective mass was confirmed in experiments using gas mixtures in a spherical cavity. Spherical resonator cavities have sharper resonance peaks when the driving frequency is swept through a gas mode and can therefore be used to confirm at a more accurate level the theory relating gas composition to sound velocity.

Figure 4. The relationship shown for effective mass as a function of frequency is for the cylindrical cavity of dimensions shown in Figure 3 at 23 degrees C. Frequencies of some common gases are shown for illustration.

Applying ARS to the proposed 3013 storage system involves some significant technical barriers. The 3013 storage system calls for placing a welded inner container within a welded outer container. In order to use ARS to probe the gases within the inner container, we have to couple the acoustic energy from the outside of the outer container to the lid of the inner container. We have tested the concept of using small metal cylinders as acoustic couplers between the inner and outer containers and have observed gas resonances. However, our results to date suggest that probing a double-container system using couplers will reduce the intensity of observed gas modes and at the same time significantly increase the number and intensity of container modes, causing reliable measurement of gas modes to become difficult, perhaps impossible. Using ARS monitoring with a singly-contained storage system, e.g., the 3013 inner container without the outer container, may have advantages. The advantages include reduced costs, early detection of changes in the gas and therefore changes within the stored material, and enhanced ability to dissipate any heat generated in the storage container. Early detection of changes in the stored material will allow problem materials to be identified and stabilized before they have a negative impact on the storage facility. Monthly surveillance of newly filled containers should identify problems well before the problems affect the container integrity. After a container shows no unanticipated changes for some time, the surveillance interval can be increased.

Collaborators on this research include D. Kirk Veirs, NMT-6; Joseph P. Baiardo, NMT-5; Clinton R. Heiple, Metallurgical Consultant, Boulder, CO; and Gerd M. Rosenblatt, Materials Sciences Division, Lawrence Berkeley National Laboratory.

Funding for this effort was made possible through the Nuclear Materials Stabilization Task Group, EM-66, of the Department of Energy.


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