Magnetic measurements detect hydrogen contaminants in plutonium

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Researchers from Condensed Matter and Magnet Science (MPA-CMMS) and Nuclear Materials Science (MST-16) used magnetization, x-ray, and neutron diffraction measurements enabled by the Lab’s materials science capabilities to demonstrate a technique for detecting low concentrations of plutonium hydride in samples. The technique, published in the Journal of Applied Physics, is relevant to plutonium applications, workers who handle plutonium, and long-term plutonium storage. Contaminants, such as oxygen, hydrogen, and carbon, can degrade the mechanical properties of plutonium, causing consequences that can negatively affect health and safety.

LANL researchers examined the effects of plutonium (Pu) metal exposed to low levels of hydrogen during the radioactive decay process. The team showed that ferromagnetic remanence – the residual magnetization left in a ferromagnetic material (a permanent magnet) after exposure to a magnetic field – could detect small quantities of hydrogen against the background of pure plutonium. Pure plutonium is non-magnetic. However, Los Alamos researchers in the early 1960s discovered that Pu acquires a magnetic moment when it reacts with hydrogen to create plutonium hydride. Therefore, magnetic measurements can be used to detect the presence of hydride formation in plutonium metal.

The researchers used the Neutron Powder Diffractometer and neutron diffraction at the Los Alamos Neutron Science Center (LANSCE) to characterize the metallic crystal structures samples of polycrystalline delta (δ)-plutonium stabilized with gallium (Ga). The samples had the expected face centered cubic (fcc) structure and lattice parameters. The scientists exposed the samples to hydrogen under partial vacuum at 450 °Celsius to ensure reproducible hydrogen solubility for the magnetization measurements. They loaded one sample to a H/Pu atom ratio of 0.01 ± 0.0003, and encapsulated the second plutonium sample without H loading.

After sealing the samples in titanium containers to prevent radioactive contamination of the surroundings or exposure of the samples to air, the team measured the magnetization of the capsules as a function of magnetic field and temperature at the National High Magnetic Field Laboratory-Pulsed Field Facility at Los Alamos. They used a commercial vibrating sample magnetometer in a physical properties measurement system. The results confirmed that the 2.0 at. % Ga stabilized H-free δ-Pu samples are non- magnetic between 4-300 K.

Figure 11. A plutonium sample is sealed in a titanium container for magnetization measurements.

The results demonstrated that commercial magnetization measurement techniques are sensitive to the conversion of tiny amounts (0.0015 mole fraction) of hydrogen in Ga stabilized δ-Pu to ferromagnetic PuHx. This easily reproducible technique is a useful quantitative diagnostic to determine the content of small amounts of PuHx in samples.

Figure 12. Magnetic moment as a function of magnetic field of δ-Pu with 1 at. % H exposure in a titanium (Ti) sample holder (B), minus the magnetization of the sample without H exposure in Ti sample holder (A). Measurements were conducted at after zero-field cooling (in H < 10−3 T) from room temperature and sweeping the magnetic field around a 6 T hysteresis loop starting from H = 0. The inset shows a zoomed view of the magnetic hysteresis, with a linear background subtracted.

Reference: “Detecting Low Concentrations of Plutonium Hydride with Magnetization Measurements,” Journal of Applied Physics 117, 053905 (2015); doi: 10.1063/1.4907216. Authors include: Jae Wook Kim (MPA-CMMS, now at Rutgers University), Eundeok Mun (MPA- CMMS, now at Simon Fraser University, Canada), Joe Baiardo, Vivien Zapf, and Chuck Mielke (MPA-CMMS); Alice Smith, Scott Richmond, Jeremy Mitchell, and Dan Schwartz (MST-16).

Laboratory Directed Research and Development (LDRD) funded the LANL work. Magnetization measurements were performed at the National High Magnetic Field Laboratory, funded by the National Science Foundation, the State of Florida, and the DOE. The neutron diffraction work was conducted on the Neutron Powder Diffractometer at the Lujan Neutron Scattering Center, funded by DOE-Basic Energy Sciences. The work supports the Lab’s Nuclear Deterrence mission area and the Materials for the Future and Science of Signatures science pillars through investigation of materials in the nation’s nuclear weapons stockpile. Technical contacts: Dan Schwartz and Chuck Mielke

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Figure 11. A plutonium sample is sealed in a titanium container for magnetization measurements.

 

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Figure 12. Magnetic moment as a function of magnetic field of δ-Pu with 1 at. % H exposure in a titanium (Ti) sample holder (B), minus the magnetization of the sample without H exposure in Ti sample holder (A). Measurements were conducted at after zero-field cooling (in H < 10−3 T) from room temperature and sweeping the magnetic field around a 6 T hysteresis loop starting from H = 0. The inset shows a zoomed view of the magnetic hysteresis, with a linear background subtracted.