Hydrogen accumulation in and at the perimeter of U–C–N–O inclusions in uranium – A SIMS analysis

• H− and C− counts are strongly correlated, in large (> µm) as well as in small carbide precipitates.
• H− counts inside large carbides, already present in the melt, is much lower than at their perimeter.
• Suggesting H− signal inside large carbides represents equilibrium with hydrogen in the melt.
• H and C solubility decrease during cooling increases H at large and in/at small forming carbides.

We use imaging secondary ion mass spectrometry (SIMS) with a Cameca NanoSIMS 50 to determine the distribution and relative concentration of H in two uranium metal samples: sample1(LANL), as cast with less than 1 wppm H, sample2(Y12), outgassed 6 h in vacuum at 630 °C to remove hydrogen. H ion counts appear almost exclusively associated with ‘carbide’ inclusions, based on H−, C− and O− ion images for uranium surfaces sputter-cleaned in situ with a 16 keV Cs+ ion beam. Two classes of inclusions are identified: small, micrometer to sub-micrometer inclusions and larger, clearly angular inclusion (⩾3 μm). In sample1(LANL) the large inclusions (⩾250/mm2) show a low H−/C− ratio inside, and have H−/C− ratios at their perimeters comparable in magnitude to that seen in μm-size inclusions. Small inclusions (∼2500/mm2) contain H more uniformly throughout and, averaged over the inclusion, the small inclusions have approximately 50 times higher relative H concentration than the large inclusions. Sample2(Y12) was found to have comparable H−/C− ratios in the large carbides, but no small inclusions were observed. Because of the matrix-sensitivity of SIMS, H/C ratios representing the actual composition of the inclusions cannot be derived from the H−/C− ratios without calibration UC samples with known H content, which are not currently available.