Recent progress in the modelling of helium and tritium behaviour in irradiated beryllium pebbles

First simultaneous detection of helium and tritium inside bubbles in beryllium

Electron energy loss spectroscopy (EELS) was applied to detect and analyze quantitatively helium (He) and tritium (³H) enclosed inside bubbles in irradiated beryllium. Both gases were formed in beryllium under neutron irradiation as a consequence of neutron-induced transmutation reactions. They were detected for the first time as pronounced peaks at 13.0 eV for ³H and 22.4 eV for He in EELS spectra collected from flat hexagonal bubbles. An adhesion of ³H or formation of thin beryllium hydride layers on the internal basal surfaces was observed. The number densities of both gases were estimated using electron scattering cross-section and intensities obtained from EELS spectra. The number density values estimated for various bubbles fluctuate from 4 to 15 at/nm³ for He and from 4 to 10 molecules/nm³ for ³H₂. Lower gas number density was measured inside large bubbles. The observed higher density of tritium at inner walls of bubbles seems to confirm very recent ab initio calculations of the interaction of hydrogen isotopes with beryllium surfaces.