A study of helium mobility in polycrystalline uranium dioxide

The mobility of Helium in polycrystalline uranium dioxide was studied by implanting samples with 3He ions at depths of approximately 1 μm and at concentrations in the region of 0.1 at.%. Samples were subsequently annealed at temperatures ranging between 700 °C and 1100 °C. Helium movement was then characterised using three different types of Nuclear Reaction Analysis (NRA) techniques based on the 3He(d,α)p reaction. The fraction of helium released from samples was measured during annealing at high temperature as a function of time. After each annealing sequence, helium depth profiles were obtained for each sample. In some cases, samples were characterised over small areas (60 × 60 μm2), using a micrometre size deuteron beam. This enables the measurement of helium distributions at the surface of samples. Using this novel approach which provides time and space dependent information relating to helium atom location, we show that grain boundaries act as effective short circuits for helium movement and release at all temperatures. Also, at temperatures above approximately 800 °C, in areas around the grain boundaries extending into the grain over distances of the order of microns, helium diffusion is high. In areas further into the grain, diffusion proceeds much more slowly presumably as a result of helium cluster formation. These observations are interpreted based on radiation damage production and annealing processes.