Helium Migration Mechanisms in Polycrystalline Uranium Dioxide

Abstract

This study aims at identifying the release mechanisms of helium in uranium dioxide. Two sets of polycrystalline UO2 sintered samples presenting different microstructures were implanted with 3He ions at concentrations in the region of 0.1 at.%. Changes in helium concentrations were monitored using two Nuclear Reaction Analysis (NRA) techniques based on the 3He(d,α)1H reaction. 3He release is measured in-situ during sample annealing at temperatures ranging between 700°C and 1000°C. Accurate helium depth profiles are generated after each annealing stage. Results that provide data for further understanding helium release mechanisms are discussed. It is found that helium diffusion appears to be enhanced above 900°C in the vicinity of grain boundaries possibly as a result of the presence of defects.

A mass spectrometry method for quantitative and kinetic analysis of gas release from nuclear materials and its application to helium desorption from UO2and fission gas release from irradiated fuel

“…The depletion of this volume, not deeper than a few micrometres, could contribute to the driving force for the helium bulk release. An analogous phenomenon was observed by Martin et al [31,32] at the UO 2 grain boundary, during an annealing experiment performed on 3 He-implanted samples. The main release occurs between 1200 and 1600 K (area B in Figure 5).…”

“…It is attributed to helium located in energetically favourable positions namely in the octahedral interstitial sites (OIS), i.e. the cavities in the middle of the oxygen cubes of the UO 2 lattice [32][33][34][35]. From these positions helium atoms diffuse to the surface via oxygen vacancy diffusion.…”

Helium solubility and behaviour in uranium dioxide

“…An analogue phenomenon was observed by Martin et al [9,10] at the UO 2 grain boundary, during an annealing experiment performed on 3 He-implanted samples.…”

Surface Blistering and Flaking of Sintered Uranium Dioxide Samples under High Dose Gas Implantation and Annealing

“…It has indeed previously been shown that above 1000°C, most of the trapped helium atoms are thermally re-soluted into the oxide matrix [1,9]. The gas should therefore be able to migrate above this temperature which would either induce its release or its capture by existing bubbles.…”