Helium diffusion in uranium and plutonium oxides

Samples of UO2, (U,Pu)O2 and PuO2 containing up to several 100 at. ppm helium were submitted to thermal annealing in a Knudsen-cell provided with a mass spectrometer. Gas release was measured on line with a great accuracy. In the examined materials helium was created by α-decay of plutonium or laboratory infused at high temperature and high pressure. The selected samples exhibited different types of lattice damage, including reactor burn-up and high α-radiation doses. Analysis of helium release as a function of temperature enabled the elementary diffusion processes to be investigated and the atomic diffusion coefficient to be deduced for a defined state of helium-in-solid. The helium diffusion coefficient has the expression:D=8×10−7exp(−46 kcal mol−1/RT) m2 s−1,however, the enthalpy of diffusion increases to 70 kcal mol−1 in high burn-up fuel where helium is apparently stabilised in uranium/oxygen vacancy clusters.