NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Talk 2.1: Micro X-ray diffraction analysis of a fully recrystallized irradiated UO2 fuel

Mélanie Chollet, Johannes Bertsch, Daniel Grolimund, Vallerie Ann Samson and Matthias Martin
  • Paul Scherrer Institute, CH–5232 Villigen PSI, Switzerland

Abstract

During irradiation, the UO2 nuclear fuel undergoes significant chemical and microstructural changes that could have significant performance and safety issues: changes in thermal conductivity, fission gas release, loss of mechanical integrity. In particular, above a certain burn-up (~60 MWd/kgU), the so-called high-burn structure (HBS) develops at the rim of the pellet. It is characterized by a high intra- and inter-granular porosity as well a reduction of grain sizes down to the sub-micrometric level. At the Paul Scherrer Institute (PSI), we have characterized the microstructure of a very high-burn-up UO2 pellet (average 80 MWd/kgU) by synchrotron micro-X-ray diffraction (XRD). Particles from the different locations of the pellet radius from the center to the periphery were collected, with sizes according to the activity limit at the micro-XAS beamline of the Swiss Light Source (PSI). After X-ray fluorescence mapping at the U-LIII edge of the samples to identify the position of the particles on the kapton sample holder, XRD patterns were recorded in transmission by a MarCCD camera using a microbeam of 1 X 1.8 micrometer2. To complete the analysis, a fresh non-irradiated UO2 sample was also characterized as a reference.

Analysis of the 2D XRD patterns show a complete re-crystallisation of the pellet even in the center which was not expected since HBS is known to preferentially develop at the edge of the pellet and higher temperatures prevailing in the center should anneal any defects. Full rings indicating very fine grains (nanometric), smaller than the beamsize, were observed in each analyzed particle. On the opposite, 2D patterns of the fresh UO2 show well defined spots in agreement with 10 micrometer grain size reported for this sample. Evolution of the full width at half maximum (FWHM) of the diffraction peaks and d-spacings as a function of pellet radius will be discussed in regards to the fresh sample and in terms of released strain and accommodation of fission gas products in the lattice. Ultimately, a comparison with the few comparable data available in the literature on lower and/or different UO2 fuel chemistry will be commented.