New methods to estimate the internal pressure of xenon in fission bubbles using SEM-EPMA-SIMS and EBSD in irradiated fuels

C. Cagna (1) , I. Zacharie-Aubrun (1) , P. Bienvenu (1) , B. Michel (1) , L. Barrallier (2)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
2 MSMP - Mechanics surfaces and materials processing

During irradiation, in Pressurized Water Reactors, gaseous fission products are created (xenon and krypton) in the fuel pellets and form bubbles distributed heterogeneously. During fast heating in an accident situation, the internal pressure of nanoscale bubbles increases, producing stresses at grain boundaries. As part of a thesis in collaboration between the CEA Cadarache and MSMP Laboratory (Arts et Metiers ParisTech, Aix-en-Provence), a research program has been undertaken to develop new methods to evaluate the internal pressure of fission gas bubbles in irradiated fuels. The first experimental method was developed for the characterization of a large number of pressurized bubbles in an irradiated UO$_2$ nuclear fuel using SIMS, EPMA and SEM. The method is now performed by selecting with EPMA an area where few pressurized bubbles are present under the sample surface. The SIMS, Secondary Ion Mass Spectrometry, is carried out to open these bubbles and collect the amount of xenon in both the matrix and the bubbles. The EPMA, enables the calibration of the SIMS in order to provide quantitative results. SEM, provides information on the number and shape of the bubbles. The quantification of xenon in the bubbles by SIMS coupled to their size measured by SEM make it possible to obtain the atomic volume. Through adapted equations of state, the pressure of xenon in these bubbles is then estimated.The second method consists in the measurement of the local deformation around a bubble by Electron Back Scatter Diffraction (EBSD). The diffraction patterns are analyzed by cross-correlation to obtain the elastic deformation and rotation tensors. In parallel, a mechanical model by finite elements is developed to determine the bubble's internal pressure. At this point, the method is calibrated with a mechanical test by four points bending of a monocrystal silicon beam. The test results are compared to a relevant mechanical model.These experimental data will provide new points of validation for the models to predict the behavior of the fuel during nominal irradiation and accidental situations.

Mots clés: Nuclar fuel; EBSD; Fission gas bubbles; Pressure

Domaines: Physique Nucléaire Expérimentale [nucl-ex]; Physique Nucléaire Théorique [nucl-th]

C. Cagna, I. Zacharie-Aubrun, P. Bienvenu, B. Michel, L. Barrallier. New methods to estimate the internal pressure of xenon in fission bubbles using SEM-EPMA-SIMS and EBSD in irradiated fuels. EMAS 2015 - 14th European Workshop on Modern Developments and Applications in Microbeam Analysis, May 2015, Portoroz, Slovenia. ⟨cea-02492538⟩