Determination of the pressure in micrometric bubbles in irradiated nuclear fuels

In oxide nuclear fuels, at high burn-up or during high temperature periods such as ramp tests, out-of- pile heating tests, or any irradiations at high linear heat rates, fission gases can form micrometric or quasi-micrometric bubbles. During nominal operations, these bubbles participate to the pellet swelling, to the decrease of the fuel thermal conductivity and are involved in the mechanisms leading to fission gas release. During events involving a temperature increase, the resulting increase in the internal pres- sure of the bubbles might play a role in fuel fragmentation and in the opening of grain boundaries. The gas densities inside these bubbles are therefore one of the useful experimental information for the un- derstanding of the fuel behaviour, and for the fuel behaviour code progress and validation. Two methods were developed to evaluate the gas density in the quasi-micrometric bubbles, using electron probe micro analyser, secondary ion mass spectrometry and focused ion beam scanning electron microscope together. The first method provides a mean gas density for all quasi-micrometric bubbles in a given area. The sec- ond method provides a gas density in a single selected bubble. In addition to the gas density, the 3D size and shape of the selected bubble is measured and can be related to the gas density result. In this work, these methods were applied to the bubbles formed in the centre of a PWR Cr doped UO 2 at 38.8 GWd/t U after a ramp test in the Osiris reactor, with a 12 h plateau at 470 W/cm, and to the bubbles formed in a PWR Cr doped UO 2 at 62.8 GWd/t U in the centre of the pellet and on the bubbles of the high burn-up structure on the rim. Both show the high pressures reached in these bubbles.CEA-DES, EDF and Framatom

Fission gases in irradiated PWR fuels : a detailed condition with a high burn rate

Au cours de l'irradiation des pastilles de combustibles nucléaires, les réactions de fission entraînent une accumulation progressive de nouveaux atomes, dont certains sont gazeux. Ces gaz de fission, et les bulles qu'ils forment, contribuent significativement au comportement du combustible, en fonctionnement nominal comme en cas de fonctionnement incidentel/accidentel. Ce travail de thèse apporte une meilleure description de l’état des gaz de fission, à l’échelle micrométrique et à fort taux de combustion, grâce à de nouvelles campagnes expérimentales de caractérisation et des méthodologies, améliorées ou nouvelles, d’acquisition, de traitement et d’analyse des données qui en résultent. Ces campagnes ont été menées en laboratoire de haute activité, avec différents équipements de microanalyse. Deux types de combustibles ont été examinés, avec des tailles de grains initiaux différentes et des taux de combustion différents. L’étude a été menée selon trois axes : la morphologie des bulles, grâce à des examens 2D et 3D au MEB-FIB, les évolutions microstructurales, grâce à des cartographies EBSD, et la quantification des gaz afin d’estimer leur densité dans les bulles, en combinant microsonde, SIMS et MEB-FIB. Ces travaux ont permis d’établir de nouvelles méthodologies d’analyse des gaz de fission et de la microstructure. Les résultats ainsi obtenus ont mené à la conception d’une représentation synthétique de l’état des gaz, selon la position radiale, le taux de combustion et la microstructure initiale du combustible. Ce travail va permettre d’enrichir, d’alimenter et de valider les codes de calcul de modélisation du comportement du combustible UO2 à fort taux de combustion.During the irradiation of nuclear fuel pellets, fission reactions lead to a progressive accumulation of new atoms, some of which are gaseous. These fission gases, and the bubbles they form, contribute significantly to the fuel’s behavior, whether during operation in nominal conditions or in incidental or accidental cases. This Ph.D. work provides a better description of the fission gases’ state at a micrometer scale of high burn up PWR UO2 fuels, thanks to experimental characterization campaigns and improved or new methodologies for data acquisition, processing and analysis. These campaigns were carried out in a high activity laboratory (LECA-STAR), with different microanalysis devices. Two types of fuels were examined, with different initial grain sizes and different burn-ups. The study was conducted along three main axes: morphology of the bubbles, thanks to 2D and mainly 3D FIB-SEM examinations, the microstructural evolutions, thanks to EBSD characterizations, and the quantification of the gases in order to estimate the pressure of the fission gases in the bubbles, by combining microprobe analysis, SIMS and SEM-FIB measurements. This work has allowed to establish new methodologies for fission gas and microstructure analysis. The combination of the results obtained in these different areas has led to a synthetic representation of the gas state, as a function of the radial position, the burn-up, and the initial microstructure of the fuel. This work will allow to enrich, feed and validate the calculation codes for the modeling of the UO2 fuel behavior at high burn-up

Restructuring in high burn-up pressurized water reactor UO2 fuel central parts: Experimental 3D characterization by focused ion beam—scanning electron microscopy

International audienceFocussed ion beam - scanning electron microscope (FIB-SEM) 3D examination was conducted on three standard UO2 and one Cr doped UO2 high burn-up pressurized water reactor (PWR) fuel samples. This work complemented other microanalysis examination, including an electron backscattered diffraction (EBSD) work on the polished surface. A parallel article giving the EBSD results was submitted simultaneously. Together, they found, in all the central area of these high burn-up samples: (i) a restructuring of the initial grains into smaller sub-grains forming low angle boundaries and with crystal orientations around that of their parent grains; and (ii) intragranular bubbles mostly situated on these low angle boundaries. The FIB-SEM 3D examination showed how such inter-sub-grain bubbles start as small compact but also small lenticular bubbles, similar to typical small intergranular lenticular bubbles. With increasing burn-up, these lenticular bubbles get thicker and locally interlink to form more complex bubbles. However, no long distance networks, between the sub-grains or between the original grains, were found. Such networks could have been a path for part of the fission gases to reach the grain boundaries, the grain edges (the intersection line of three grain boundaries), and the rod free volumes. These FIB-SEM 3D examinations brought details on the intragranular and intergranular bubbles situation for each studied volume. The distribution of the intragranular bubbles according to their sizes and shapes was exposed. The central restructuring, studied in this work, is likely to play a role in the increase of the fission gas release fractions at high burn-up. This work is an incentive to study further this restructuring and the bubbles formed, combining different approaches