Modelling of the stress fields in oxide pellets, in order to prevent the formation of surface cracks
Résumé
Nearly all the nuclear fuel presently used in commercial power reactors (especially PWR, BWR and SFR ones) is handled under the shape of oxide pellets, and manufactured by die pressing and then sintering.The requirements related to the manufacturing and use of these pellets include the absence of cracks and other surface defects. These cracks may originate from the die pressing and die ejection, or during the sintering stage. Millions of pellets are routinely fabricated each one undergoes surface condition checks in order to verify the absence of surface flaws. Thus, it is very important to understand how the powder condition (homogeneity, grain and aggregate size) and the pressing parameters (tool dimensions, kinetics of the moving parts, tool wear) can influence the surface conditions of the pellets. To help this understanding we develop a numerical tool to forecast the levels of stresses (which creates and open these cracks), depending on these parameters.We use the Cam-Clay model which is implemented in Cast3M software to model a complete die pressing and ejection sequence. The powder properties are deduced from prior instrumented pressings; the cohesion change which occurs during the pressing is taken into account.We calculate the order of magnitude of the stresses at the surface of alumina pellets, these stresses coming fromThe die pressing and ejection sequenceThe sintering stage, supposing an homogenous powderThe sintering of microscopic inclusions in the pellets, whose density differences induce local stresses.With the help of a straightforward sintering model (sintering to a homogenous target-density), we can calculate the new stress fields that occur in the sintered pellet. This is done with two very different start hypotheses, i.e. single-phase powder, or two-phase powder (typical of mixed-oxide fuel). Two-phase powder induces microscopic inclusions and density inhomogeneities in the compacted pellet, which leads to extra (and often detrimental) stresses.We recap the link which can be established (with the classical rupture theory) between the surface stresses, and the likelihood of crack opening, by the consideration of typical grain-size flaws.