Atom probe tomography of Zr-alloys using in nuclear fuel cladding
Résumé
Zirconium-based alloys are used for nuclear fuel cladding and act as the first containment barrier of the fission product tubes because of their low thermal neutron absorption cross section and their good mechanical properties. During in-service operation in water reactors, the Zr-based cladding tube is subject simultaneously to oxidation, hydrogen pick-up and to neutron irradiation. These phenomena have consequences on the microstructure and affect the alloy properties during its service lifetime. This presentation will give an overview of different microstructure evolutions that can occur on Zr-alloys and particularly by investigating the distribution of several alloying elements by atom probe tomography. Firstly, we will focus on the influence of the hydrogen pick-up coming either from the metal oxidation that reduces water and liberates atomic hydrogen or directly from the fuel. A consequence of this increase in hydrogen concentration is the precipitation of zirconium hydrides and these hydrides are known to be mechanically brittle. To deepen our understanding of the formation mechanism of hydrides in this system, we investigate by atom probe tomography different samples as Zr-Sn alloys exhibiting large grains microstructure and hydrogen/deuterium charged with particular attention to the grain boundary. In addition to this study, a technological solution to decrease the Zr corrosion, both in nominal and hypothetical accidental conditions at high temperature will be presented. Indeed, it has been shown that the deposition of a thin Cr coatings on Zr-based nuclear fuel claddings provide a significant reduction in the oxidation-induced embrittlement of the nuclear fuel cladding, especially upon accidental conditions (and therefore decrease the associated gaseous hydrogen production). To guarantee the successful adhesive strength of the Cr coatings, the interface between Cr and Zr has to be finely characterized via atom probe tomography. In the last part, evolution of the microstructure and the micro-chemistry in the Zr-Nb alloy under irradiation will be presented. The results will focus on the chemical investigation of the Zr-matrix, beta-Nb precipitates, Laves phases, radiation-enhanced precipitates, solute dissolution from the precipitates into the matrix or potential segregation related to -component loops. These APT observations are correlated with TEM results obtained on the same materials in order to give a complete overview of the microstructural evolution under irradiation.