Oxygen, Hydrogen and Main Alloying Chemical Elements Partitioning Upon Alpha←→Beta Phase Transformation in Zirconium Alloys

Abstract

Due to their adequate properties, zirconium alloys are the reference materials for the nuclear fuel cladding tubes of Light Water Reactors (LWR). During some hypothetical accidental High Temperature (HT) transients, the materials should experience heavy steam oxidation and deep metallurgical evolutions. This promotes Alpha-Beta phase transformations and an associated strong partitioning of oxygen/hydrogen and of the main chemical alloying elements (Nb, Sn, Fe and Cr). Moreover, it has been shown quite recently that such chemical elements partitioning during on-cooling Beta-to-Alpha transformation can strongly impact the residual mechanical properties of HT oxidized materials. Thus, it appeared that it was important to better quantify and, if possible, to compute the quite complex phase equilibrium that occurs in multi-alloyed zirconium materials in the presence of both oxygen and hydrogen. For that, systematic studies have been performed on industrial alloys, charged with oxygen and/or hydrogen. After applying different heating/cooling scenarii, both Electron Microprobe using Wave Dispersive Spectrometry (WDS) and Nuclear Microprobe using Elastic Recoil Detection Analysis (ERDA) have been applied. Finally, to support the observed chemical elements partitioning between the Alpha and Beta allotropic phases, some thermodynamic calculations have been performed thanks to the development and the use of a specific thermodynamic database for zirconium alloys called “Zircobase".

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

“…Chemical microanalysis was performed by EPMA on transverse cross-sections of Zircaloy-4 and M5Framatome samples oxidized 3270 and 7500 s at 1000°C and then quenched. In the case of Zircaloy-4, iron and chromium were mainly concentrated into the prior-βZr layer and the concentration of tin was higher in the αZr(O) than in the prior-βZr layer, as expected after oxidation at 1000°C (but not for higher oxidation temperatures) as discussed in [34]. Fig.…”

Cladding burst behavior of Fe-based alloys under LOCA

“…Therefore separate creep rates, in the distinct α, α+β, and β regions need to be considered for accurate consideration of cladding response. Furthermore, the alloying elements [17], hydrogen and oxygen content in the alloy [18,19], and the alloy microstructure [20] all affect these creep rates.…”

Phase transformations during cooling from the βZr phase temperature domain in several hydrogen-enriched zirconium alloys studied by in situ and ex situ neutron diffraction

“…As shown in Section 3.1.4, there is a microchemical partition between the pro-eutectoid α Zr phase and the β Zr phase during the on-cooling β Zr to α Zr transformation. Note that no significant partitioning of tin is expected in Zircaloy-4 for the conditions studied here [50]. .…”