Secondary hydriding of zirconium-based fuel claddings at high temperature (LOCA conditions). Part 2: Effect of high hydrogen contents on metallurgical and mechanical properties. Part 1: Multi-scale study of hydrogen distribution

T. Le Hong (1) , Matthieu Le Saux (2) , J.-C. Brachet (1) , J. Crepin (3)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
2 SRMA - Service des Recherches Métallurgiques Appliquées
3 CDM - Centre des Matériaux

During oxidization in steam at High Temperature (HT) under hypothetical Loss-Of-Coolant Accident (LOCA) conditions, zirconium alloy fuel cladding tubes can absorb, in some conditions (e.g. after burst occurrence and during subsequent HT steam oxidation), an important amount of hydrogen, up to thousands of wt.ppm locally. The behavior of the cladding can be affected by this secondary hydriding since hydrogen is known to modify metallurgical and mechanical properties of zirconium alloys. The purpose of this study is to refine the description of secondary hydriding and to improve both experimental database and understanding of the effect of high hydrogen contents up to 3000 wt.ppm, on metallurgical and mechanical properties of zirconium alloys, during and after cooling/quenching from high temperatures ($\beta _{Zr}$ domain). In a first part, we will illustrate a recent multi-scale analysis of hydrogen partitioning within Zr-based claddings having experienced secondary hydriding at HT. The objective was to map quantitatively, at different scales, the spatial distribution of hydrogen (and oxygen) within M5 clad segments having experienced ballooning and burst at HT followed by steam oxidation at 1100 and 1200DC, and final direct water quenching down to Room Temperature (RT). This was done by coupling information from several advanced analytical techniques available at CEA Saclay neutron radiography/tomography, electron probe micro analysis, micro elastic recoil detection analysis, and micro laser induced breakdown spectroscopy. Finally, the hardness of the prior-$\beta _{Zr}$ structure was correlated to the local oxygen and hydrogen contents, reaching locally 1-1.2 wt.% and 3000-4000 wt.ppm, respectively. In a second part, we will illustrate a recent study performed on model materials, homogeneously charged in hydrogen at ~1000-3000 wt.ppm, produced from Zircaloy-4 and M5 cladding tubes. Samples were also prepared from nearly-pure Zr (Van Arkel process) in order to isolate the effect of hydrogen. The samples were heat-treated at ~1000DC to characterize the properties of the transformed $\beta _{Zr}$ phase and were then cooled down to RT, rapidly (direct water quenching) and slowly to study the potential effects of cooling scenario. Neutron and X-ray diffraction analyses were realized on these model materials to study their structure as a function of chemical composition of the alloy, hydrogen content and cooling scenario. The results were compared to thermodynamic equilibrium predictions using the Thermo-Calc software with the Zircobase thermodynamic database. The mechanical properties of the (prior-) $\beta _{Zr}$ structure were investigated by performing uniaxial tensile tests at various temperature between 700DC and 20DC upon cooling from the $\beta _{Zr}$ temperature domain, on model materials prepared from Zircaloy-4 tubes containing various hydrogen contents, up to 3000 wt.ppm. Material microstructure and fracture surface of the tested specimens were observed in order to identify the failure mechanisms.

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

T. Le Hong, Matthieu Le Saux, J.-C. Brachet, J. Crepin. Secondary hydriding of zirconium-based fuel claddings at high temperature (LOCA conditions). Part 2: Effect of high hydrogen contents on metallurgical and mechanical properties. Part 1: Multi-scale study of hydrogen distribution. 23rd QUENCH workshop 2017, Oct 2017, Karlsruhe, Germany. ⟨cea-02417311⟩