To simulate the end-of-life behavior of cladding tubes during the first phase of a LOCA transient, one may assume that the main effect of a long service exposure on the cladding deformation behavior during LOCA arises from the hydrogen uptake associated with the cladding oxidation at high burn-up. Thus, the recent metallurgical studies and EDGAR [2] tests performed on pre-hydrided Zr-base alloys are presented. The influence of hydrogen has been studied for concentrations ranging from ∼ 100 up to ∼1000 (weight) ppm on FramatomeANP low-tin Zy-4, M4 (ZrSnFeV), and M5™(ZrNbO) alloys. The decrease of the α/β phase transformation temperatures with the increase of the hydrogen content is noticeable and has been quantified, and then modeled, for both quasi-equilibrium (calorimetry) and dynamic (dilatometry) conditions for heating rates up to 100°C/s. Some complementary microstructural examinations on hydrided samples, beforehand partially transformed into β phase, have been performed to get a better insight of the metallurgical features associated with the hydrogen effects. Finally, the EDGAR thermal-mechanical test results are presented and discussed. The alloys have been tested under steady state conditions of pressure and temperature, on the one hand, and with continuous heating (thermal ramps) on the other. The results show that the mechanical behavior cannot be explained solely by the effect of hydrogen on the shift of the α/β phase transformation temperatures, but that hydrogen modifies also the creep behavior and the burst criterion, especially in the a domain, and in the lower α+β temperature range. As a result, hydrogen decreases the creep strength and the ductility of the materials, the effect being greater for higher hydrogen content. All these data are used to model the thermal-mechanical behavior of the hydrided cladding tubes in order to simulate the LOCA behavior of the clad after long-term in-service exposure. Finally, preliminary thermal ramp tests under uniaxial loading performed on irradiated Zy-4 are presented and compared to the behavior of non-irradiated as-received and hydrided Zy-4. These last experiments were made to validate the assumption that the main effect of a long service exposure on the cladding deformation behavior during the first phase of LOCA is mainly linked to the hydrogen uptake associated with the cladding oxidation.
It is now well acknowledged that, after a prototypical loss of coolant accident (LOCA) transient, the resultant mechanical properties of fuel cladding tubes depend strongly on the oxygen content of the residual prior-β layer, as this phase is the only metallic part of the high-temperature oxidized cladding that may show some residual ductility. The aim of this study is to obtain relevant information on the evolution of the mechanical properties, on the one hand, of the prior-β structure as a function of the oxygen content, assuming that there is a critical oxygen content that leads to a ductile-to-brittle failure mode transition at low testing temperatures (20–135°C); and on the other hand, of the α(O) structure as a function of the oxygen content. Sheets of Zircaloy-4, 1 to 3 mm thick, and M5®M5® is a registered trademark of AREVA-NP. advanced alloys from AREVA NP have been studied. To obtain different oxygen contents, they were oxidized at high temperature and then annealed under vacuum in order to reduce the oxide layer. Systematic post-treatment measurements of the oxygen concentration and of its homogeneity within the sheet thickness were performed. The different prior-β and α(O) structures thus obtained have homogeneous oxygen content between ∼0.14 wt. % and 0.9 wt. % and ∼2 wt. % and 7 wt. %, respectively. Such oxygen concentration ranges cover the solubility values that are expected in the β phase and in the α(O) phase at high temperatures typical of LOCA transients. Detailed microstructure investigations were subsequently performed on the prior-β structures since it is considered to be the most important layer when regarding the post-quench mechanical behavior of the material. Continuous cooling temperature (CCT) phase diagrams as a function of the oxygen content were established to correctly interpret the results. Electron backscattered diffraction (EBSD) analysis has then allowed the crystallographic orientations and the morphology of prior-β phase sub-grains to be determined. For each considered prior-β grain, it was possible to interpret the data by taking into account the “Bürgers” crystallographic relationship between the parent β phase and the resultant α phase. Complementary electron probe microanalysis (EPMA) was also used. These last experiments have shown a spatial fluctuation of the oxygen content within the microstructure that depends both on the nominal oxygen content and on the cooling rate. Nanohardness measurements were also performed and correlated with this oxygen spatial partition. These measurements proved to be useful for the understanding of the tensile macroscopic mechanical behavior. Finally, on the one hand, tensile tests were performed on prior-β phase at testing temperatures ranging from −100°C up to 260°C. The ductile-to-brittle temperature transition and the mechanical constitutive laws as a function of the oxygen content were then described. These tests show the existence of a ductile-to-brittle failure mode transition at 20°C for a critical oxygen concentration of ∼0.5 wt. %. A detailed fractographic analysis was performed to assess the failure mechanism. On the other hand, four-point bending tests were conducted on α(O) phase at 25°C and 135°C in order to obtain behavior laws. Preliminary finite element calculations were performed to simulate ring compression tests carried out on multi-layered high-temperature oxidized cladding tubes.
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