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.