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.
The main objective of this paper is to describe the effects of a long service exposure of the PWR fuel cladding tubes on their thermal-mechanical properties during and after a hypothetical LOCA transient. Within this prospect, specific studies have been performed: on one hand, thermal ramp tests under uniaxial stress loading on as-received, pre-hydrided and irradiated samples of Zy-4 and M5™, and on the other hand, mechanical tests after high temperature oxidation and quench on as-received and prehydrided Zy-4 and M5™. The main conclusions are: (1) In service, hydrogen pick-up impacts the thermal ramp behavior under uniaxial stress loading of cladding tubes upon the first phase of LOCA transient, and the effect of the irradiation defects can be ignored for the conditions tested. (2) As-received Zy-4 and M5™ achieve comparable post-quench mechanical behavior at 1000, 1100, and 1200°C for representative times of LOCA transient (1800 s). (3) Oxygen has the most important effect in embrittlement of as-received Zr alloys at 1100 and 1200°C. (4) Hydriding has no effect on the oxidation kinetics at 1200°C, but after quenching the hydrided materials become brittle at a lower weight gain than as-received materials; this embrittlement is due both to an intrinsic hydrogen-embrittlement effect and to the oxygen content increase resulting from the effect of hydrogen on its solubility in prior-Beta phase.
Functions that can be allocated to cladding during interim storage depend on the evolution of cladding properties with time. The fuel rod cladding is strained by the end of-life internal fuel rod pressure (40–60 bars NTP) and the potential release of fission gases and helium during dry storage. Within the temperature range that is expected during dry interim storage, long-term creep under over-pressure of the cladding might probably be a relevant strain mechanism, which could lead to breaching. Creep experiments were carried out on 4 cycles irradiated Cold Worked Stress Relieved (CWSR) Zircaloy-4 cladding under internal pressure within the temperature range of 470–520°C for up to 10 days, in order to estimate the eventual effect of a transient period at higher temperature on creep behavior during storage. Therefore, some of the tests consist of two periods: the first period at high temperature (470°C), followed by a second period at a lower temperature (320–400°C). A metallurgical characterization (TEM, optical microscopy) was carried out after the tests. A significant impact of the stress level is observed at the temperature of 470°C on creep strain. Tertiary creep is reached after a few days for 100 or 120 MPa. The effect of a first period at 470°C on the next creep behavior at 400°C for 150 MPa is confirmed. The probable induced annealing of irradiation defects contributes to increase the secondary creep rate at 400°C. Moreover, the creep kinetics of the tests conducted to rupture show in all the cases a ductile rupture with ballooning instability, which might be partially the result of an annealing of irradiation defects. The microscopic characterization confirms the hypothesis of a partial annealing of the irradiation defects after a period of 10 days at 470°C, which leads to a microstructure intermediate between irradiated and as-received CWSR condition, while a temperature of 520°C leads to a microstructure that looks like recrystallized Zircaloy-4 condition. After the creep test, hydrides morphology, distribution, and orientation appear rather different from usual post-irradiation hydrides characteristics. The hydrides are distributed uniformly throughout the thickness of the tube. A cooling under mechanical loading influences the hydrides precipitation, particularly by leading to a radial “reorientation.” A stress level of 80 MPa during cooling is sufficient to lead to radial hydrides formation as hydrogen precipites in the cladding.
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