The behavior of PWR fuel rod cladding under LOCA conditions strongly depends on the interactions between the metallurgical evolution (phase transformation) and the thermomechanical behavior at high temperature. FRAMATOME needs for the qualification of advanced zirconium alloys have motivated an extensive study of the phase transformation kinetics and the development of a new test facility, EDGAR-2, to perform thermomechanical tests. The first part of the paper deals with an experimental study and modeling of the α↔β phase transformation kinetics. High-temperature/high-sensitivity calorimeter and fast dilatometer facilities have been used to measure the on-heating α⃗β and on cooling β⃗α phase transformations from near equilibrium conditions up to LOCA conditions (heating-cooling rates up to 100°C/s). The equilibrium fraction of α/β phase as a function of temperature is derived from calorimetric measurements using the Zhu and Devletian model and described using a modified Johnson-Mehl-Avrami equation. Modeling of the α↔β kinetics is given by the differential Holt equation. The present results show that the Holt model gives a good description of the α⃗β kinetics upon heating but is not able to describe accurately the β⃗α phase transformation upon cooling, probably because it does not take into account the occurrence of the partial martensitic transformation during cooling, especially for the higher cooling rates. An important feature of the current study is that, despite the α/β equilibrium temperatures for M4 (ZrSnFeV) and M5 (ZrNbO) alloys being lower than that of Zy-4, for kinetic reasons the α to β phase transformation occurs in the same temperature range for the three alloys in the case of fast thermal transients (i.e., 10°C/s). This observation could be related to the slower thermal diffusion of Nb and V atoms compared to that of Fe and Cr. This slower diffusion explains why the thermalmechanical behavior of new M4 and M5 alloys is quite equivalent to Zy-4 in LOCA conditions. The second part of this paper deals with the thermomechanical tests. The EDGAR-2 test facility performs single rod tests under any internal pressure and clad temperature transients in steam environment. The advanced zirconium alloys M4 and M5 have been tested under steady-state conditions of pressure and temperature, continuous-heating and constant-pressure conditions for various heating rates and pressure, and some LOCA representative pressure temperature transients. The experimental program covers the LOCA conditions, and the rupture occurs from a few seconds up to 2000 seconds. The results are compared with those of the SRA optimized Zy-4 clad using the Monkman-Grant correlation, rupture ductility versus burst temperature, and burst criteria. EDGAR models for the prediction of the evolution of the transformed β-phase volume fraction and of the diametral deformation, time to rupture, and uniform diametral rupture elongation in typical LOCA conditions are derived for advanced alloys from the two parts of this study. These models may easily be implemented in most accident simulation computerized codes for safety analysis.
Within the scope of the optimization of the M5 cladding tubes made of ternary alloy (Zr, Nb, O), an extensive program of investigation and industrial development has been undertaken. The various possible factors and potential causes of variability have been thoroughly analyzed with the aid of industrial-scale or laboratory ingots from the point of view of their impact on the finished product properties. In this way, all the chemical composition variabilities of the alloying elements (Nb, O) and impurities (Fe, S, C) have been studied through variable-composition ingots. Also, a number of manufacturing process variants (number of melts, quench, extrusion, heat treatments, pilgering …) have been studied. In some cases, it was possible to investigate the combined effect of two types of parameters (sulfur-process and iron-process interactions). For each of the products manufactured in this way, systematic characterization of: creep, microstructure (optical microscopy, TEM), corrosion (autoclave) tests was accomplished. In each case, the influence of each variability parameter was tested, and in many cases correlations with the out-of-pile characteristics of the finished tubes were established. Lastly, for some variables (process, S content, …) the effect of irradiation was more specifically analyzed. These investigations pointed to a new and very important factor, the effect of sulfur concentration on the in-pile operating properties, especially creep and growth. This set of results constitutes a database covering the whole industrial variability range of this alloy, allowing Framatome to embark upon its industrial development phase and to offer M5 cladding tube on the market. This product has been irradiated over a wide range of PWR service and environmental conditions in Europe and the U.S. The improvements in corrosion (Factors 3 to 4), hydriding (Factors 5 to 6), and creep and growth (Factors 2 to 3) data after five cycles (55 GWd/tU) show impressive gains over optimized low-tin Zircaloy-4.
Previous papers pointed out the influence of long-term service exposures on the thermal-mechanical behavior of Zr alloys in LOCA conditions and, especially, the impact of in-service hydrogen pick-up on post-quench mechanical properties. Moreover, the oxide layer grown under in-service conditions was occasionally expected to have a protective effect against high temperature oxidation. Finally, the oxygen and hydrogen distributions within the prior-β layer appear as a key parameter with regard to the residual ductility of the alloy, especially as a function of the cooling scenario. The objective of the study presented here was to further investigate the influence of these parameters on the post-quench mechanical properties. Unirradiated Zircaloy-4 and M5® cladding tubes were consequently hydrided up to different concentration levels, then oxidized at high temperature (1000–1200°C) up to at least 10 % measured equivalent cladding reacted (ECR) and directly quenched to room temperature (RT). Ring compression tests (RCT), 3-point bending tests (3PBT) at RT and 135°C, as well as impact tests at RT were then performed to determine the evolution of the post-quench mechanical properties of Zircaloy-4 and M5® alloys with H content. Similarly, specimens preoxidized out-of-pile were also submitted to high temperature oxidation and direct quench, as well as to post-quench ring compression tests. Along with calculations of oxygen diffusion in the metal, results from those tests allowed us to estimate the assumed protective effect of the pretransient oxide layer. Finally, using specimens in the as-received condition or hydrided to typical end-of-life H contents, the effect of temperature history after oxidation at 1200°C was studied, i.e., at the end of the high temperature isothermal oxidation, samples were either submitted to direct quenching to RT or to slow cooling to different final quenching temperatures. It was thus demonstrated that the cooling scenario has a significant impact on the post-quench mechanical properties. All test samples were investigated by means of fractographic examinations to assess the type of failure mode. Moreover, a deep metallurgical analysis has been performed: SEM and image analysis were used for accurate phase thickness measurements, nuclear and electron microprobes for quantitative mapping of hydrogen and oxygen. It proved that the oxygen and hydrogen contents and their distribution in the prior-β layer have a first-order influence on the residual ductility. From all the results obtained on as-received and hydrided samples directly quenched from the oxidation temperature, it was then possible to derive a relationship between structural parameters, i.e., oxygen and hydrogen contents and thickness of the prior-β layer, and the post-quench impact properties at RT.
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.
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.
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