Effect of surface roughness on the texture and oxidation behavior of Zircaloy-4 cladding tube
Introduction
Fuel cladding tube is one of the most critical parts in a nuclear reactor's core. To avoid any possible accidents, it should be manufactured in a very strict clearance and have high structural integrity. Conventional pressure water reactors like CANDU use Zircaloy-4 as a cladding tube. These tubes are manufactured by a cold extrusion-like process called pilgering. In this process reciprocal movement of grooved dies with a static mandrel forms the tube. Thus, the wall thickness and the inner diameter reduce progressively. Abe and Furugen [1] described and evaluated the workability in pilgering process by performing systematic pilgering tests and establishing plastic deformation state during the process. Furugen and Hayashi [2] carried out a comprehensive study on pilgering process and determined stress strain and roll separating forces during pilgering using the theory of plasticity. Montmitonnet et al. [3] carried out a rigorous 3D elastic-plastic simulation of cold pilgering of Zircaloy tubes. They emphasized that the pigering process is a cold deformation process where a complex stress system is applied to the tube. In such a process surface conditions (e.g. roughness) would be affected during consecutive multi-pass processing routes. Since the final tube has a relatively rough surface further polishing is required in order to get a bright and smooth surface. This polishing treatment can change the substrate texture thus affecting the formation of an oxide layer, particularly in the nucleation stage. On the other hand, increasing/decreasing surface area by rough/fine polishing will directly change the oxidation kinetic, which can subsequently affect the hydrogen ingress and hydrides precipitation. As zirconium hydrides are brittle phases that adversely affect the mechanical properties, polishing treatments may have considerable effects on the service performance of the Zircaloy-4 cladding tubes.
To date, there is no in-depth study of surface polishing effects on the properties of the cladding tubes. The literature shows that researchers mostly agree about the general effects of surface roughness such as its influence on oxidation kinetic, its affinity to react with contaminants, coolant flow and fluid turbulence on the studied surface. Huntz et al. [4] studied the effect of surface roughness on Ni oxidation. They showed that the surface roughness can affects the oxidation, particularly at initial stages. The effects of thermohydraulics are not in the scope of this paper, but have been studied elsewhere. Guillen and Yoder [5] studied the thermal hydraulic effect of fuel plate surface roughness with analytical approaches which followed by simulation. They claimed that if the fuel surface roughness exceeds 1.5 μm, the coolant flow rate drops. This will increase the fuel temperature dramatically. They emphasized that the order of magnitude of surface roughness is very decisive in fuel temperature, coolant flow rate and temperature. Krogstadt and Antonia [6] stated that surface profile significantly affects the flow turbulence. Despite these thermohydraulic reports, there are few studies that address the effects of surface roughness on metals oxidation. Uran et al. [7] studied the influence of surface roughness on the oxide thickness and residual stress on Fe–Cr–Al alloys. They found that surface roughness can alter the sequence of oxidation of the alloying elements. In another study, Evans [8] reported that by increasing surface roughness, weight gain increased. However, zirconium is a different case. Oxidation of zirconium and its alloys will form a surface layer of zirconium oxide. It is well known that zirconium oxide, known as zirconia, has three different allotropes: monoclinic, cubic and tetragonal. Monoclinic and tetragonal allotropes are stable below and above 1000 °C respectively. Therefore, in the air and in-reactor oxidation regimes, monoclinic and tetragonal oxides are more pronounced. The amount of tetragonal phase has a significant effect on the oxidation behavior; however there are controversial data in the literature on this effect. While some studies report a higher oxidation rate and lower protectiveness for the oxide with higher amounts of tetragonal phase, other researchers report that lower amounts of tetragonal phase will result in a more protective oxide layer. Lin et al. [9] analyzed the zirconium oxide formed on Zircaloy-4 and Zr–2.5Nb during heavy water oxidation. They showed that higher tetragonal percentage leads to better oxidation resistance. Yilmazbayhan et al. [10] studied the corrosion behavior of zirconium alloys formed in pure water. They found that the samples with higher tetragonal fraction had higher corrosion rate. Qin et al. [11] stated that many complicated factors like tetragonal percentage, compressive stress near the interface, internal stress induced by tetragonal to monoclinic transformation can affect the oxidation resistant. However, researchers agree that the tetragonal phase has an effect on the protectiveness of the oxide layer. Since the zirconia layer works as a protective barrier against hydrogen ingress and further oxidation, the structure of zirconium oxide is important. Despite many research papers about the zirconium oxide structure, many questions still remain open.
There is no detailed study of the effects of surface condition on oxidation resistance and oxides allotropies of zirconium alloys. Surface conditioning could be used to modify an industrial processing route if it could improve the oxidation resistance of fuel cladding. It could also be applied at the final stage of manufacturing without changing tube-forming processes. Because the focus of this research is on the oxidation kinetic and oxide structure (texture and allotropies), air oxidation tests were conducted in order to limit the hydrogen ingress and hydride formation effects and also to characterize the air oxidation kinetic.
Section snippets
Experimental procedure
We used Zircaloy-4 cladding tubes with the composition of 1.45%wt Sn, 0.24%wt Fe, 0.13%wt Cr, 0.1%wt O, and balance Zr. In order to explore the effects of surface roughness, we first characterized various surface conditions and then oxidized the material. Table 1 illustrates the conditions of the samples, which include both, tube that was unpolished and tube with its final surface finished after the pilgering.
The other sample (G) was polished in our laboratory using 60 grit SiC abrasive paper.
Surface profile and roughness
Table 2 illustrates the measurement of various surface roughness parameters. All the reported numbers are the average of five measurements on different regions of the samples. Ra and Rq are the arithmetic average and root mean square of surface asperities and valleys, respectively. Rsk (Skewness) shows the number of asperities and valleys regarding the base surface profile, while Rku (Kurtosis) shows the sharpness of these asperities and valleys. Rsk is negative if the surface has more valleys
Texture
As shown in previous sections, after a certain time the weight gain and oxidation rate for the samples with different surface roughness changed in a similar way. In other words, the effect of surface roughness is significant mostly for short oxidation times. Such conclusion can also be derived from the results presented in Fig. 4.
In this section we discuss the effect of surface roughness on oxide texture. We conducted oxidation tests on sets of Zircaloy-4 tube samples at 700 °C for 1, 3, 5, 8,
Conclusion
We studied surface roughness effects on the oxidation in air and steam at specific temperatures. We observed some slight differences regarding oxidation kinetics and weight gain. In almost all tests the ground (roughest) sample had the highest weight gain. The surface roughness effect was more pronounced for the initial weight gain and oxidation rate while becoming less important for longer oxidation times. This means that, at the early stages of the oxidation, the higher the Zircaloy-4 surface
Acknowledgments
The authors express their gratitude to Cameco Corporation for their encouragement and kind support in the present investigation. The authors also would like to acknowledge Cameco CRD-NSERC program. Valuable discussions with Farhad Fathieh are greatly appreciated.
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