Oxidation Behavior and Mechanical Property of Cr ...

Proceedings of WRFPM 2014 Sendai, Japan, Sep. 14-17, 2014 Paper No. 100054

Oxidation Behavior and Mechanical Property of Cr-coated Zirconium Cladding prepared by 3D Laser Coating Hyun-Gil Kim*, Il-Hyun Kim, Yang-Il Jung, Dong-Jun Park, Jung-Hwan Park, Jeong-Yong Park, Yang-Hyun Koo Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong, Daejeon, 305-353, Republic of Korea Tel. +82-42-868-2522, Fax. +82-42-862-0432, E-mail: [email protected] ABSTRACT: To decrease the high-temperature oxidation rate of zirconium cladding, a coating technology for a zirconium cladding surface was applied in this study. Cr was selected as a coating material on the surface coating of commercial zirconium cladding. A 3D laser coating technology supplying Cr powder was selected as a coating method, as this method can be applied to a long tube shape without high vacuum and high-temperature annealing treatment during the coating process. Many parameters such as the laser beam power, inert gas flow, and Cr powder supply were studied to obtain a good coating layer property. After coating of Cr on the zirconium cladding surface, the coated samples were evaluated for the microstructural characteristics for a cross-sectional direction, and evaluated for the mechanical properties for the hoop direction. The coated layer properties such as the crack, pore, and homogeneity were intensively inspected using SEM after the various parameter studies during the 3D laser coating to make a soundness-coated layer on the zirconium cladding. The adhesion property of the Cr-coated layer was evaluated from the mechanical testing of compression and tensile tests for the ring shaped cladding samples. In addition, the LOCA simulated test was performed for the partially Cr-coated zirconium cladding tubes to identify the oxidation and adhesion performances. From this result, it is known that a Cr-coated layer by 3D laser coating successfully acts as a corrosion barrier layer to resist the high-temperature oxidation, and is shown to have a good adhesion property on the zirconium cladding. KEYWORDS: accident, fuel cladding, 3D laser coating, oxidation, LOCA

I. INTRODUCTION From the Fukushima accident, a hydrogen explosion is one of the major impact factors on reactor safety during the loss of coolant accident (LOCA) in LWR’s. Hydrogen is produced by the oxidation reaction of Zr-based alloy components such as cladding, a guide tube, and a spacer grid in a fuel assembly, and that acts as an explosion source. Thus, a decrease in the oxidation rate of Zr-based alloys at high-temperature is a key issue to improve the accident tolerance of fuel assembly. At present, the development of accident tolerant fuel (ATF) is a major concern in the nuclear research fields in the present time 1-7). The research concepts for enhanced ATF cladding development consists of Mo-Zr cladding 2), cladding coating 3, 4), iron-based alloy cladding 5, 6) , and SiC/SiCf cladding 7). As a mid-term application the coating and iron-based alloy, and Mo-Zr claddings can be considered, whereas the SiC cladding is considered a long-term application. The major benefit of cladding coating is the economics because the commercial Zr-based alloy and the manufacturing utility can be continuously used among the concepts for the mid-term application. In addition, the cladding coating on Zr-based alloys has a high melting point, high neutron economy, and high tritium permeation when compared to the iron-based alloy cladding. However, the challenge of the cladding coating on Zr-based alloys is a lower oxidation resistance and mechanical strength at higher temperature than the iron-based alloy, as well as the technical issues regarding the adhesion property between the Zr-based

alloy and the coated materials. A coating technology has been widely used in the industrial material improvements to increase the corrosion and wear resistances, as those resistances can easily use a coating without a change in the base materials. In the ATF, it was reported that the weakness in the corrosion and oxidation resistance of Mo-alloys is addressed using an outer layer coating of Zr-alloy or Al-containing stainless steel 2). The Cr alloy coating prepared by physical vapor deposition (PVD) is able to drastically reduce the high-temperature steam oxidation rate and prevent massive hydriding during high temperature oxidation 3). On the other hand, the complex coating technology, which was composed of a plasma spray and laser beam scanning, was applied to make a stable Si coated layer on Zr-based alloy up to 1200oC 4). However, a complex technology cannot be recommended because the manufacturing cost will be increased and the quality control of the product is difficult. Thus, the development of a simplified coating technology is necessary in the present time. As a new coating method, a 3D laser coating technology supplying Cr powders is considered to make a coated cladding because it is possible to make a coated layer on the tubular cladding surface by controlling the 3-diminational axis in this study. We are systematically studying the laser beam power, inert gas flow, cooling of the cladding tube, and powder control as key points to develop 3D laser coating technology. After Cr-coating on the Zr-based cladding, ring compression and ring tensile tests were performed to evaluate the


adhesion property between a coated layer and Zr-based alloy tube at room temperature (RT), and a high-temperature oxidation test was conducted to evaluate the oxidation behavior at 1200oC of the coated tube samples.

II. EXPERIMENTAL PROCEDURES 1. Coating Materials A commercial Zircaloy-4 (Zr-1.5Sn-0.2Fe-0.1Cr in wt.%) cladding tube was used as a substrate with a dimension of 9.5 mm in diameter, and 0.57mm in thickness. A Zircaloy-4 cladding tube was cut in 350 mm from 4 m, and cleaned by alcohol to remove any stains or contamination on the surface, and then dried. The selection of the coated materials was based on the neutron cross-section, thermal conductivity, thermal expansion, melting point, phase transformation behavior, and high-temperature oxidation rate. From a high-temperature oxidation test at 1200oC in a previous study 4), it is known that pure Cr metal shows very superior oxidation resistance to Zircaloy-4 under a high-temperature steam environment. To apply 3D laser coating technology, Cr powder (3N purity) was used as the coating material, and the mean size of the powder was about 63 m. 2. Coating Methods Various coating methods such as plasma spray (PS), chemical/physical vapor deposition (CVD/PVD), and laser beam scanning (LBS) supplied with powder have been applied in commercial industry. Among them, we considered direct coating methods on the surface of fuel assembly components having a complex shape (cladding tube, guide tube, and spacer grid). Since the length of the cladding and guide tube is 4 m, a coating method having a high vacuum control and a temperature increase more than the recrystallization of zirconium-based alloys is not acceptable from an economical and technical consideration. In addition, the coating can be applied to an irregular surface shape, because the coated area is not flat (tube and irregularly formed grid; which are represented as a 3-dimensional geometry). For these reasons, a 3D laser coating method supplied with powder as a coating material was considered in this study. A schematic drawing of the 3D laser coating method supplied with powder was reported in a previous report 8). The constructed 3D coating equipment for Cr coating on Zircaloy-4 cladding tube is shown in Fig. 1. This equipment consists of three major parts: lens, powder supplier, and nozzle. A continuous wave (CW) diode laser with a maximum power of 300 W (PF-1500F model; HBL Co.) was used to coat Cr powder on a Zircaloy-4 cladding tube. Cr powder can be continuously supplied by a powder supplier during the coating process. Coating parameters such as the laser power, specimen velocity, powder injection, and gas flow were developed based on the parameter study. Finally, the applied power for the LBS treatment ranged from 100 to 160 W, and the scanning speed was 14 mm/s. To prevent oxidation during the 3D laser coating, an inert gas (Ar) was continuously bellowed into the melting zone.

Fig. 1 Surface appearance of the main equipment for 3D laser coating supplied with Cr pwder 3. Adhesion Property Test Ring compression and tensile tests were carried out to evaluate the ductility and adhesion property of the Cr-coated cladding at room temperature. In the ring compression and tensile tests, the Cr-coated cladding was cut to a 2 mm length and compressed using an Instron (Model 3366) at a loading rate of 1 mm per minute. Fig. 2 shows a schematic drawing and photos for both the ring tensile and compression test methods. From these tests, the fracture and peeling properties of a Cr-coated layer can be evaluated.

Fig. 2 Shematic drawing and photos of ring tensile and compression tests to evaluate the adhesion property


4. High Temperature Oxidation Test A Zircaloy-4 cladding tube as a reference and the Cr-coated Zircaloy-4 cladding tube prepared by 3D laser coating were been cut to 10 mm in axial length, and the cut surface was carefully ground with silicon carbide (SiC) paper. The prepared samples were washed using ultrasonic cleaning in alcohol for 5 min, and dried. The dried samples were mounted in the test equipment for high-temperature oxidation. The temperature of the samples rose 50oC/min, and the temperature was maintained at 1200oC for 2000 s. During the heating ramp, only Ar was continuously supplied to prevent the air oxidation, and a mixed gas of steam and Ar was supplied at the target test temperature, and the supplied volume of the mixed gas was 10 ml/min. A high-temperature oxidation test was performed on two samples for each condition. The oxidation behavior of the reference and Cr-coated samples was evaluated through a cross-sectional observation for the oxide thickness using SEM after high-temperature oxidation test.

III. RESULTS AND DISCUSSION 1. Cr powder coating on Zircaloy-4 cladding tube using 3D laser coating technology

because the melted Cr particles were attached on the coated surface during the coating. However, this rough surface can be easily controlled by a grinding process using SiC paper. In this work, the sample tests were performed using the as-coated state (not polished), and the sample test for the final surface treated state will be included in the next test plan. After the cross-section observation using SEM, the Cr-coated layer was well-deposited on the Zircaloy-4 cladding surface like in a previous study on the Cr coating on the Zircaloy-4 plate 8). However, the Cr-coated layer was controlled such that the thickness formed on the cladding tube was lower than 100 m, and the incorporated Cr particles can be removed in this work. When compared to the previous result 8), the decrease in the Cr-coating layer thickness can be obtained by a decrease in the Cr powder size from 90 to 60 m, although the powder control was difficult because of the decrease in the powder size. 2. Ring tensile and compression test to evaluate the adhesion property of Cr-coated Zircaloy-4 cladding tube An adhesion property is a key issue in the coating technology, because the application of the surface coating is impossible when the coated layer is spalled or peeled out from the base material. The adhesion property of the fuel cladding is a very import factor, since the coated layer can be easily damaged by the severe reactor environment during normal operation and under accident conditions.

Fig. 3 3D laser coating process and surface appearance of Cr-coated Zircaloy-4 cladding Fig. 3 shows a 3D laser coating process supplying Cr powders on a commercial Zircaloy-4 cladding tube. In this case the laser beam power was 120 W, and scan speed was 14 mm/s. The Cr powder and Ar gas were continuously supplied as a mixed state during the coating process. To obtain a good coating property, a key technology of 3D laser coating such as the control program, powder supplier, and coating nozzle was developed at KAERI. From this process, the Cr-coated layer with a 100 mm length can be made on a Zircaloy-4 cladding tube surface without cark formation of the coated layer, surface oxidation, or deformation of the cladding tube. The coated area showed as a rough surface,

Fig. 4 Strain and stress curves of ring tensile and compression tests performed at room temperature and appearance of tested samples


The limitation of creep deformation on the inner and outer direction of a cladding tube is 1% for the cladding hoop direction during normal operation. In addition, it is known that severe ballooning deformation of a cladding tube is shown under accident conditions. Thus, the coated layer can maintain the integrity of the hoop direction deformation of the cladding tube. As an evaluation method of the adhesion property of a Cr-coated cladding tube using 3D laser coating for the hoop direction, ring tensile and compression tests was used in this work. Fig. 4 shows the engineering stress and strain curves in both tests and the appearance of the samples after the tests. The variation of stress-strain curves was similar between the reference and Cr-coated cladding, although the strength of Cr-coated samples was somewhat increased when compared with that of the reference samples in both the tensile and compression tests. Peeling or spalling phenomenon was not observed in the tested sample, although the Cr-coated layer was cracked after the tests. To identify the time to crack initiation, the sample surface observation was performed during the compression test, and it was identified that the crack initiation was started when the strain was passed about 5%. It is thought that the crack formation of the Cr-coated layer was caused by the brittle characteristics of the Cr element as well as the influence of the laser treatment. A fast cooled structure and the inter-diffusion zone between the coated material of Cr and Zircaloy-4 substrate can be obtained by the Cr coating process using 3D laser coating supplied with powder. The hoop strength of the coated sample was increased by the fast cooled structure such as martensite, and the good adhesion property was caused by the formation of the inter-diffusion layer between Cr and Zircaloy-4.

status. To resolve this problem, the coating process is tuned by the control of various coating parameters such as the laser power, powder size, and coating speed.

3. High-temperature oxidation behavior of Cr-coated Zircaloy-4 cladding tube Fig. 5 shows the cross-sectional SEM observation of the Cr-coated Zircaloy-4 cladding using 3D laser coating and the reference Zircaloy-4 cladding after the high-temperature oxidation test at 1200oC for 2000 s in a steam environment. It was clearly identified that the Cr-coated layer was maintained without spallation or severe oxidation, whereas the ZrO2 layer of about 113 micron in thickness was formed on the uncoated Zircaloy-4 cladding tube surface. This oxidation behavior of Cr-coated Zircaloy-4 cladding samples was very similar to the previous test results for the Cr-coated Zircaloy-4 sheet samples 8). Since the -Zr(O) phase layer was not formed at the interface between the Zircaloy-4 matrix and Cr-coated layer from the SEM micrograph, the oxygen diffusion through the Cr-coated layer was effectively prevented during the high-temperature oxidation test. From the comparison of the oxide thickness between the Cr-coated layer and none coated region of Zircaloy-4 cladding tube, the oxidation resistance of Cr-coated layer was much higher than the Zircaloy-4 matrix. In addition, it was known that the adhesion property of the Cr-coated layer on the Zircaloy-4 cladding tube was reasonable to resist the spalling problem owing to the thermal expansion and oxidation in the LOCA condition. However, the control of the uniform layer thickness is necessary to improve the quality of the coated layer in this

A 3D laser coating technique supplied with Cr powders was developed to decrease the high-temperature oxidation rate in a steam environment, and the Cr-coated Zircaloy-4 cladding tube 100 mm length can be successfully manufactured in this study. From the ring tensile and compression tests, the peeling or spalling phenomenon of the Cr-coated layer attached on Zircaloy-4 cladding tube was not observed, although the Cr-coated layer was cracked after the 5% strain during the ring compression test. The Cr-coated layer showed a good oxidation resistance without severe damage such as peeling and spalling after the high-temperature oxidation test. From this study, the hydrogen generation of zirconium alloy caused by an excess oxidation reaction in a high-temperature steam environment can be considerably reduced by the application of Cr coating using 3D laser coating technology supplied with Cr powder. However, the coating thickness and homogeneity for applying the fuel cladding will be determined after adjusting various factors such as the laser power, powder size, and coating speed.

Fig. 5 Cross-sectional SEM observation of the Cr-coated Zircaloy-4 cladding using 3D laser coating and the reference Zircaloy-4 cladding after the high-temperature oxidation test at 1200oC for 2000 s in a steam environment.

IV. CONCLUSIONS

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012M2A8A5000702)


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