Nuclear Engineering and Technology 50 (2018) 724e730 Contents lists available at ScienceDirect Nuclear Engineering and Technology journal homepage: www.elsevier.com/locate/net Original Article Effect of CrN barrier on fuel-clad chemical interaction Dongkyu Kim a, b, Kangsoo Lee b, **, Young Soo Yoon b, * a b U.C. San Diego, Department of Mechanical and Aerospace Engineering, 9500 Gilman Dr, La Jolla, CA, 92093, USA Gachon University, Department of Environment and Energy Engineering, Seongnamdaero 1342, Gyeonggi-do, 461-710, Republic of Korea a r t i c l e i n f o a b s t r a c t Article history: Received 29 November 2017 Received in revised form 2 February 2018 Accepted 12 February 2018 Available online 28 March 2018 Chromium and chromium nitride were selected as potential barriers to prevent fuel-clad chemical interaction (FCCI) between the cladding and the fuel material. In this study, ferritic/martensitic HT-9 steel and misch metal were used to simulate the reaction between the cladding and fuel ﬁssion product, respectively. Radio frequency magnetron sputtering was used to deposit Cr and CrN ﬁlms onto the cladding, and the gas ﬂow rates of argon and nitrogen were ﬁxed at certain values for each sample to control the deposition rate and the crystal structure of the ﬁlms. The samples were heated for 24 h at 933 K through the diffusion couple test, and considerable amount of interdiffusion (max. thickness: 550 mm) occurred at the interface between HT-9 and misch metal when the argon and nitrogen were used individually. The elemental contents of misch metal were detected at the HT-9 through energy dispersive X-ray spectroscopy due to the interdiffusion. However, the specimens that were sputtered by mixed gases (Ar and N2) exhibited excellent resistance to FCCI. The thickness of these CrN ﬁlms were only 4 mm, but these ﬁlms effectively prevented the FCCI due to their high adhesion strength (frictional force 1,200 mm) and dense columnar microstructures. © 2018 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: CrN Fuel-Clad Chemical Interaction Barrier Interdiffusion Scratch Test 1. Introduction The majority of nuclear industry uses pressurized water reactors, but many people think that sodium-cooled fast reactor (SFR) will become the next generation nuclear reactor because of its ability to recycle spent nuclear fuels such as uranium, plutonium, and lanthanide through pyroprocessing . SFR has many advantages over pressurized water reactor. SFR has high thermal conductivity, which creates a reservoir that prevents overheating . Also, the neutrons in SFR lose less energy compared to hydrogen and oxygen atoms found in water when atoms collide due to the weight difference. SFR is energy efﬁcient, but the drawback of sodium is its chemical reactivity. When the temperature of the fuelcladding interface reaches 650 C (923 K), the interdiffusion occurs between actinide elements (nuclear ﬁssion products) and the cladding [1,2]. This phenomenon is called fuel-clad chemical interaction (FCCI), and this lowers the melting point of the cladding through eutectic reaction [3e7]. The FCCI causes the thickness of * Corresponding author. ** Corresponding author. E-mail address: email@example.com (Y.S. Yoon). the cladding to become thinner, which end up weakening the mechanical strength of the cladding. One of the common methods to prevent diffusion is coating the inner cladding tube with a thin layer [8e12]. So far, many researchers conducted experiments with Zr barriers in the nuclear industry. However, Zr-based coatings are not efﬁcient diffusion barriers compared to Cr-based coatings. Lee et al. deposited Zr ﬁlm onto HT-9 clad using radio frequency magnetron and tested its performance as a FCCI barrier. However, the barrier did not entirely prevent the interdiffusion between the misch metal and the HT-9, so they had to add a 25 mm-thick Zr foil, which made the FCCI barrier thicker overall . Jee et al. created Zr ﬁlm FCCI barriers at multiple temperatures using hydrothermal crystallization. The barriers did not perfectly prevent the interdiffusion from occurring, and the experiment gave inconsistent results at various temperatures . In this study, Cr-based coatings were considered as a substitute for Zr-based coatings. Among Cr-based coatings, the CrN coatings particularly have good mechanical and chemical properties, such as high adhesion strength, high hardness, good conductivity, and excellent chemical, thermal, and wear resistance [14e21]. To deposit Cr and CrN layers, radio frequency magnetron sputtering was chosen due to its high deposition rate and good uniformity [22e24]. HT-9 was used as cladding and went through diffusion https://doi.org/10.1016/j.net.2018.02.008 1738-5733/© 2018 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). D. Kim et al. / Nuclear Engineering and Technology 50 (2018) 724e730 725 couple test to assess the FCCI behavior [25e28]. HT-9 (Table 1) was chosen as a candidate for SFR cladding due to its excellent irradiation resistance . The misch metal was used to simulate the nuclear fuel due to its high content of cerium and lanthanum. The gas ﬂow rate of nitrogen and argon were adjusted differently for each sample to ﬁnd the best CrN barrier during sputtering. The interdiffusion between the misch metal and HT9 was evaluated through diffusion couple tests. 2. Experimental procedure 2.1. Material and preparation HT-9, a ferritic/martensitic steel, is a commonly used material in nuclear facilities as a fuel cladding. The HT-9 disk was used to test the FCCI through the diffusion couple test. The diameter of the HT-9 is 8 mm, and its thickness is 1.9 mm. The speciﬁc composition of the HT-9 that was used for the experiment is shown in Table 1. Before the sputtering process, the HT-9 disks were chemically etched by ultrasonication in a mixture composed with 95 mL of deionized (DI) water, 2 mL of hydrogen ﬂuoride, and 3 mL of nitric acid (HNO3) for 30 s to increase the adhesion. Once the etching was done, the Zr plate, HT-9, and the silicon wafer inside the acetone were cleaned by ultrasonication for 15 minute. The Cr and CrN ﬁlm was deposited on these cleaned materials at 100 W under the pressure of 5 m torr for a certain amount of time depending on its gas composition as shown in Table 2. Total of four types of samples were made using the mass ﬂow controller. The ratio of the argon and nitrogen changed for each sample, but the total ﬂow rate was kept at 40 sccm. The substrate plate rotated at a speed of 40 rpm during the process to create an even deposition layer. Table 1 Chemical properties (wt %) of the HT-9 steel. Fe Cr Mo Si Ni W V C Other 84.4 11.9 1.03 0.69 0.62 0.48 0.30 0.21 0.37 Fig. 2. XRD spectra of Cr and CrN ﬁlms with different nitrogen ﬂow rate. (A) 100% N2 (B) 70% N2, (C) 30% N2, (D) 0% N2. XRD, X-ray diffraction. 2.2. Diffusion couple test The diffusion couple test was conducted to measure Cr and CrN barriers' ability to prevent FCCI between the HT-9 and the rare earth element. The misch metal, which mainly consisted of Ce: 75% and La: 25%, is a good candidate to simulate the interdiffusion behavior between rare earth material and the clad. The HT-9 and the misch metal were inserted into a screw jig as shown in Fig. 1, and then these materials were covered with tantalum foil (thickness 0.25 mm, 99.9%, Aldrich) to prevent the reaction between the jig and the misch metal. Once they were clamped, the samples were put into the furnace inside argon (purity: 99.999%)-ﬁlled glove box maintained at humidity level below 2 ppm to prevent the misch metal from oxidizing. The samples were heated for 24 h at 933 K, which is the peak inner cladding wall temperature for SFR [30,31].” After heating, the samples were taken out to be cooled. 2.3. Material characterization Table 2 Deposition parameters under a constant total gas ﬂow rate of 40 sccm and the properties of each sample. Sample N2 (%) Ar (%) N2 (sccm) Ar (sccm) Time (min.) Thickness (mm) 1 2 3 4 0 30 70 100 100 70 30 0 0 12 28 40 40 28 12 0 144 289 442 562 4.18 4.16 4.14 3.85 The glass substrates deposited with Cr and CrN ﬁlm were prepared to analyze the crystal structures of the Cr and CrN ﬁlm on the HT-9 using X-ray diffraction (XRD, Rigaku RINT). To characterize the FCCI through ﬁeld-emission scanning electron microscope (FESEM, Hitachi S-4200), the HT-9 samples were cut into half, and the cross sections of the samples were polished by silicon carbide paper. The images of the cross section and the surface of the Cr and Fig. 1. Schematic illustration of the specimen for the diffusion couple test. (A) Diagonal view. (B) Enlarged diagonal view. 726 D. Kim et al. / Nuclear Engineering and Technology 50 (2018) 724e730 Fig. 3. FE-SEM images of Cr and CrN ﬁlms deposited by RF magnetron sputtering. (A) For surface, 0% N2. (B) Cross section, 30% N2. (C) Surface, 30% N2. (D) Cross section, 70% N2. (E) Surface, 70% N2. (F) Cross section, 100% N2. (G) Surface, 0% N2. (H) Cross section, 100% N2. FE-SEM, ﬁeld-emission scanning electron microscope; RF, radio frequency. CrN ﬁlm were captured as shown in Fig. 3 during the FE-SEM analysis. To test the adhesion of barriers, the zirconium plates deposited with Cr and CrN ﬁlms were scratched by the nano scratch tester (CSM Instruments). The frictional forces onto the Cr and CrN barriers were drawn as graphs along the grind marks of these coatings. Critical loads for each samples were detected from sharp peaks of the frictional force graphs, and these loads assessed the coating adhesion . The critical loads were collated into a graph as shown in Fig. 4. 3. Results and discussion 3.1. Structural characterization The crystalline structures of the four samples at Table 2 were determined by XRD. The main purpose of XRD was to determine whether the barriers are composed of CrN or Cr2N. The XRD patterns of Cr and CrN ﬁlms for each sample are shown in Fig. 2. The peaks of Cr foil (N2 0%) in the diffraction patterns clearly correspond D. Kim et al. / Nuclear Engineering and Technology 50 (2018) 724e730 Fig. 4. Scratch test images and graph of Cr and CrN ﬁlm with different N2 ﬂow content. (A) 0% N2. (B) 30% N2. (C) 70% N2. (D) 100% N2. (E) The frictional force of Cr and CrN ﬁlms with different N2 coated by RF magnetron sputtering. RF, radio frequency. Fig. 5. Magniﬁed FE-SEM images of cross section of the Sample 1 after diffusion couple test. (A) At low magniﬁcation (50). (B) Enlarged image of (A) at 200 magniﬁcation. (C) EDS line proﬁles for elements content (500). EDS, energy dispersive X-ray spectroscopy; FE-SEM, ﬁeld-emission scanning electron microscope. to crystal system of cubic structure (JCPDS, No. 01-085-1336). Sample 3 and 4 (N2 70%, N2 100%) clearly have four peaks that correspond to the crystal system of cubic system (JCPDS, No. 65-2899), but Sample 2 (N2 30%,) does not have a peak at 43.594 at the horizontal axis of Fig. 2. Also, unlike the other two samples, the intensity of the remaining peaks for Sample 2 also does not match the cubic system (JCPDS, No. 65-2899). The CrN may have not been formed completely due to its lack of N2 gas compared to other two samples. 727 Fig. 6. Magniﬁed FE-SEM images of cross section of the Sample 2 after diffusion couple test. (A) At low magniﬁcation (50). (B) Enlarged image of (A) at 500 magniﬁcation. (C) EDS line proﬁles for elements content (500). EDS, energy dispersive X-ray spectroscopy; FE-SEM, ﬁeld-emission scanning electron microscope. Fig. 7. Magniﬁed FE-SEM images of cross section of the Sample 3 after diffusion couple test. (A) At low magniﬁcation (50). (B) Enlarged image of (A) 500 magniﬁcation. (C) EDS line proﬁles for elements content (500). EDS, energy dispersive X-ray spectroscopy; FE-SEM, ﬁeld-emission scanning electron microscope. The FE-SEM analysis of Cr and CrN thin ﬁlms for each sample is shown in Fig. 3. The images of the surface and the cross section of Cr and CrN barriers for different specimens are shown in Fig. 3. Fig. 3B shows thick columnar microstructures of the Cr barrier, and it does not have any major defects such as pinholes. However, the columnar microstructures are very thick, and they are not exactly perpendicular to the silicon wafer. Owing to the thickness and shape of the microstructures, the top (surface) of the Cr layer 728 D. Kim et al. / Nuclear Engineering and Technology 50 (2018) 724e730 is very rough, and the microstructures are not very compact as shown in Fig. 3A. This is mainly due to the fast deposition rate of Cr ﬁlm. It took less than ~2.5 h to deposit 4.18 mm of Cr ﬁlm, but the surface was rough due to its high speed. There is a noticeable difference between samples with Cr and CrN barriers. Adding nitride signiﬁcantly decreased the deposition rate. In Fig. 3D, F, H, the columnar microstructures of the ﬁlms are much thinner compared to the ones in Fig. 3B. The CrN barriers became denser and the surfaces became smoother as the deposition rate went down. The adhesion strength of the Cr and CrN barriers were analyzed by comparing the frictional forces during the scratch test. Fig. 4AeD show the images of the scratched plates and the location of the measured frictional forces. Fig. 4E compares the frictional forces of the specimens with different nitrogen ﬂow content from Table 2 as a graph. Overall, samples with CrN ﬁlms had higher adhesion strengths compared to the Cr ﬁlm. The frictional force the CrN ﬁlm can withstand went up signiﬁcantly when the nitrogen ﬂow content was 30% but went down as the nitrogen ﬂow content increased. Sample 4 only used nitrogen for sputtering, but its adhesion strength was similar to Sample 1, which did not use any nitrogen. As a result, it has been found that proper ratio of nitride and argon increases the adhesion strength. 3.2. Reaction between misch metal and HT-9 Figs. 5e8 exhibit the results of the diffusion couple test for each specimen from Table 2 at 933 K. Fig. 5 shows the cross-sectional FESEM image of misch metal and HT-9 with Cr barrier (sample 1: Ar 100%). In Fig. 5A, most of the Cr barrier got destroyed by the interdiffusion that occurred throughout most parts of the HT-9. As shown in Fig. 5B, the thickness of the interdiffusion layer is 292.5 mm, which is more than 70 times the thickness of the Cr barrier. In Fig. 3C, energy dispersive X-ray spectroscopy (EDS) is used for more accurate analysis. The elemental content of the misch metal and HT-9 throughout the interdiffusion is drawn as a graph. Between 100 and 150 mm, there is a Cr peak, which indicates the Cr barrier. However, a lot of the Ce and La content can be seen at the side of HT9 on the graph. Although there was a Cr barrier, many of the Ce and La content moved from the misch metal to HT-9 and the Fe content moved from HT-9 to misch metal due to the interdiffusion. However, the Cr within the HT-9 did not move over to misch metal. Instead, some Cr peaks with high intensity appeared at the side of the HT-9. When the diffusion occurred, the chromium that fell apart from the HT-9 formed clusters at some points of the HT-9. As shown in Fig. 2A and B, the columnar gaps of the Cr ﬁlm are thick, and their surface is rough. The element content of misch metal and HT-9 may have pierced through the columnar gaps within the Cr ﬁlm. Figs. 6 and 7 demonstrate the cross-sectional FE-SEM images of misch metal, Sample 2 (Ar 70%, N2 30%), and Sample 3 (Ar 30%, N2 70%). Unlike the Cr barrier in Fig. 5A, there are no signs of interdiffusion in Figs. 6A and 7A. As shown in the EDS graphs of Figs. 6C and 7C, none of the Ce and La content moved from misch metal to HT-9, and the Fe content remained at the side of HT-9 as well. The CrN ﬁlms of the two specimens successfully prevented the interdiffusion from occurring at SFR operation temperature of 933 K. In Figs. 6B and 7B, the thin CrN ﬁlms are clearly visible between the misch metal and HT-9. The Cr peaks of the CrN ﬁlms are conﬁrmed at Figs. 6C and 7C, but the nitrogen is not included in the graphs. Unfortunately, nitrogen's atomic number is 7, and FE-SEM cannot detect elements with atomic number lower than the oxygen (atomic number: 8). However, this was not a major issue since the nitrogen content of the CrN barriers were conﬁrmed from the XRD at Fig. 2. The dense CrN ﬁlms, formed by thin columnar microstructures, were effective barriers for diffusion. Fig. 8 shows the cross-sectional FE-SEM image of misch metal and HT-9 with CrN barrier (Sample 1: N2 100%). In Fig. 8A, almost half of the Cr barrier got destroyed by interdiffusion. As shown in Fig. 8B, the thickness of the interdiffusion layer is 549.4 mm, which is almost twice the thickness of the Cr barrier at Fig. 5A. On top of that, the CrN ﬁlm was not even visible in Sample 4, so it was necessary to analyze where the elements were distributed through EDS. In Fig. 8C, the peaks for each element are all over the place. Unlike the Ce content, which diffused equally throughout the Line EDS, most of the La content was detected at the side of HT-9. As expected, some of the Fe peaks are detected at the misch metal because of interdiffusion. Unfortunately, the location of CrN ﬁlm was not detected through EDS analysis. It is very likely that the CrN barrier got destroyed completely because of interdiffusion. Not only the CrN ﬁlm was not visible but also there were multiple Cr peaks detected at the side of HT-9. There is a high chance that the Cr diffused throughout HT-9 as the CrN ﬁlm dispersed. Among all the samples listed in Table 2, only Sample 1 allowed interdiffusion to occur throughout the entire interface of misch metal and HT-9, as shown in Fig. 5A. To strengthen the Cr barrier, a Cr foil (thickness: 25 mm, purity 99.99%, Goodfellow) was inserted inside the steel jig between misch metal and HT-9 (Sample 1) shown in Fig. 1A during the diffusion couple test. However, cutting chromium foil into the shape of HT-9 was very difﬁcult because it was very brittle. As shown in Fig. 9A, the Cr foil had multiple cracks before it was cut, and these cracks are clearly visible through the optical microscope image in Fig. 9B. Fig. 9C demonstrates the crosssectional FE-SEM images of misch metal, Sample 1 (Ar 100%), and Cr foil, and the Cr foil completely was clearly visible in the image because of its thickness. However, the Cr foil was not visible at the bottom part of the interface of HT-9 and misch metal. It was presumed that the portion of the chromium foil fell off because of the compression force of the bolts as they got clamped to the steel jig. As expected, the interdiffusion occurred in this region, and the EDS analysis had to be done. The line EDS was drawn at the selected location in Fig. 9D, and the distribution of the elements were drawn as a graph in Fig. 9E. In Fig. 9E, cerium, lanthanum, and iron were Fig. 8. Magniﬁed FE-SEM images of cross section of the Sample 4 after diffusion couple test. (A) At low magniﬁcation (50). (B) Enlarged image of (A) at 200 magniﬁcation. (C) EDS line proﬁles for elements content (500). EDS, energy dispersive X-ray spectroscopy; FE-SEM, ﬁeld-emission scanning electron microscope. D. Kim et al. / Nuclear Engineering and Technology 50 (2018) 724e730 729 (Sample 2: Ar 70%, N2 30%/Sample 3: Ar 70%, N2 30%) that successfully prevented FCCI had high adhesion strength, and these specimens used mixed gases (N2 and Ar) for magnetron sputtering. As the nitrogen ﬂow rate of the samples increased, the deposition rate went down and the barriers became denser and smoother. Owing to different N2 ﬂow rates, the microstructures of the samples were all different, and this affected the adhesion strength of the specimens. These results indicate that the gas ratios of N2 and Ar during sputtering affect the adhesion strength, and the adhesion strength affects the performance of FCCI barrier. These results suggest that the CrN thin ﬁlm has the potential to serve as an effective diffusion barrier for cladding with careful adjustment of the gas ratio. Conﬂicts of interest The authors do not have any conﬂicts of interest to declare. Acknowledgments The authors would like to thank the support by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A5A1013919). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.net.2018.02.008. References Fig. 9. Images of Cr foil. (A) Digital image. (B) Optical microscope (40 magniﬁcation). Magniﬁed FE-SEM images of cross section of the Sample 1 with Cr foil after diffusion couple test. (C) At low magniﬁcation (50). (D) Enlarged image of (C) at 300 magniﬁcation. (E) EDS line proﬁles for elements content (500). EDS, energy dispersive X-ray spectroscopy; FE-SEM, ﬁeld-emission scanning electron microscope. diffused throughout the entire region due to the interaction. Multiple chromium peaks were detected past 150 mm, which conﬁrms that the Cr ﬁlm got destroyed. However, the chromium peaks were not detected at the center (between 100 mm and 150 mm) of the image. The Cr foil has a thickness 25 mm, but it was not visible even when the image was magniﬁed by 500 using scanning electron microscope. The EDS analysis veriﬁed the previous presumption of Cr foil falling-off due to its brittleness. This experiment conﬁrmed that the Cr foil is not an efﬁcient barrier and reafﬁrmed that the Cr ﬁlm lacks the ability to prevent interdiffusion. As a result, it was concluded that CrN ﬁlm with the appropriate nitride ratio is the best barrier to prevent diffusion. 4. Conclusion The diffusion couple test was conducted to experiment the performance of FCCI barrier at the interface between the HT-9 clad and misch metal at the SFR operating temperature. Samples 1, 2, 3, and 4 displayed diffusion behaviors, and FCCI occurred in specimens (Sample 1: Ar 100%/Sample 4: N2 100%) with low adhesion strength in the scratch test. 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