MICROSTRUCTURAL EVOLUTION IN CERIUM DIOXIDE IRRADIATED WITH HEAVY IONS AT HIGH TEMPERATURE

Transcription

1 MICROSTRUCTURAL EVOLUTION IN CERIUM DIOXIDE IRRADIATED WITH HEAVY IONS AT HIGH TEMPERATURE Takeshi Mihara, Department of Nuclear Engineering and Management School of Engineering The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo , Japan, Hiroaki Abe, Takeo Iwai, Department of Nuclear Professional School School of Engineering The University of Tokyo 2-22, Shirakata Shirane, Tokai, Naka, Ibaraki , Japan Takeshi Sonoda Central Research Institute of Electric Power Industry (CRIEPI) , Iwado-kita, Komae-shi, Tokyo , Japan and Eiichi Wakai Japan Atomic Energy Agency (JAEA) 4-49, Muramatsu, Tokai, Naka, Ibaraki , Japan ABSTRACT The nuclear fuel is subjected to high irradiation dose. Then the fuel, uranium dioxide (UO 2 ) with a cubic fluorite structure, leads to high burn-up structure over the dose. The structure is characterized as formation of subdivided grains and coarsened bubbles, which is attributable to deterioration of the nuclear fuels performance. Therefore, it is necessary to suppress its formation. Cerium dioxide (CeO 2 ) is expected as the simulation of UO 2 under energetic particle irradiation because both possess the same structure. CeO 2 is a well-studied oxide because of its technological applications, such as a major component in the catalysts, stable capacitors and so on. In order to clarify behavior of lattice defects in the structure, changes of crystal structure and surface morphology in CeO 2 were investigated. Polycrystalline samples were irradiated with 300keV-Xe, 12MeV-O and other ions at temperatures from 370 to 1000 K. Microstructural evolution and composition change was analyzed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffractometry (XRD). Grain boundary etching, embossed shapes formation and bubble formation were observed at high temperature, however blistering was not detected. The tendency of surface features remarkably changed at 1000 K because of faster vacancies diffusion. Raman spectra and diffraction data indicated that oxygen vacancies accumulate at the early stage and reach the balance of their formation and recovery, and the formation is correlated with electronic excitation. Key Words: cerium dioxide, Ion irradiation, Raman spectroscopy, X-ray diffraction, scanning electron microscopy

2 1. INTRODUCTION The development of high burnup fuels is required to reduce the total amount of spent nuclear fuels and nuclear energy costs. Characteristics of high burn-up structure or Rim structure [1-3] are coarsened bubbles and subdivided grain, all of which result in FP releases, fuel temperature increase, swelling and so on. These phenomena are attributable to pellet-clad interaction and deteriorate the properties of nuclear fuels. Therefore, it is necessary to restrain the high burnup structure formation. There have been models for high burnup structure formation [4-6]. However, the formation mechanism still remains a matter of discussion because fuel pellets were usually investigated after irradiations. In this study, cerium dioxide (CeO 2 ) was selected so as to simulate the behaviors of uranium dioxide and as a first report the effect of ion-irradiation was investigated. The purpose of the study is that surface morphology change and composition change of CeO 2 are clarified. 2. EXPERIMENTAL METHODS High purity (99.99 %) CeO 2 powders was pressed and sintered at 1670 K for 8 hours in air. The bulk materials were cut into disks of 9mmφ 1mmt followed by optical polish to achieve specim ens for ion irradiations and the density was 95 % of the theoretical dentisy. Ion irradiation has performed at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) in JAEA. The samples were irradiated with single 300 kev Xe +, 18 MeV I 5+, 350 kev O + and 12 MeV O 3+ ion so as to simulate fission products and self ion. Sample temperatures under irradiation were measured by infrared radiation thermometer and were feed back to minimize beam heating effect. Irradiation conditions of CeO 2 specimens shows in Table I. Irradiated samples were investigated with scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), Raman spectroscopy and x-ray diffractometry (XRD). Table I. Irradiation conditions of CeO 2 specimens Ion 300 kev Xe + 18 MeV I kev O + 12MeV O 3+ Flux [i/cm 2 s] 2.9~ ~ ~ ~ Fluence[i/cm 2 ] ~ ~ ~ Irradiation temperature[k] 388, 633, 893, , , RESULTS AND DISCUSSION The surface for CeO 2 disks irradiated with 300 kev Xe + was observed by SEM Surface morphology features Fig. 1 shows SEM images of the surface for CeO 2 disks irradiated to i/cm 2. Surface morphology features under ion irradiations are described in the following. Priority etching of grain boundary by sputtering was observed in Fig. 1(b)-(d). And rounded dents were observed in 2/6

3 Fig. 1. This results from sputtering and surface diffusion. Transgranular embossed shapes was observed in Fig. 1(b)-(d). The shapes result from self-organization by sputtering in competition with surface diffusion. (a) 388 K (b) 633 K (c) 893 K (d) 1003 K Figure 1. SEM images of the surface for CeO 2 disks irradiated to ions/cm 2 (a) 388 K; (b) 633 K; (c) 893 K; (d) 1003 K Irradiation temperature dependence of surface morphology Fig. 1 shows SEM images of the surface for CeO 2 disks irradiated at ions/cm 2 as a function of irradiation temperature. This figure show that grain boundaries were etched with increasing temperature, but the effect was recovered at 1003 K. And embossed shapes were remarkable with increasing temperature, but the variation between 893 K and 1003 K was unclear. We suggested that the characteristic variations at 1003 K indicate lattice defects migration evident Coarsened Xe bubbles formation at high temperature Fig. 2(a) shows a SEM images of the surface irradiated to a fluence of ions/cm 2 at 1003 K. Dot contrasts collecting around pits were observed. Fig. 2 showed that the contrasts corresponded with Xe element distribution by EDS. These results suggests that Xe bubbles are located immediately below surface of pits, however blistering which is one of main ionirradiated surface effect was not observed. 3/6

4 (a)sem (b) Xe-L Figure 3. SEM image of the surface irradiated to fluence of ions/cm 2 at 388 K Figure 2. (a) SEM image of the surface irradiated at 1003 K; (b) Xe-L line element mapping by EDS Flaking at low temperature Fig. 3 shows a SEM image of the surface irradiated to fluence of ions/cm 2 at 388 K. Flaking was observed, but blistering was not observed. This result suggests that bubbles below surface don t lead to blistering and generate flaking in CeO Structure analysis Raman spectra The main peak of the Raman spectrum in CeO 2 is F 2g triple degenerate vibrational mode and corresponds with vibration of oxygen sublattice. Measurement region by Raman spectroscopy correspond with ion-implanted region (300 kev Xe +, 350 kev O + ) and directly below surface (18 MeV I 5+, 12 MeV O 3+ ) compared with vacancy and ion rage distribution calculated by the SRIM-2006 code as shown in Fig. 4. Fig. 5 shows F 2g mode shift in CeO 2 irradiated with 300 kev Xe +, 350 kev O +, 18 MeV I 5+, 12 MeV O 3+. F 2g mode shift in irradiated CeO 2 varied toward lower frequency in initial irradiation period in Fig. 5. In high fluence region F 2g mode shift in CeO 2 irradiated with 350 kev O + varied toward higher frequency in Fig. 5(a). The one in CeO 2 irradiated with high energy ion at 773 K varied toward lower frequency and the one at 1073 K varied toward higher frequency in Fig. 5(b). Lower frequency variations are reported to result from effect of oxygen vacancies [7]. A higher sputtering rate by 300 kev Xe + causes for those results and its variation corresponds with surface morphology by SEM observation. The variation toward higher frequency in CeO 2 irradiated with 350 kev O + indicates that interstitial atoms accumulate. Continuous variation toward lower frequency at 773 K indicates that vacancies accumulate. On the other hand variation toward higher frequency at 1073 K indicates that vacancies annihilate. 4/6

5 (a) (b) Vacancy (%) Ion Range (%) Vacancy (%) Ion Range (%) Depth, d/μ m Depth, d/nm Figure 4. Depth profiles of vacancy (open symbols) and ion range (solid symbols) distribution of CeO 2 irradiated with (a) 18 MeV I 5+ (triangle), 12 MeV O 3+ (square), (b) 350 kev O + (a) F 2g mode frequency, k/cm kev Xe K (b) 350 kev O K kev O K MeV I K MeV O K 12MeV O K Fluence, φ/10 16 i/cm 2 Fluence, φ/10 16 i/cm 2 F 2g mode frequency, k/cm -1 Figure 5. F 2g mode shift in CeO 2 irradiated with (a) 300 kev Xe +, 350 kev O + ; (b) 18 MeV I 5+, 12 MeV O X-ray diffraction Fig. 6 show that lattice constant for CeO 2 irradiated with 18 MeV I 5+, 12 MeV O 3+ at a function of fluence. Lattice constant increased in initial irradiation with increasing fluence. And then lattice constant showed a tendency to decrease. Lattice constant for CeO 2 irradiated with 12 MeV O 3+ was asymptotic with increasing fluence Lattice parameter, a/nm MeV O K 12 MeV O K 18 MeV I K Fluence, φ/10 16 i/cm 2 Figure 6. Lattice constant for CeO 2 irradiated with 18 MeV I 5+, 12 MeV O 3+ at a function of fluence 5/6

6 Raman spectra and XRD results indicate following phenomena. In initial irradiation vacancies and interstitial atoms accumulate. While vacancies continuously accumulate near surface of CeO 2 irradiated with high energy ion at 773 K, vacancies annihilate at 1073 K. Interstitial atom accumulate near surface of CeO 2 irradiated with 350 kev O + at 773 K. Then, defect productions and annihilations are asymptotic. 3. CONCLUSIONS Disks of CeO 2 irradiated at irradiation temperatures between 388 K and 1073 K, with 300 kev Xe +, 18 MeV I 5+, 350 kev O + and 12 MeV O 3+. The changes in surface morphology and lattice defects in these pellets were studied by SEM/EDS observation, Raman spectroscopy and XRD. The results were summarized as follows: 1. The surface of irradiated CeO 2 showed grain boundary etching, rounded dents and transgranular embossed shapes. 2. Xe bubbles, the size of 100 nm, below surface of pits were observed, and were formed as surface move by etching (893 K, 1003 K). Flaking was observed, and was formed by Xe accumulation (388K). 3. The variations of SEM, the Raman peak and lattice parameter at about 1000 K give suggesti ons that the oxygen vacancies more diffuse over the temperature. 4. The variations of the Raman peak and lattice parameter in irradiated CeO 2 indicate that interstitial atoms accumulate (350 kev O + ) and Production of defects is balanced out by annihilation (12MeV O 3+ ). ACKNOWLEDGMENTS A part of this study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. REFERENCES 1. Hj. Matzke; J. Nucl. Mater. 189 (1992) I.L.F. Ray, Hj. Matzke, H.A. Thiele and M. Kinoshita; J. Nucl. Mater. 245 (1997) M. Kinoshita, T. Kameyama, S. Kitajima and Hj. Matzke; J. Nucl. Mater. 252 (1998) T. Sonoda, M. Kinoshita, I.L.F. Ray, T. Wiss, H. Thiele, D. Pellottiero, V.V. Rondinella and Hj. Matzke; Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) K. Nogita and K. Une; J. Nucl. Mater. 226 (1995) J. Spino, K. Vennix and M. Coquerelle; J. Nucl. Mater. 231 (1996) S. Patil, S. Seal, Y. Guo, A. Schulte and J. Norwood; Appl. Phys. Lett. 88 (2006) /6