铈掺杂诱导U_3O_8晶型变化
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摘要
铈是核反应的重要裂变元素,其氧化物CeO_2容易与UO_2燃料形成U_(1-x)Ce_xO_2固溶体。铀铈固溶体会因为U4+发生氧化而生成(U_(1-x)Ce_x)3O_(8-δ)混合氧化物。Ce在U_3O_8中的存在会改变U_3O_8母体的局域结构,影响着(U_(1-x)Ce_x)3O_(8-δ)产物的物理化学性质。研究铀铈混合氧化物的局域结构,可以帮助理解处于苛刻环境中乏燃料的性状。使用共沉淀方法制备了铈掺杂的U_3O_8混合氧化物样品,即(U_(1-y)Ce_y)3O_(8-δ)(y=0,0.05,0.20),利用粉末X射线衍射(X-ray Diffraction,XRD)获取其物相信息,借助同步辐射X射线吸收精细结构(X-ray Absorption Fine Structure,XAFS)技术来获取(U_(1-y)Ce_y)3O_(8-δ)中铀和铈的局域结构。结果表明:与掺杂前的U_3O_8具有C2mm空间群不同,掺入Ce后的(U_(1-y)Ce_y)3O_(8-δ)转变成了与U_3O_8的P62m空间群一样的晶型,Ce的存在还导致铀的平均价态升高,层内最近邻Ce-U(Ce)的原子距离减小。
[Background] Cerium is one of the important products of the mixed oxide(MOX) reaction, and CeO_2 forms U_(1-x)Ce_xO_2 solid solution with UO_2. In the post-treatment of spent fuel, UO_2 may be oxidized to U_3O_8 resulting in an increase in spent fuel volume. The structure and volume of spent fuel are concerned with the solubility, valence and electronic structure of uranium-cerium solid solutions. [Purpose] This paper aims to study the local structure of Ce-substituted U_3O_8 by X-ray absorption fine structure(XAFS) technique. [Methods] First of all,(U_(1-y)Ce_y)3 O_(8-δ)(y=0, 0.05, 0.20) mixed oxides were prepared by coprecipitation method. Then, powder X-ray diffraction(XRD) and Xray absorption fine spectroscopy(XAFS) characterizations at U L3 edge and Ce K edge have been undertaken to study the chemical changes of uranium-cerium oxides. [Results] The XRD data shows that the Ce-substituted U_3O_8 has the P62 m space group which is different with α-U_3O_8. The U L3-edge X-ray absorption near edge structure(XANES) data of Ce-substituted U_3O_8 shows a slight increase in the uranium average valance. The X-ray absorption near edge structure(EXAFS) data shows that the neighboring atoms of Ce cations are far away from those of U cations. [Conclusions] The Ce cations replaced the U site in U_3O_8, and the U_3O_8 sample was changed from the original C2 mm to P62 m space group. The lattice volume was increased and the local structure around U cation was changed by adjusting the atomic distance and increasing the valence of U.
[Background] Cerium is one of the important products of the mixed oxide(MOX) reaction, and CeO_2 forms U_(1-x)Ce_xO_2 solid solution with UO_2. In the post-treatment of spent fuel, UO_2 may be oxidized to U_3O_8 resulting in an increase in spent fuel volume. The structure and volume of spent fuel are concerned with the solubility, valence and electronic structure of uranium-cerium solid solutions. [Purpose] This paper aims to study the local structure of Ce-substituted U_3O_8 by X-ray absorption fine structure(XAFS) technique. [Methods] First of all,(U_(1-y)Ce_y)3 O_(8-δ)(y=0, 0.05, 0.20) mixed oxides were prepared by coprecipitation method. Then, powder X-ray diffraction(XRD) and Xray absorption fine spectroscopy(XAFS) characterizations at U L3 edge and Ce K edge have been undertaken to study the chemical changes of uranium-cerium oxides. [Results] The XRD data shows that the Ce-substituted U_3O_8 has the P62 m space group which is different with α-U_3O_8. The U L3-edge X-ray absorption near edge structure(XANES) data of Ce-substituted U_3O_8 shows a slight increase in the uranium average valance. The X-ray absorption near edge structure(EXAFS) data shows that the neighboring atoms of Ce cations are far away from those of U cations. [Conclusions] The Ce cations replaced the U site in U_3O_8, and the U_3O_8 sample was changed from the original C2 mm to P62 m space group. The lattice volume was increased and the local structure around U cation was changed by adjusting the atomic distance and increasing the valence of U.
引文
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2 Duan P Q,Bao H L,Li J,et al.In-situ high-energyresolution X-ray absorption spectroscopy for UO2oxidation at SSRF[J].Nuclear Science and Techniques2017,28(1):2.DOI:10.1007/s41365-016-0155-x.
3 Xiao H,Long C,Tian X,et al.Influences of Zr,Ce and Ba fission products on the surface properties of UO2atomistic simulations[J].Surface Science,2016,649:1-6DOI:10.1016/j.susc.2016.01.014.
4 Cao H J,Bao H L,Lin X.Differential interplay between Ce and U on local structures of U1-xCexO2solid solutions probed by X-ray absorption spectroscopy[J].Journal of Nuclear Materials,2019,515:238-244.DOI:10.1016/j jnucmat.2018.12.042.
5 Prieur D,Martel L,Vigier J F,et al.Aliovalent cation substitution in UO2:electronic and local structures of U1-yLayO2±xsolid solutions[J].Inorganic Chemistry,2018 57:1535-1544.DOI:10.1021/acs.inorgchem.7b02839.
6 Ha Y K,Lee J,Kim J G,et al.Effect of Ce doping on UO2structure and its oxidation behavior[J].Journal of Nuclear Materials,2016,480:429-435.DOI:10.1016/j.jnucmat.2016.08.026.
7 Rodriguez-penalonga L,Soria B Y M.A review of the nuclear fuel cycle strategies and the spent nuclear fuel management technologies[J].Energies,2017,10:1235.DOI:10.3390/en10081235.
8 Perriot R,Liu X Y,Stanek C R,et al.Diffusion of Zr,Ru,Ce,Y,La,Sr and Ba fission products in UO2[J].Journal of Nuclear Materials,2015,459:90-96.DOI:10.1016/j.jnucmat.2015.01.001.
9 Al-Salik Y,Al-Shankiti I,Idriss H.Core level spectroscopy of oxidized and reduced CexU1-xO2materials[J].Journal of Electron Spectroscopy and Related Phenomena,2014,194:66-73.DOI:10.1016/j.elspec.2013.11.013.
10 Rehr J J,Kas J J,Vila F D,et al.Parameter-free calculations of X-ray spectra with FEFF9[J].Physical Chemistry Chemical Physics,2010,12:5503-5513.DOI:10.1039/b926434e.
11 Shi W Q,Yuan L Y,Wang C Z,et al.Exploring actinide materials through synchrotron radiation techniques[J].Advanced Materials,2014,26:7807-7848.DOI:10.1002/adma.201304323.
12 Chollet M,Martin P,Degueldre C,et al.Neptunium characterization in uranium dioxide fuel:combining a XAFS and a thermodynamic approach[J].Journal of Alloys and Compouns,2016,662:448-454.DOI:10.1016/j.jallcom.2015.12.005.
13 Caisso M,Roussel P,Auwer C D,et al.Evidence of trivalent am substitution into U3O8[J].Inorganic Chemistry,2016,55:10438-10444.DOI:10.1021/acs.inorgchem.6b01672.
14 Ravel B,Newville M.ATHENA,ARTEMIS,HEPHAESTUS:data analysis for X-ray absorption spectroscopy using IFEFFIT[J].Journal of Synchrotron Radiation,2005,12:537-541.DOI:10.1107/S0909049505012719.
15 Yu H S,Wei X J,Li J,et al.The XAFS beamline of SSRF[J].Nuclear Science and Techniques,2015,26:050102.DOI:10.13538/j.1001-8042/nst.26.050102.
16 Loopstra B O.The phase transition inα-U3O8at 210°C[J].Journal of Applied Crystallogrphy,1970,3:94-96.DOI:10.1107/S002188987000571X.
17 Shannon R D.Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[J].Acta Crystallographica Section A,1976,32:751-767.DOI:10.1107/S0567739476001551.
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