铀在Nd_2Zr_2O_7烧绿石中的固溶量及重离子辐照效应

Nd_2Zr_2O_7烧绿石因其稳定的物理化学性质和辐照稳定性可以作为高放废物中锕系核素的固化基材.通过溶胶凝胶—喷雾热解—高温烧结方法制备了含铀的Nd_2Zr_2O_7烧绿石固化体;开展了Nd_2Zr_2O_7和Nd_(1.9)U_(0.1)Zr_2O_7固化体的重离子辐照实验,辐照剂量为1 dpa和3 dpa;利用X射线衍射和Raman光谱对固化体结构进行了分析.研究发现铀在Nd_2Zr_2O_7烧绿石体系的固溶量仅为10 at%,高价态铀掺杂导致固化体结构向无序化转变.重离子辐照实验表明, Nd_2Zr_2O_7烧绿石基材具有较高的抗辐照稳定性;而Nd_(1.9)U_(0.1)Zr_2O_7在较低辐照剂量下,固化体烧绿石体系结构破坏,重离子辐照诱导固化体结构转变为更加无序化的萤石结构.低固溶量和抗辐照能力减弱主要是由于锕系核素烧绿石固化体的结构无序化所致.

Solubility and ion-irradiation effects of uranium in Nd2Zr2O7 pyrochlore

Nd₂Zr₂O₇ pyrochlore with higher physicochemical and radiation stability has been considered as a host matrix for actinide immobilization of high level radioactive wastes. Uranium is a constituent and the decay-daughter product of high level radioactive wastes. It is necessary to study the solubility and ion-irradiation effect of uranium in Nd₂Zr₂O₇ pyrochlore. The solubility of U is studied by the A site substitution in the pyrochlore structure. A series of uranium-doped zirconate pyrochlore compositions is prepared by the sol-gel-spray pyrolysis-high temperature sintering method. The structures of immobilization are studied by using X-ray diffraction (XRD) and Raman spectroscopy. The XRD and Raman spectroscopy studies reveal that the solubility limit of uranium in Nd₂Zr₂O₇ is estimated at 10 at%. The lattice parameter of pyrochlore decreases with U content increasing, which is due to lower ionic radius of U. The immobilization structure changes from order pyrochlore to disorder structure. Further addition of U content leads to the separation of U₃O₈ phase in the immobilization. The U ions with high valance may be substituted at A or B site in Nd₂Zr₂O₇ pyrochlore, which results in the A–O and B–O bond destruction. In order to keep the balance of charge, extra O ions should enter into the vacancy site, the structure of pyrochlore maybe transforms into a disorder structure. The radiation resistance of immobilization is investigated by ion-beam irradiation with 2 MeV Kr¹⁵⁺ ions at room temperature. The Nd₂Zr₂O₇ and Nd_1.9U_0.1Zr₂O₇ are irradiated at doses of 1 dpa and 3 dpa, respectively. Analyses of the XRD and Raman spectroscopy data show that the Nd₂Zr₂O₇ pyrochlore remains full pyrochlore structure even at a higher irradiation dose, which suggests that the Nd₂Zr₂O₇ exhibits higher radiation resistance as potential immobilization. In contrast, the Nd_1.9U_0.1Zr₂O₇ immobilization shows the weaker radiation resistance, the pyrochlore structure completely transforms into a disorder fluorite structure. The A–O and B–O bonds of Nd_1.9U_0.1Zr₂O₇ pyrochlore structure are easy to destroy under ion irradiation conditions due to the disorder of pyrochlore. At the same time, the excess O ions are rearranged in U-rich pyrochlore after irradiation. Bond destruction and ion rearrangement of pyrochlore structure result in the full disorder fluorite structure. The actinides-doped pyrochlore structure is modified due to the change in physicochemical propertyof actinide, which results in the reductionof the solubility limit and radiation resistance.