Ce4+替代Pu4+的模拟固化体(Gd1-xCex)2Zr2O7+x的合成及结构演变

Gd₂Zr₂O₇中Gd具有很大的中子吸收截面, 其烧绿石结构-缺陷萤石结构的转变能较低, 使其成为理想的核废料固化基材. 使用硝酸盐为原料, 添加少量NaF作助熔剂, 在较低温度下(和传统高温固相反应相比), 合成了烧绿石型Gd₂Zr₂O₇. 以Ce⁴⁺模拟Pu⁴⁺, 研究了Gd₂Zr₂O₇对锕系核素的固化, 并合成了系列模拟固化体(Gd_1-xCe_x)₂Zr₂O_7+x (0≤x≤0.6). 采用粉末X射线衍射(XRD)对系列样品进行了表征. 结果表明: 随着x值的增大,样品从烧绿石结构向缺陷萤石结构转变, 且晶胞大小基本保持恒定, 但当x=0.6时, 衍射峰明显宽化, 晶格畸变比较严重, 晶格稳定性降低. 当x=1时, 即用Ce⁴⁺完全取代Gd³⁺进行合成, 不能得到Ce₂Zr₂O₈, 产物发生了相分离, 为四方结构的(Zr_0.88Ce_0.12)O₂和萤石结构的(Ce_0.75Zr_0.25)O₂的混合物. 模拟固化体的浸出率测试表明: 当x≤0.2时, 各元素浸出率均很低, 但当x≥0.4时, 各元素的浸出率明显升高, 说明以Gd₂Zr₂O₇作为固化Pu⁴⁺的基材, Pu⁴⁺掺入量不宜高于40%.

Synthesis and Structural Transformations of (Gd1-xCex)2Zr2O7+x: An Analogue for Pu Immobilization

Gd₂Zr₂O₇ is a well known host for nuclear waste immobilization because of the high neutron absorption cross section of Gd and low energy transformation between ordered pyrochlore and disordered defect-fluorite structures. Pyrochlore Gd₂Zr₂O₇ was synthesized at relatively low temperature (compared with traditional high temperature solid-state reaction) using Gd(NO₃)₃·nH₂O, Zr(NO₃)₄·nH₂O as a starting material and a small amount of NaF as fluxing agent. Ce⁴⁺ was used as an analogue for Pu⁴⁺ and its immobilization behavior in Gd₂Zr₂O₇ was studied in a series of solidified forms comprising (Gd_1-xCe_x)₂Zr₂O_7+x (0≤x≤0.6). Powder X-ray diffraction (XRD) data showed that the sample structure transformed from pyrochlore to defect-fluorite type with increasing x but maintained constant unit cell volumes. As x was increased to 0.6, the diffraction peaks showed broadening, suggesting considerable lattice distortion. When x=1, i.e., all Gd³⁺ are placed by Ce⁴⁺, the product was not Ce₂Zr₂O₈, but a phase separated mixture of tetragonal (Zr_0.88Ce_0.12)O₂ and an ideal fluorite (Ce_0.75Zr_0.25)O₂. Leach rate measurements indicated that the leach rate of Gd³⁺, Zr⁴⁺, Ce⁴⁺ was low when x≤0.2, but increased significantly when x≥0.4. This suggests that the substitution rate of Pu⁴⁺ for Gd³⁺ should not be more than 40% when Gd₂Zr₂O₇ used as the host matrix for Pu⁴⁺.