Ce^(4+)替代Pu^(4+)的模拟固化体(Gd_(1-x)Ce_x)_2Zr_2O_(7+x)的合成及结构演变

Synthesis and Structural Transformations of (Gd_(1-x)Ce_x)_2Zr_2O_(7+x) : An Analogue for Pu Immobilization

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(Gd1-xCex)2Zr2O7+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(Zr0.88Ce0.12)O2和萤石结构的(Ce0.75Zr0.25)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%.

Gd2Zr207 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 Gd2Zr207 was synthesized at relatively low temperature （compared with traditional high temperature solid-state reaction） using Gd（NO3）3. nH20, Zr（NO3）,. nH20 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 Gd2Zr207 was studied in a series of solidified forms comprising （Gdl-xCex）2Zr2OT＋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=l, i.e all Gd3＋ was placed by Ce4＋, the product was not Ce2Zr2Os, but a phase separated mixture of tetragonal （Zr0.88Ce0.t2）O2 and an ideal fluorite （Ce0.75sZr0.25）O2. Leach rate measurements indicated that the leach rate of Gd4＋, 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 Gd2Zr20, is used as the host matrix for Pu4＋.