燃耗对水法乏燃料后处理工艺中钚行为的影响

Effects of Burnup Degree on Behaviors of Plutonium During PUREX Reprocessing

随反应堆燃耗加深,核燃料在反应堆中产生更多的裂变产物和复杂的微结构变化,使乏燃料后处理过程中次级沉淀和界面污物等的形成变得越来越严重,也使钚在此过程中的行为变得越来越复杂。依据Pu(Ⅳ)与Zr4+结构相似(均为四价和八配位体),考察硝酸溶液中钼锆沉淀行为和锆与溶剂辐解产物HDBP形成界面物的机制,初步探讨钚在次级沉淀和界面物形成过程中的行为。结果表明:溶液硝酸浓度对锆钼沉淀的形成有明显影响;沉淀物中钼锆的物质的量比为2,结构式为Zr(MoO_4)_2·xH_2O,次级沉淀过程中,钚产生共沉淀,推测沉淀物的分子式为Pu(MoO_4)_2·xH_2O;Zr^(4+)与HDBP主要形成Zr(NO_3)_x(DBP)_n和Zr(NO_3)_x·n(DBP·HDBP)配合物,前者易形成界面物,后者易溶于有机溶剂;它们的生成量与HDBP浓度有关,HDBP浓度较低时,生成的主要是Zr(NO_3)_x(DBP)_n,随HDBP浓度增大,易形成界面物;HDBP浓度进一步增大,生成的主要是Zr(NO_3)_x·n(DBP·HDBP),界面物消失;在Pu(Ⅳ)与HDBP反应过程中,随HDBP浓度逐渐增大,界面形成过程与Zr^(4+)相似;同时,Pu(Ⅳ)与HDBP和H2MBP形成配合物,对钚还原反萃取过程有一定影响。

With increasing of burnup of NPP fuel,irradiated nuclear fuel forms more radionuclides and changes complex microstructure. It is quite obvious that the behaviors of post-precipitation and formation of interfacial crud will be more serious, and behaviors of plutonium will be more complex. Based on the similar structure of tetravalency and octahedral coordination geometry of Pu（Ⅳ） and Zr^4＋ ,the precipitation of Mo and Zr in nitric acid solution and the forming mechanism of interfacial crud by Zr and HDBP were investigated, and the behavior of plutonium during forming post-precipitation and interfacial crud were discussed. The results show that the concentration of nitric acid has an obvious effect on the precipitation of Mo and Zr. The molar ratio of Mo and Zr in precipitation is 2, and the structural formula may be Zr（MoO4 ）2 · xH2O. It can be deduced that plutonium is also co-precipitated with Mo and the precipitation formula is Pu（MoO4）2· xHz O. Zr^4＋ and HDBP forms mainly two types of complex,i. e. ,Zr（NO3）x （DBP）n and Zr（NO3）x n （DBP · HDBP）. The former tends to form interfacial crud and the latter is easily dissolved in organic solvents. The amount of complex is affected by the concentration of HDBP. The main production is Zr（NO3）x （DBP）n for low concentration of HDBP and the interracial crud tends to increase with the concentration of HDBP. When more HDBP forming,the main production is Zr（NO3）x· n（DBP · HDBP） and the interracial crud disappears. The similar results are also observed in the case of Pu（Ⅳ） when the concentration of HDBP increasing, and the complex formed by Pu（Ⅳ） and HDBP or H2MBP is found,which has an effect on the reduction strip.