基于Am/Cm分离的芳胺基多孔树脂制备及其模型化合物的动态辐解机理研究

The long-term radiotoxicity in high level liquid waste (HLLW) is governed by the existence of actinde elements (Am, Cm). The separation of americium and curium is compilcated because of their similar chemical properties in solution. The commercial anion exchange resins which contain alkylamino groups have poor stability against radiation. Furthermore, the main disadvantage of impregnated resin is their low stability due to the leakage of the extractant from the inorgnic support. Radiation induced grafting method for preparation of exchange resin has advantages of controlling the composition and micro-structure of resin. By designing grafting system which includes monomers with arylamino groups, we will prepare exchange resins with different surface area and pore volume through optimizing type and concentration of crosslinking agent and chain transfer agent. At last we will clearly indicate the relationship between grafting conditions, micro-structure of resin and adsorption kinetics. Moreover, arylamines and alkylamines will be selected as model compounds for resins. According to operating environment of resin, the pulse radiolysis technique will be used to clarify the degradation processes and degradation rate of model compounds. The results will elucidate the degradation mechanism from aspect of dynamic molecular reactions. The whole research project which combines with experimental study and mechanism analysis will provide experimental and theoretical base for modifying amino resins used for separation of Am and Cm.

次锕系元素（Am，Cm）是HLLW中对环境具有潜在放射性危害的决定核素，Am与Cm的化学性质极为相似，相互分离非常困难。已商业化分离树脂因含有烷胺基，辐射稳定性差。以多孔无机材料为载体的灌装型树脂存在吸附剂易泄漏缺陷。多孔材料辐射接枝改性制备交换树脂具备灵活调控树脂组成及孔道微观结构的优势。本项目设计含芳胺基单体的辐射接枝体系，通过改变交联剂、链转移剂类型和用量制备出不同孔体积和比表面积的树脂，最终明确接枝条件-树脂微观结构-核素吸附动力学三者关系。接着,以分子结构含芳胺基或烷胺基的化合物作为树脂的模型化合物，模拟树脂吸附分离Am/Cm时所处的环境，设计合理的辐解体系，采用脉冲辐解瞬态分析技术揭示含（不含）芳胺基树脂的辐射降解过程和降解速率，从动态分子反应的角度阐明不同结构树脂辐射降解机理。本项目结合实验研究和机理分析，将对烷胺基树脂进行结构改性并应用于Am/Cm分离提供实验和理论依据。

由于某些镧系同位素具有很高的中子吸收截面，会成为锕系嬗变过程中的中子毒物，并且镧系元素在HLLW中的含量远高于Am和Cm，因此，从Tc等长寿命裂变产物共存 的HLLW中选择分离镧系元素是高放废液“分离-嬗变”的关键步骤之一，而且分离回收后的镧系元素因特殊的化学和物理性质可广泛应用在各工业领域。本项目采用多孔聚苯乙烯-二乙烯苯树脂（PS）为基体，分别采用化学改性和辐射接枝法成功合成了吡啶基阴离子交换树脂（PS-N）和二酰胺基螯合树脂（PS-TMMA），树脂上TMMA的化学键合量为0.486 mmol/g。考察pH，固液比和接触时间对PS-TMMA吸附镧系、锕系元素和99Tc的影响， 其中pH=7时可实现镧系元素、锕系元素（以Am为代表）和99Tc的完全分离。镧系元素在包含PS-TMMA色谱柱上的穿透点和饱和吸附点顺序均为La，Ce，Pr，Nd，Eu，Gd，Dy，Yb。淋洗液pH=3.03，0.01 mol/L 2-羟基异丁酸对各镧系元素的回收率为40.5%(La)，45%(Ce)，58.0%(Pr)，54.2%(Nd)，75.7%(Eu)，81.5%(Gd)，87%(Dy)，100%(Yb)，并且轻镧系元素和重镧系元素可实现分离。PS-TMMA对镧系、锕系元素的吸附机理发现是基于TMMA羰基氧与元素间的络合作用。. 项目中制备的多孔树脂将用于HLLW中相关元素的选择性分离，具备良好的辐照稳定性很重要。选取PS-N作为芳香型树脂代表，详细研究了吸收剂量，pH和NO3-浓度对溶液中NH4+浓度和树脂离子交换容量的影响，结果表明一定浓度的硝酸或硝酸盐的存在能够减缓PS-N树脂的辐解，当吸收剂量为100 kGy，5 mol/L HNO3溶液中PS-N的离子交换基团损失率仅为2%。水的辐解产物水合电子攻击使PS-N分解成吡啶和聚苯甲酰自由基，吡啶被水溶液中的氢氧自由基氧化分解成NH4+、甲酸和草酸等。随吸收剂量增加，甲酸生成量逐渐降低，而乙二酸的生成量逐渐增大，归因于甲酸被氢氧自由基进一步降解成水和二氧化碳。PS-N树脂辐解气相产中物，易燃气体CH4和H2的生成量远低于商用树脂Amberlite IRN400。PS-N树脂辐照分解的研究为吸附实验设计和实际工程应用中预言树脂的辐射损伤提供理论和数据依据。