双功能基阴离子交换吸附——还原解吸法从高放废液中分离锝的研究

The long-term radiotoxicity in high level liquid waste (HLLW) is governed by the existence of several long-lived radionuclides, among which the most significant nuclides are the long life fission product elements (99Tc and 129I) and minor actinides (Np, Am and Cm). From the viewpoints of minimizing the long-term radiological risk to the environment and facilitating the management of HLLW, separation of some long-lived radionuclides is much more essential. . 99Tc is a long-lived, β-emitting fission product with a half-life of 2×105 years and a relatively high fission yield (6.3% of 235U). Removal of this isotope from spent nuclear fuel waste streams is important in view of its long lifetime, toxicity, and high mobility in the environment. Most of researches on separation of Tc utilize liquid-liquid extraction technology. A large amount of the secondary waste, which is difficult for treatment and disposal, will be generated by the extraction process. In addition, the liquid-liquid extraction process is basically beneficial for a large-scale process. Compared to U and Pu, the Tc and MA are significantly less abundant in the spent fuel. In this project, to develop an advanced technology which uses compacted equipment and generates less radioactive waste for separation of Tc, a new aqueous reprocessing based ion exchange separation will be established which is characterized by fast adsorption kinetics and low pressure drop in a packed column. . The adsorption affinity between anion exchanger and TcO4- is so high that it is difficult to elute the adsorbed TcO4- by using a diluted HNO3. However, concentrated HNO3 with a strong oxidation will result in damage to the exchanger. In this project, we will prepare novel exchangers with bi-functional groups which are immobilized in porous silica (SiO2) or chemically bonded in porous silica and polymer composite particles (SiO2-P). Adsorption behavior, desorption behavior, resistant behavior against irradiation and HNO3 of anion exchangers will be evaluated in detail. We will examine the reductive elution of Tc(VII) from anion exchanger by using reducing agents. Adsorption and separation behavior of various elements contained in simulating HLLW will be studied experimentally and theoretically. We will select Re as simulated nuclide to study the separation mechanism by means of X-ray absorption fine-structure spectroscopy. At last the structure of anion exchangers will be optimized. This work is performed to establish the advanced techniques for efficient adsorption and elution of Tc. All research results will provide scientific evidence for partitioning and transmutation of Tc.

高放废液中99Tc和次锕系元素的存在是决定其长期潜在放射性危害的关键因素。99Tc的半衰期为21万年，具备高移动性和高裂变产额，所以在环境放射性安全评价方面亦是关键核素。将其从高放废液中分离出来进行嬗变处理是消除其放射毒性的唯一途径。但Tc 的化学行为复杂，至今没有找到具有实用前景的分离方法。本项目旨在探讨阴离子交换技术分离Tc的新方法。首先采用原位聚合或辐射接枝法在硅基载体上引入弱碱和强碱性双功能交换基，并实现功能基与载体的键合，合成对Tc吸附选择性高、稳定性好的交换剂，考察两种方案制备的交换剂对Tc 及其共存元素的分离性能。洗脱过程中引入还原剂，解决Tc易被吸附而难被解吸的技术瓶颈。探讨吸附/解吸机理，并评价交换剂的辐照和化学稳定性，以建立从模拟高放废液中分离Tc 的最优化技术体系。该技术的研究不仅为制备高稳定性的交换剂提供新思路，而且可明确不同价态的Tc在交换剂上的吸附/解吸机理。

99Tc是一种长寿命放射性核素，其在核电站运行过程中的裂变产额高达6%，因此研究如何将其从乏燃料后处理过程中分离出来具有重要意义。铼作为与锝同处于VIIB族的元素，与锝具有相似的化学性质，因此常被用来作为锝的非放射性模拟元素；另一方面，铼本身是稀有的、具有巨大应用价值的金属元素，因此研究如何提升分离吸附铼的工艺也具有重大意义。项目主要采取高分子材料辐射接枝改性法和悬浮聚合法合成了两种类型含叔胺、季胺基团的阴离子交换树脂，并基于这两种树脂研究了它们对于铼和锝的吸附解吸性能。.首先，采用γ-共辐照接枝法，以聚苯乙烯白球（PS）作为骨架材料，4-乙烯基吡啶（4-VP）作为接枝单体设计辐照反应体系，一步法制备了一种高离子交换容量、对TcO4-或ReO4- 吸附选择性高的接枝共聚型吸附剂（PS-g-4VP）。接着在此基础上，通过与碘乙烷（IE）进行季铵化反应，制备了一种强碱性阴离子交换树脂PS-g-4VP-IE。为了优化合成条件，分别考察了单体浓度，吸收剂量，季铵化时间等因素对接枝率和胺化率的影响规律。在辐射接枝制备阴离子交换树脂的基础上，采用第二种方法--悬浮聚合制备出多孔型树脂，在优化4-乙烯吡啶单体用量，引发剂类型、溶剂类型及配比的条件下合成一种高比表面积弱碱性阴离子交换树脂4VP-DVB。在此基础上，经过季铵化处理，得到一种强碱性阴离子交换树脂4VP- DVBQ。.在研究两种树脂吸附铼的静态实验过程中,分别考察了水溶液酸度、吸附温度、吸附时间等影响因素，结果表明PS-g-4VP-IE树脂的吸附平衡时间约为30 min，理论饱和吸附容量约为252 mg/g，这些性能都优于树脂PS-g-4VP。运用静态和动态柱吸附优化了树脂对TcO4-或ReO4-的最佳吸附和解吸条件，针对叔胺和季胺型两种树脂，进行吸附动力学、吸附机理研究。在解吸实验中，引入还原性解吸剂，实现了低酸度下还原解吸吸附后ReO4-，从而避免高酸解吸环境对树脂结构的破坏，相关研究结果为正在进行的TcO4-的吸附和解吸提供强有力的实验数据支持。.在研究对于99Tc的吸附实验中，相应实验结果与铼基本一致，体现了用铼模拟锝的合理性；同时，两种方法制备的树脂都表现出了对于99Tc的良好吸附--解吸性能，为它们在乏燃料后处理中的应用奠定了基础。