同步辐射技术研究附银分子筛吸附剂对放射性含碘气体的吸附性能

Purification of radioactive iodine contained off-gas is an important process for the nuclear spent fuel reprocessing. Renewable Ag/zeolite sorbents are the most effective iodine removal materials. The adsorption technology was suggested as follows: 1) absorption of I2 and CH3I in the off-gas on Ag/X and Ag/MOR; 2) regeneration of Ag/X and Ag/MOR by H2 reduction at 500oC when in saturation, 3) absorption of HI gas produced in reduction by Pb/X with iodine loading at 400mg/g; 4) conversion of the produced PbI2 into Pb10(VO4)6I2, which was the terminal radioactive waste land-filled finally. This project produced less amount of nuclear waste with high economy effectiveness. Currently the sorbents Ag/X and Ag/MOR have two problems need to be resolved. The first one is that Ag loading on the sorbents is higher than 40wt%, while the utilization of Ag is lower than 50%. A lot of Ag is inactive to the adsorption of I2 and CH3I, the reason could be likely that Ag species are in different states and located in different position in the zeolite. In order to study the influence of Ag state to the iodine adsorption, the Extended X-ray Adsorption Fine Structure (EXAFS) measurement will be chosen for observations of the local structure around Ag+ ions or Ag atoms, and the XRD with Rietveld refinement strategies be chosen to explore the Ag location in the framework of zeolite 13X and MOR. On this basis, we could improve the preparation process in order to obtain the sorbents with better performance. The second problem is that the adsorbed iodine species, which can not be desorpted entirely from Ag/X and Ag/MOR. To resolve this problem, we will probe the local structure around the adsorbed iodine species with EXAFS, and study the relation between iodine states with H2 reduction performance by H2-TPR technique. Then, we could improve the regeneration process of sorbent in order to use Pb contained material as the terminal radioactive waste instead of Ag. In the light of the special conditions for the off-gas in nuclear spent fuel reprocessing, the influences of temperature, humidity, γ-ray radiation and the coexisted nitric acid vapor and the oxidizing gas NO2, will be studied deeply to discover the safety threshold for the application of Ag/zeolite sorbents. This project will not only provide an advanced iodine removal material for the nuclear spent fuel reprocessing plant in China, but also play an important role on the development of nuclear air cleaning disciplines.

放射性碘废气净化是核能乏燃料后处理重要工艺环节，可再生型附银分子筛是最有效的除碘吸附剂。理想技术路线为：AgX或Ag/MOR吸附I2及CH3I→H2还原解析→PbX吸附HI→含铅终端废物，该技术核废物量小、经济性好。Ag负载量大、利用率低及碘解析不完全是亟待解决的问题。本课题采用EXAFS、XRD结构精修与多种技术结合，研究AgX和Ag/MOR表面Ag物种结合状态及其在分子筛中的位置分布，探明对碘吸附最有效的Ag物种，藉此反馈调节吸附剂制备过程，获得Ag利用率高的吸附剂。采用EXAFS研究吸附态碘物种的结构与其H2还原解析性能的相关性，藉此改进吸附剂再生工艺，实现以Pb代替Ag作为碘终端废弃物。针对乏燃料后处理废气特殊工况，研究温湿度、硝酸蒸汽、氧化性气体NO2及γ辐照对吸附性能的影响，探索吸附剂使用的安全阈值。该项研究既可为我国后处理厂提供先进除碘材料，也对核空气净化学科发展意义重大。

放射性碘废气净化是核能乏燃料后处理的重要工艺环节，可再生型附银分子筛吸附剂是最有效的除碘技术，该技术路线核废物量小、经济性好。美国和德国都对该技术进行了研究，AgX吸附剂已在日本东海村乏燃料后处理厂实际应用，而我国至今尚没有附银分子筛型除碘吸附剂。本课题旨在研制出应用于我国后处理厂的附银分子筛型吸附剂，通过活性物种Ag结构的深入分析，以降低非活性Ag的份额，提高Ag的利用率。主要结果如下：1.首先分别采用AgNO3和Ag(NH3)2+为离子交换剂，制备出Ag/X和Ag/MOR吸附剂，采用XRF测定Ag交换度，发现：Ag+半径小，可进入13X分子筛中的不同孔道与Na+充分交换，获得100%交换度的Ag/X吸附剂；但离子半径大的Ag(NH3)2+只能进入13X分子筛中较大的孔道，最高交换度接近50wt%，据此可实现13X中不同晶体点位Na+离子的可控离子交换。分子筛MOR中具有12元环直通孔道，不能阻止Ag(NH3)2+进入，因此采用相同浓度的Ag+和Ag(NH3)2+都能够获得100%交换度，而且Ag(NH3)2+易于在MOR表面吸附，所获吸附剂同时存在离子交换态Ag物种和表面吸附态Ag物种。2.采用UV光谱、XPS和饿歇能谱、EXAFS等研究了Ag/X和Ag/MOR上Ag物种结构。结果表明：Ag/MOR上主要以Ag+或Ag2O的状态存在，未见还原态Ag单质的吸收峰。Ag/13X上，低交换度样品中主要是以Ag+或Ag2O的状态存在，随着交换度增大，Ag还原显著，主要以原子态Ag团簇结构为主。不同交换离子形成的Ag原子团簇结构不同，AgNO3交换样品主要以Ag0团簇为主；而Ag(NH3)2+交换样品以Agmn+团簇结构存在。3.Ag/13X上甲基碘的吸附容量高于Ag/MOR，其中Ag+交换度为47%的样品吸附容量最高为54mL/g，Ag(NH3)2+交换度为37.30%的样品容量最高为55mL/g，交换度小于20%样品吸附容量都较小。Ag/MOR体系，无论以AgNO3还是以Ag(NH3)2+为交换剂获得的两组样品穿透时间短，吸附容量普遍较小。 对活性Ag物种结构与其对元素碘蒸汽和甲基碘的吸附性能相关性进行研究，发现：Ag/MOR和低交换度的Ag/13X上Ag主要以离子态存在，有利于元素碘吸附；而高交换度Ag/13X上的Ag0团簇或Agmn+团簇结构对甲基碘的吸附有利。