Recent activities in the field of nuclear waste management

Buffer construction using bentonite pellets as filling material is a promising technology for enhancing the ease of repository operation. In this study, a test of such technology was conducted in a full-scale simulated disposal drift, using a filling system which utilizes a screw conveyor system. The simulated drift, which contained two dummy overpacks, was configured as a half-cross-section model with a height of 2.22m and a length of 6.0 m. The average dry density of the buffer obtained in the test was 1.29 Mg/m³, with an angle of repose of 35 to 40 degrees. These test results indicate that buffer construction using a screw conveyor system for pellet emplacement in a waste disposal drift is a promising technology for repositories for high level radioactive wastes.     

模拟高放玻璃体在高温湿气中的蚀变行为

模拟地下水穿透地质处置多重屏障情形下，地下水会以高温蒸气的形式与高放玻璃体发生反应，加速玻璃体的蚀变。采用纯水和特定的盐溶液作为浸泡剂以控制体系的相对湿度分别在100%和90%左右。模拟高放玻璃体样片用聚四氟乙烯线悬挂在浸泡剂上方并在低氧箱中平衡，分别在120 ℃和150 ℃的干燥箱内反应。通过测试浸泡液中各元素的浓度和分析玻璃体表面的腐蚀产物，以表征玻璃的蚀变速率。实验发现，温度对玻璃腐蚀的影响最为明显，湿度的影响较弱。玻璃体中大多数元素在150 ℃下的浸出速率高于120 ℃下的浸出速率2～3个数量级。90 d时，玻璃体在150 ℃下已观察到显著的腐蚀。900 d后，150 ℃下玻璃体已基本完全腐蚀；而在120 ℃下的玻璃腐蚀程度很小，900 d时腐蚀厚度仍小于50 μm。150 ℃时随着腐蚀的发生，玻璃表面的SiO2腐蚀层逐渐形成以钙的硅酸盐为主的二次矿相。     

腐殖酸对Tc价态影响研究

在模拟高放废物深地质处置环境的低氧(O2含量小于5×10-6)、低Tc(Ⅶ)初始浓度(10-8 mol/L)条件下,研究腐殖酸浓度对Tc(Ⅶ)价态影响。结果显示,腐殖酸对99Tc液闪测定存在淬灭效应,但当腐殖酸浓度在40 mol/L以下,保持闪烁液体积为样品体积20倍可消除影响;在此基础上分别用四苯砷氯-氯仿萃取法和LaCl3沉淀腐殖酸法独立进行腐殖酸-Tc(Ⅶ)体系中锝价态研究,所得Tc(Ⅳ)和Tc(Ⅶ)价态分布结果基本吻合,表明两种研究方法均可用于腐殖酸-Tc(Ⅶ)体系中低浓度Tc价态研究;研究发现平衡350天的不同浓度Fluka腐殖酸-Tc(Ⅶ)体系中,Tc(Ⅶ)浓度长期保持稳定,而Tc(Ⅳ)浓度很低,表明即使在低氧气氛中,Fluka腐殖酸也未对Tc(Ⅶ)表现出明显还原行为,腐殖酸在地下水中的存在可能是高放废物地质处置中Tc(Ⅶ)阻滞的不利因素。     

模拟高放玻璃体在高温湿气中的蚀变行为

模拟地下水穿透地质处置多重屏障情形下，地下水会以高温蒸气的形式与高放玻璃体发生反应，加速玻璃体的蚀变。采用纯水和特定的盐溶液作为浸泡剂以控制体系的相对湿度分别在100%和90%左右。模拟高放玻璃体样片用聚四氟乙烯线悬挂在浸泡剂上方并在低氧箱中平衡，分别在120 ℃和150 ℃的干燥箱内反应。通过测试浸泡液中各元素的浓度和分析玻璃体表面的腐蚀产物，以表征玻璃的蚀变速率。实验发现，温度对玻璃腐蚀的影响最为明显，湿度的影响较弱。玻璃体中大多数元素在150 ℃下的浸出速率高于120 ℃下的浸出速率2～3个数量级。90 d时，玻璃体在150 ℃下已观察到显著的腐蚀。900 d后，150 ℃下玻璃体已基本完全腐蚀；而在120 ℃下的玻璃腐蚀程度很小，900 d时腐蚀厚度仍小于50 μm。150 ℃时随着腐蚀的发生，玻璃表面的SiO2腐蚀层逐渐形成以钙的硅酸盐为主的二次矿相。     

高放废物处置库预选缓冲/回填材料液塑限测定试验研究

深地质处置被国际上公认为处置高放废物的最有效可行的办法。缓冲/回填材料是高放废物深地质处置中非常重要的一道人工屏障。经过筛选,内蒙古高庙子钠基膨润土被确定为我国高放废物深地质处置的首选基料。通过对该钠基膨润土的落锥式和碟振动式两种液塑限测定方法的比较,确定利用碟振动式仪法测试更合理,测得液限为310%,塑限值为39.0%,塑性指数为271.0%。结果表明,内蒙古高庙子钠基膨润土具有高的水容量、荷重下的高致密性、低渗透性、较高的强度和较强的水稳定性。     

高放废物处置评价中应用的有关程序

对有关在高放废物处置中应用的程序进行了调研,根据应用方向将这程序分成源顶计算程序、地下水与核素迁移程序、热－湿－力耦合程序及其它程序4类。     

低氧条件下浸出剂和温度对固化体浸出行为的影响

采用MCC-1静态浸出法研究了在低氧条件下,两种温度（150℃和90℃）和两种浸出剂（模拟地下水和去离子水）在浸泡546d后对玻璃固化体浸出行为的影响。实验结果显示,高温会加速玻璃固化体的质量浸出率,增加表面层的厚度和加速表面生成物形成。玻璃固化体浸出反应活化能计算结果显示：在高温条件下模拟地下水对玻璃固化体腐蚀比去离子水严重。玻璃固化体主要元素浸出机理研究显示,高温条件下浸出剂的差异会对元素浸出行为产生影响。     

Consequence Analysis of Fluvial Erosion Scenarios for an HLW Repository

In Japan, uplift/erosion scenarios must be analysed even if they occur far in the future, as no assessment cut-off times have yet been defined. For this purpose, the geological record of river terrace formation and their long-term evolution have been studied further, with the aim of constructing a better defined fluvial erosion model than the original, rather simple cyclic fluvial erosion model developed in a previous study. The developed conceptual model, based on a generalization of field observations in relevant settings, allows the consequences of erosion of the repository to be assessed just as before, using comparisons with natural radionuclide fluxes. Results illustrate the effectiveness and robustness of the H12 HLW disposal system and indicate that erosion scenarios are unlikely to be critical for distinguishing between different sites that lie in similar mid-range river settings.     

Boehmite Actual Waste Dissolution Studies

Abstract

The U.S. Department of Energy plans to vitrify approximately 60,000 metric tons of high-level waste (HLW) sludge from underground storage tanks at the Hanford Nuclear Reservation. To reduce the volume of HLW requiring treatment, a goal has been set to remove a significant quantity of the aluminum, which comprises nearly 70 percent of the sludge. Aluminum is found in the form of gibbsite, sodium aluminate and boehmite. Gibbsite and sodium aluminate can be easily dissolved by washing the waste stream with caustic. Boehmite, which comprises nearly half of the total aluminum, is more resistant to caustic dissolution and requires higher treatment temperatures and hydroxide concentrations.

Samples were taken from four Hanford tanks and homogenized in order to give a sample that is representative of REDOX (Reduction Oxidation process for Pu recovery) sludge solids. Bench scale testing was performed on the homogenized waste to study the dissolution of boehmite. Dissolution was studied at three different hydroxide concentrations, with each concentration being run at three different temperatures. Samples were taken periodically over the 170 hour runs in order to determine leaching kinetics. Results of the dissolution studies and implications for the proposed processing of these wastes will be discussed.     

Influence of heating and water-exposure on the liquid limits of GMZ01 and MX80 bentonites

Bentonite has been considered as a buffer material for embedding canisters with high-level radioactive waste（HLW） in deep geological repositories.GMZ bentonite deposit,located in Xinghe County,Inner Mongolia Autonomous Region,China was proposed as a buffer/backfill material for HLW repository in China.The liquid limits of natural Na-bentonite GMZ01 and commercial Na-bentonite MX80,which are previously heated at 80 ℃ and 95 ℃,respectively,and exposed to water for different times are measured.It is observed that the liquid limit of GMZ01 increases slightly at the beginning,and then decreases as the heating time increases,while the liquid limit of MX80 decreases with the heating time.The liquid limits of both GMZ01 and MX80 decrease with increasing water-exposure time.After the samples are heated at 80℃and 95 ℃ for several months,the mineralogical composition of GMZ01 does not exhibit evident change,whereas MX80 experiences some changes.In addition,the chemical composition,cation exchange capacity（CEC） and exchangeable cation of all the samples do not change significantly.