不同质地土壤对核素Sr-90阻滞及迁移的影响

为了给放射性废物填埋场选址、设计、建造和安全评估提供科学支撑,选择3种代表性质地的土壤(砂土、粉质壤土、黏土),采用室内动态土柱法,通过3个土柱体剖面Sr-90的浓度分布曲线和浓度峰迁移距离,研究Sr-90在不同质地土壤介质中的吸附特性和迁移规律。砂土、粉质壤土、黏土的喷淋量分别为60 m L/d、52 m L/d、60 m L/d,试验时间分别为102 d、390 d、390 d。试验结束后Sr-90浓度峰在砂土、粉质壤土、黏土中垂直向下迁移了46.2 cm、3.0 cm、1.2 cm。Sr-90在砂土柱剖面的浓度分布曲线存在明显的不对称性和拖尾,即Sr-90从源层上洗脱下来后,受到砂土介质的吸附作用,待浓度峰值通过后,更多的Sr-90从之前的砂土介质中解吸出来,使得很长时间段内土柱体中保持相对较高的浓度,该现象在粉质壤土和黏土不明显。应用HYDRUS-1D软件建立平衡吸附、非平衡吸附两种模式下的核素迁移数值模型,发现砂土柱中两种模式的计算结果差别较大,粉质壤土、黏土柱两种模式的计算结果却逐渐接近,非平衡吸附模式考虑一级速率系数β能更好地描述浓度分布曲线的"拖尾"。Sr-90在砂土、粉质壤土、黏土中的一级速率系数β分别为0.56 d-1、13.2 d-1、42.0 d-1,随β增大,吸附、解吸速率加快,越容易达到平衡。

Effect of different types of soil media on Sr-90 migration

The paper has chosen 3 representative soil media (sand,silt loam and clay) to establish the soil columns in the laboratory condition in hoping to reflect the migration capability of the Sr-90 relative in the different soil media and provide a scientific support for the selection,design,construction and safety assessment of the radioactive waste disposal sites.For this purpose,we have been engaged in a study of the adsorption characteristics and migration regularity of the said Sr-90 relative in the different soil conditions through the concentration distribution curve and the peak migration distance of Sr-90 in the 3 soil columns.The spray quantities of the sand column,the silt loam column,the clay column account for 60 mL/d,52 mL/d,60 mL/d,respectively,with the corresponding experimental time lengths being 102 d,390 d,390 d.Through the experiments,it has been found that the concentration peaks of Sr-90 migration are 46.2 cm,3.0 cm,1.2 cm in the sandy column,and those of the silt loam and the clay condition.Beyond the above quoted concentration peaks,the concentration distribution curves of Sr-90 in the sand column tends to exhibit asymmetrical and "tailing" tendency,which implies the beginning of the declining trend of the said Sr-90 migration from the source layer and get adsorbed by the sand.And,in turn,when the concentration peak goes beyond the observation point,more Sr-90 relative would be getting adsorbed from the sand and those sand particles left over,making the concentration of Sr-90 in the sand column stay rather high for a long time duration.However,except for the sand column,no concentration distribution curves of Sr-90 cannot be found as a kind of "tailing" phenomena in the silt loam and the clay column.Furthermore,we have managed to establish an equilibrium and nonequilibrium adsorption numerical model aided by the known HYDRUS-1D software for the radioactive nuclide migration.Except for the silt loam and clay,the calculation results gained through the two models turn out to vary greatly in the aforementioned sand columns.Of the two,the non-equilibrium adsorption model proves to be able to describe the tailing phenomenon better,for it has taken into consideration the first-order rate coefficientsβ,whichare0.56d-1,13.2d-1,42.0d-1inthesand,and in the silty loam and the clay with the increase of β.In spite of this,the calculation results of the equilibrium adsorption and the non-equilibrium adsorption can be made gradually closer to each other in the silt loam and clay,which implies that,the faster the adsorption and desorption rate would be,the easier the equilibrium status can be achieved.