Cs和I在某拟建核电厂液态流出物长距离排放管线周围土壤中的扩散行为

Diffusion Behaviors of Cs and I in Soil Surrounding Long-distance Effluent Pipeline of Proposed Nuclear Power Plant

裂变产物^(137)Cs和129/^(131)I是液态流出物中重点关注的核素,其产额高、迁移能力强,具有生物有效性,是核设施辐射环境安全评价的重要指标。本文以某拟建核电厂液态流出物长距离排放管线下方浅层土壤及模拟地下水为实验材料,采用稳定同位素(^(133)Cs和^(127)I)替代开展贯穿扩散实验,通过自定义软件Pycharm对扩散数据进行拟合。结果表明,不同采样点的浅层土壤通过吸附作用阻滞Cs^(+)扩散,Cs^(+)的有效扩散系数为3.87×10^(-11)~4.31×10^(-11)m^(2)/s;而对于阴离子I^(-),其所有采样点的固液分配系数K d均趋近于0(1.75×10^(-4)~2.72×10^(-7)L/g)、有效扩散系数为1.97×10^(-11)~2.61×10^(-11)m^(2)/s,表明弱吸附核素^(129/131)I难以被土壤阻滞,易迁移至地下水中。由于采样点土壤的主要成分均为石英、斜长石、钙长石,因此土壤表面电荷是影响Cs^(+)和I^(-)在该区域扩散的主控因素。

In the assessment of environmental radiation risk,^(137)Cs and 129/^(131)I,as high-yield fission products,are crucial indicators in the radioactive effluent of nuclear power plants.Due to the strong mobility and bioavailability of Cs^(+) and I^(-)in the groundwater-soil system,their diffusion behaviors attracted much attention.In this work,the shallow soil samples under the long-distance effluent pipeline of a proposed nuclear power plant in China were used to study the diffusion behaviors of stable analogue(^(133)Cs and^(127)I)of radioisotopes.The experimental data were fitted by Pycharm,which shows that the shallow soil at different sampling sites retards Cs+through the adsorption process.The effective diffusion coefficient of Cs+ranges from 3.87×10^(-11)m^(2)/s to 4.31×10^(-11)m^(2)/s,the range of effective diffusion coefficient of I^(-)is 1.97×10^(-11)m^(2)/s to 2.61×10^(-11)m^(2)/s,illustrating the diffusion of Cs+and I^(-)are normal diffusion.The through time of Cs+ranges from 29.24 d to 34.48 d,and the solid-liquid distribution coefficient(K d)ranges from 3.34×10^(-2)L/g to 4.27×10^(-2)L/g.On the contrary,the average through time of I^(-)is 0.30 d,which is much less than that of Cs^(+),and the solid-liquid distribution coefficient(K_(d))of I^(-)tends to be zero(1.75×10^(-4)-2.72×10^(-7)L/g),indicating that the weakly adsorbed^(129/131)I is difficult to be fixed by soil,and illustrating a faster diffusion of I^(-)than Cs^(+).The diffusion coefficient of Cs+and I^(-)in SK02 soil is the largest,which indicating that the migration rate of Cs+and I^(-)is faster in SK02.The differences of Cs^(+)and I^(-)diffusion behaviors are affected by soil adsorption.Illite,biotite and montmorillonite in soil can provide stong affinity adsorption sites for Cs^(+),on the contrary,it is difficult to provide effective adsorption sites for I^(-).Since the main components of the soil samples are quartz,plagioclase and anorthite,when the concentration of Cs+is higher than the capacity of strong affinity sites,a small amount of clay minerals in the soil will no longer control the diffusion of Cs^(+).In the meantime,the negative charge of the soil surface can attract Cs+and reject I^(-),so in the long-term diffusion process,the surface charge is the dominant factor that controlling the diffusion behavior of Cs+and I^(-)in this area.