放射性核素迁移的包气带-饱和带耦合模拟研究

Study on vadose-saturation zone coupling simulation of radionuclide migration

为了使地下水环评结果更加真实,通过试验和拟三维方法数值模拟开展包气带-饱和带耦合模拟工作。为进一步说明包气带对核素迁移的迟滞(延迟和滞留)作用,开展了不考虑包气带的模拟并与耦合模拟结果对比。结果表明:(1)由于包气带对核素迁移的迟滞作用,吸附性较强的Se-79、Pd-107、Cs-135和Sn-126迁移至潜水面处的浓度远小于泄漏的放射性液体核素浓度,所致公众食入年待积有效剂量已小于剂量限值0.1 mSv,只有吸附性较弱的C-14所致公众食入年待积有效剂量大于0.1 mSv,需进行耦合模拟;(2)C-14在对流作用下不断向下游迁移,在弥散作用下污染范围逐渐增大,在地下水稀释及衰变、吸附作用下浓度逐渐降低。通过耦合模拟得到的地下水中放射性核素所致公众食入年待积有效剂量小于剂量限值0.1 mSv;(3)包气带对核素迁移的迟滞作用使地下水中C-14的浓度大大降低、迁移距离明显变小:地下水中C-14峰值浓度在考虑包气带时为2.15 Bq/L,不考虑包气带时为9.4×10^(4) Bq/L;事故发生后第1、5和10年,考虑包气带时C-14迁移距离分别为1.5 m、4 m和4.2 m,不考虑包气带时分别为3 m、12 m和17 m。

In order to obtain more real environmental impact assessment results for groundwater,a simulation study on vadose-saturation zone coupling was carried out by using quasi-three-dimensional method based on experiment and numerical simulation.To further explain the hysteresis effect of vadose medium on nuclides migration,simulation without considering the vadose medium was carried out and the results was compared with that of the coupling simulation.The results show that:(1)For strong adsorbed Se-79,Pd-107,Cs-135 and Sn-126,their concentration arriving at water table is far less than the leaked liquid due to the hysteresis effect.And their annual committed effective dose for public will be less than the dose limit of 0.1 mSv.For weakly adsorbed C-14,the annual committed effective dose for public will be more than 0.1 mSv and it requires coupling simulation.(2)C-14 migrates to downstream by convection,and pollution range increases through dispersion.At the same time the C-14 concentration decreases by dilution,decay and adsorption,therefore the annual committed effective dose for public will be less than 0.1 mSv.(3)The concentration of C-14 in groundwater decreases greatly and the migration distance decreases obviously because of the hysteresis effect of vadose medium on nuclides migration.The peak concentration of C-14 in groundwater is 2.15 Bq/L taking vadose zone into consideration;while leaving out the vadose zone,the peak concentration of C-14 will be 9.4×10^(4) Bq/L.The migration distance of C-14 is 1.5 m,4 m and 4.2 m respectively in 1,5 and 10 years after the accident,taking vadose zone into consideration;while leaving out the vadose zone,the migration distance of C-14 will be 3 m,12 m and 17 m respectively.