北山地下水氧化还原电势及其对可变价核素迁移的影响

氧化还原电势(Eh)是影响高放废物处置库长期安全性能的重要参数之一。然而,直接测定地下水的Eh值面临较多不确定性因素。北山花岗岩中存在一定量的含铁矿物,由此可能控制地下水中铁的含量。本工作基于北山三号井400m深处的地下水,水中铁质量浓度为0.033mg/L及钻孔岩芯中Fe~(2+)/Fe_(tot)的比值为96.86%条件下,提出了一个推测北山地下水氧化还原电势的可能方法,计算得到北山三号井地下水的Eh=83.2mV。该值与利用针铁矿的沉淀饱和指数估算得到的101.8 mV相接近,但需进一步开展现场的Eh测量工作来验证及比对。依据此Eh,利用PHREEQC程序及OECD/NEA发布的最新热力学数据,并加入MUO_2(CO_3)_3~(2-)和M_2UO_2(CO_3)_3~0(M=Ca、Mg、Sr)的稳定常数,计算了可变价核素U、~(99)Tc、~(79)Se和Np在北山地下水中的形态分布和溶解度。结果表明,U和Tc的溶解度相对较高(约10~(-5)~10~(-4) mol/L),Se的溶解度相对较低(约10~(-8) mol/L),Np的溶解度则极低(约10~(-18) mol/L);此外,溶解态的U、Tc和Se主要以阴离子形式存在,具有较强的迁移性。另一方面,北山花岗岩富含二价铁离子,因此需进一步开展其对可变价核素还原沉淀作用的实验研究,以综合评价处置场的安全性能。

Investigation of Redox Potential of Beishan Site and Its Impact on Mobility of Redox-Sensitive Radionuclides

The redox potential(Eh) is one of the key parameters that can affect the long-term safety of the HLW repository.However,direct determination of the Eh value of the groundwater faces many uncertainties.Beishan granite contains a certain amount of iron-containing minerals,which may control the concentration of aqueous iron in the groundwater.In this study,based on the aqueous iron of 0.033 mg/L analyzed in the groundwater of BS03 borehole at 400 m deep and the Fe2+/Fetot ratio of 96.86％ found in the corresponding rock core,a possible method for deducing the redox potential of Beishan groundwater is proposed,and a value of 83.2 mV is calculated for the groundwater of BS03 borehole.This value is comparable to 101.8 mV that is estimated using the precipitation saturation index of goethite,but in situ Eh measurement should be carried out for verification and comparison.In light of the obtained Eh,the speciation and solubility of redox-sensitive radionuclides,i.e.,U,99Tc,79Se,and Np are calculated using PHREEQC with the Nuclear Energy Agency thermodynamic database by adding stability constants of MUO2 (CO3)32-and M2UO2 (CO3)30 (M=Ca,Mg,Sr).The results indicate that the solubility is expected to be relatively high (≈10 5-10-4 mol/L) for U and Tc,whereas relatively low for Se (≈10-8 mol/L) and fairly low for Np (≈10-18 mol/L).Furthermore,the dissolved U,Tc,and Se mainly exist as anions that have strong mobility in geological media.On the other hand,because of the abundance of ferrous iron in Beishan granite,in order to have a comprehensive evaluation on the safety performance of the disposal site,further experimental works concerning the retardation of these redox-sensitive radionuclides via heterogeneous reduction are needed.