模拟高放废物地质处置条件下锝在去离子水体系中的胶体行为

Technetium Colloidal Behavior in Deionized Water System under Simulated HLW Geological Disposal Condition

^(99)Tc是高放废物地质处置中重点关注的关键核素,深地质处置条件下其在地下水中的胶体行为对其迁移行为可能存在较大影响,亟需系统开展研究。本文在模拟高放废物地质处置的低氧和低浓(氧气含量小于5 ppm、^(99)Tc初始浓度10^(-9)~10^(-5) mol/L)条件下,首先探讨了用膜过滤法研究锝胶体行为的可行性,发现pH值和离子强度等对^(99)Tc的液闪测定无明显影响,且过滤器对Tc(Ⅳ)和Tc(Ⅶ)均无显著吸附行为,基于此初步建立了锝胶体行为研究的膜过滤实验方法,并采用该方法研究了去离子水体系中锝的胶体行为。结果表明:Tc(Ⅶ)不易发生水解反应,在低氧条件下无明显胶体行为,并能长期稳定地存在于体系中,可能具有较强的迁移能力;Tc(Ⅳ)易水解而聚集为胶体,在较宽pH值范围内(pH=4~11)以真胶体形式在398 d实验周期内较长期地稳定存在于体系中,其浓度高于以离子形态存在的Tc(Ⅳ)的数倍甚至1个数量级,所形成的胶体可能会提高地质处置条件下Tc(Ⅳ)的迁移能力,但体系离子强度升高会使Tc(Ⅳ)胶体易凝聚为大颗粒物质甚至沉淀,导致锝以真胶体形态存在和迁移的可能性降低,因此高放废物处置中,锝胶体行为对其迁移行为的影响应予以足够重视和研究。

^(99)Tc is a key radionuclide in the safety case of deep geological disposal of high-level radioactive waste(HLW)due to its long half-life of 2.13×10^(5) years and high yield in nuclear reactors.The formation of technetium colloid in groundwater was believed to play a potential important role on its migration and transport with ground-water to biosphere under deep geological disposal conditions.In this paper,the membrane filtration experimental method was established firstly for the study of the colloidal behavior of technetium in aqueous solutions.All experiments were performed in an argon glove box to maintain the anaerobic conditions(O_(2) concentrations lower than 5 ppm)which will be expected in the HLW repository and initial technetium concentrations in the range from 10^(-9) mol/L to 10^(-5) mol/L,and liquid scintillation spectrophotometry was used to determine ^(99)Tc.It is found that pH value and ionic strength have no significant effect on the liquid scintillation determination of ^(99)Tc,and the filter has no obvious adsorption behavior for different oxidation states of technetium(Tc(Ⅳ)and Tc(Ⅶ)).The experimental studies of technetium colloidal behavior in deionized water system were conducted by using this experimental method.The results indicate that Tc(Ⅶ)has no significantly colloidal behavior even under anaerobic conditions due to its low hydrolysis ability,and can exist stably in the system for a long time,therefore it is a potentially higher mobile species.As Tc(Ⅳ)is trend to hydrolyze to form oxic polymers easily,which then aggregate immediately to form the intrinsic colloids over a wide ranging of pH values from 4 to 11 under the anaerobic conditions.Moreover,it is found that the Tc(Ⅶ)intrinsic colloid would exist stably during the experimental period of 398 days in the pH range of 4-11,and its concentrations may be several times or even one order of magnitude higher than the Tc(Ⅳ)presented as the ionic form.These results seem mean that the formation of Tc(Ⅶ)colloid may enhance the migration capacity of Tc(Ⅳ)which is considered to be relatively immobile,then potentially facilitating transport of Tc(Ⅳ)under geological disposal conditions.But the next experimental results demonstrate that with the increasing of the ionic strength,the Tc(Ⅳ)colloid would aggregate into large particles even continue to form precipitation.This experimental results illustrate that higher ionic strength in aqueous systems may eliminate the dangerous of Tc(Ⅳ)transfer to biosphere with groundwater in the form of intrinsic colloid.The effect of technetium colloidal on its migration behavior needs to be take into account,and some important formation mechanism of technetium colloidal also needs to be clarified and confirmed.Therefore,it is necessary to carry out more researches on radionuclide colloids in the research and development projects of geological disposal of HLW in China.