高放废物处置库北山预选区深部地下水成因机制研究

【作者】 周志超；

【导师】 王驹； 郭永海；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2014， 博士

【摘要】 目前国际上公认的技术上最为可行的高放废物安全处置方法为深地质处置，即以地质体作为天然屏障，并在地下处置库中设置多重人工屏障，从而实现高放废物与人类生存环境的长期或者永久隔离。基岩裂隙介质对核废物的屏障功能以及作为核素迁移载体的基岩裂隙水的水文地质特征对高放废物处置库的安全性能极为重要。鉴于高放废物具有强放射性、长半衰期及高释热率等特点，在场址筛选和安全评价中，往往更注重于处置库场址水文地质长时间尺度的系统特征。因此，开展高放废物处置库北山预选区深部地下水成因机制研究，获取地下水水化学与同位素等所蕴涵的水文地质信息，对理解和把握处置库场址地下水的补给来源和滞留时间、长时间尺度的地下水系统特征及深部地下水循环模式与动力学特征具有重要的意义。论文研究目标是揭示我国高放废物处置库甘肃北山预选区深部地下水成因机制，为我国高放废物地质处置库场址的选址提供依据。为此主要开展三个方面的研究工作：（1）地下水水化学与同位素分布特征研究；（2）浅部介质的渗透特性与地下水补给研究；（3）地下水测年及深部地下水的成因机制研究。论文研究以我国高放废物地质处置的工程需求及关键水文地质问题为导向，围绕北山预选区深部地下水成因机制这一核心问题开展研究。首先在收集和总结前人研究成果的基础上，详细分析研究区的水文地质背景；通过开展深部环境原状地下水取样、地下水的水化学与同位素分布特征、地下水的分层监测、氯质量平衡法的地下水补给估算、同位素测年等综合手段和方法，重点研究地下水的补给及水化学同位素特征、测定深部地下水的年龄、识别地下水的循环更新速率及补给来源，提出地下水补给机制和流动模式，深化研究区深部地下水的成因机制的认识，解决深部地下水的年龄和成因机制等关键水文地质问题，为我国高放废物地质处置研发工作提供了重要的科学依据。通过研究得出如下成果和认识：1.采用国际先进的双栓塞水文地质试验系统，历时近4个月的深部地下水分层连续抽水试验，在我国高放废物处置库甘肃北山预选区的BS15号钻孔，成功地采集到330m～340m和390m～400m两个深度段的原状地下水样，并开展了地下水的水化学、同位素及惰性气体测试工作。2.通过渗透性试验定量获得了区内不同地貌单元的渗透参数，平滩地带渗透性最弱，平均渗透系数为7.69×10-6m/s，沟谷和缓坡地带平均渗透系数分别为4.0×10-5m/s和1.6×10-5m/s，构造影响带的渗透性最强，渗透系数达10-4m/s的量级。基于包气带的氯质量平衡法估算的地下水补给量在第四系较厚的沟谷中仅1.57mm/a，基于饱和带氯质量平衡法估算的区域浅部地下水补给量为0.28mm/a，不足大气降水量的1%。3.重点阐明了区内大气降水、浅部和深部地下水的水化学及同位素特征，浅部地下水水化学类型以Na-Cl-SO4型和Na-Ca-Cl-SO4型为主，深部地下水水化学类型以Na-Cl-SO4型为主，深部地下水经历了相对较长的水流路径和更长时间的水岩作用，地下水循环速率较慢；地下水样中的δD和δ18O大部分位于降水线的右下侧，且沿着斜率为5的蒸发线分布；地下水的补给高程范围均落在研究区范围附近，研究区大气降水补给的平均月气温为13.6℃，浅部地下水的主要补给来源于本地雨季的大气降水，深部地下水的氚含量都小于5TU，氚含量较低，说明深部地下水并非现代成因为主的大气降水所补给。4.联合使用多种同位素（2H、18O、3H、14C、85Kr、81Kr）及环境示踪剂（4He），并结合研究区水文地质条件、水化学和同位素特征，对研究区BS15号孔深部地下水的年龄进行了综合分析与校正计算。其中，深部地下水的14C年龄分布在5.7ka～8.7ka；在国内首次测得了地下水中的81Kr的数据，其81Kr年龄达25ka和46ka；基于地下水溶解气体及深部原位岩石的取样和测试，采用4He积累法所估算的深部地下水平均滞留时间为3.8ka和5.0ka。定量评价了区内地下水的更新速率，其中浅部地下水的更新速率为0.28mm/a，深部地下水的流速为9.8×10-4m/a～9.5m/a，深部地下水的更新速率为0.006%～0.012%。5.结合地下水的水化学、同位素、水文地质试验及地下水动态监测等数据，提出了深部地下水同时接受侧向古地下水和浅部年轻水的补给机制，深部地下水以侧向补给的老水为主，所占比例大于80%的成因模式，且无深部壳源和幔源的补给来源，总结了研究区地下水的补给和流动模式，揭示了区内深部地下水的成因机制。本文的研究，不仅深化了对预选区场址水文地质条件的认识，而且在国内首次开展了81Kr同位素测年研究，根据地下水水化学演化特征、氯离子示踪估算及同位素测年研究，定量评价了地下水补给量及更新速率，从新老水混合的角度认识和提取同位素水文地质信息，认识了深部地下水的年龄属性，从水循环的角度提出了地下水的补给机制、流动模式及成因机制，在学术思路及研究方法上具有一定的创新性，为推动我国高放废物地质处置工作具有重要的研究意义。更多还原

【Abstract】 Deep geological disposal of high-level radioactive waste is currently consideredas the most technically feasible method for the final safe isolation of high-levelradioactive waste from human environment. That is, select a natural geological barrierand set multiple engineered barriers in the underground repository in order to achievelong-term or permanent isolation of HLW from human environment. In safetyassessments, groundwater flow is the most important factor in transportingradionuclides released from wastes to the biosphere. Radioactive wastes should beplaced in a site where groundwater flow is very low or stagnant. As the high levelradioactive waste is characterized by highly radioactive, long half-life and high heatrelease, we often focus more on the long time-scale behavior and performance of thesite hydro-geological system in site selection and safety assessment. The chemical andisotopic compositions in groundwater contain a wealth of hydro-geologicalinformation, they can provide the most direct and effective means for the study ofgroundwater recharge sources and residence time, long time scales of regionalgroundwater system characteristics, cyclical pattern and dynamic characteristics of thedeep groundwater.The study aims to reveal the deep groundwater formation mechanism of HLWrepository in Beishan preselected area, and provide a basis for site selecting andevaluation. The thesis is mainly focused on the following aspects:(1) hydrochemistryand isotope distribution characteristics of groundwater;(2) penetration characteristicsof the medium and groundwater recharge;(3) groundwater dating and deepgroundwater formation mechanism research.In geological disposal of high level radioactive waste engineering requirementsand key hydro-geological problem-oriented, the thesis study on the issues about deepgroundwater formation mechanism of Beishan preselected area. In summing up theresults of previous research and progress, and based on regional hydro-geologicalbackground, it mainly through analysis bedrock fractures and permeabilitycharacteristics, groundwater chemistry and isotopic distribution of water, Multilevel groundwater monitoring, application of the chloride ion tracer method in estimation ofgroundwater recharge in unsaturated zone and saturated zone, groundwater datingtechniques to carry out the related research. It focuses on the study of groundwaterrecharge, groundwater cycle and its renewal characteristics, dating groundwater age,and proposed groundwater recharge and flow patterns.This study attains the following achievements:1. This study adopts the international advanced technology of the double-packerhydraulic test system, by pumping test for nearly four months and using the isolationof double packer, water-sampling campaign was conducted in the sections of330m～340m and390m～400m. The real groundwater samples were taken from differentdepth in deep environment of BS15, and the chemistry of groundwater, isotopes andnoble gas were tested.2. The permeability characters under different geomorphic units wereobtained by permeability tests, the permeability coefficient is minimum inlow-lying land, the average permeability coefficient of low-lying land is7.69×10-6m/s, the average permeability coefficient of the valley and gentleslope zone is1.6×10-5m/s and4.0×10-5m/s, the impact structure zone has thestrongest permeability, and the coefficient can reach to10-4m/s. The chloride iontracer method was used to estimate groundwater recharge in unsaturated zone andsaturated zone, The results showed that the recharge rate in valley zone is only1.57mm/a, and the recharge rate in shallow groundwater is0.28mm/a, less than1%ofprecipitation.3. The study focus to illustrate the chemistry and isotopic characteristics ofprecipitation, shallow groundwater and deep groundwater in the region, the shallowgroundwater chemical mainly type is Na-Cl-SO4-and Na-Ca-Cl-SO4, and the deepgroundwater chemical mainly type is Na-Cl-SO4. The deep groundwater flow wasthrough a relatively longer path and longer water-rock interaction, the groundwatercirculation rate was slower. Hydrogen and oxygen isotope data line in the vicinity ofthe meteoric water line, indicating the shallow groundwater originates from the localprecipitation. 4. Through the groundwater chemistry and isotopic analysis, with considerationof hydro-geological conditions, the age of deep groundwater in BS15was analyze byusing a variety of isotopes (2H,18O,3H,14C,81Kr,85Kr) and environmental tracer(4He).According to the result of carbon-14age dating, the deep groundwater age between5.7ka to8.7ka.Krypton-81data of groundwater were first got from checkout in home,and the result of krypton-81age dating were25ka and46ka.The groundwaterresidence time estimated from the4He accumulation rate were3.8ka and5.0ka. Thegroundwater renewal rate for shallow groundwater was0.28mm/a, and the deepgroundwater flow rate was between9.8×10-4m/a and9.5m/a, the deep groundwaterrenewal rate was between0.006%and0.012%.5. Combined with groundwater data of chemistry, isotope, penetration testing andgroundwater monitoring, it proposed formation mechanism of deep groundwater, asthe recharge of deep groundwater is lateral deep ancient groundwater and the youngshallow groundwater, the recharge from fossil groundwater is greater than80%and ithas no deep crust and mantle sources, shallow groundwater recharge is mainly fromprecipitation during the rainy season. It revealed the formation mechanism of the deepgroundwater and provided realistic basis for selecting and evaluating potential sites.The study would deepen the understanding of hydro-geology conditions in thepreselected area; this investigation on krypton-81dating of groundwater was firstcarried out in our country. The integrated quantitative evaluation on recharge andrenewal rate of groundwater was made by isotope analysis and chloride ion tracermethod. We tested three methods for dating the deep groundwater, the mechanism ofgroundwater recharge, groundwater flow pattern and formation mechanism wasproposed. In conclusion, the studies of this dissertation reach the anticipate targetswith some innovation and integrity, and has important significance in promotingChina’s HLW geological disposal.更多还原