粤北下庄铀矿田中铀酰矿物的成分特征——对核废料处置库中UO_2氧化行为的启示

The Compositional Characteristic of Uranyl Minerals in Xiazhuang Uranium Ore Field: Insight for the Oxidation Behavior of UO_2 in Nuclear Waste Repository

下庄铀矿为一花岗岩型铀矿,矿田地处湿热气候条件下,沥青铀矿普遍产于破碎带中,这种特定的产出环境致使该区沥青铀矿经受了强烈的风化,形成种类繁多、数量丰富的铀酰矿物;而我国高放废物地质处置库拟建在花岗岩体中。因此,下庄铀矿田是开展核废料氧化的天然类比研究的理想地区,并对我国的高放废物地质处置库的安全性评价有重要的指导作用。下庄铀矿田的铀酰矿物组合为铀酰氢氧化物、铀酰硅酸盐和铀酰磷酸盐,包括柱铀矿、黄钙铀矿、calciouranoite、红铀矿、富硅铀酰相、硅钙铀矿、钙铀云母和盈江铀矿等。根据它们的空间分布特征可划分成两个风化系列,即硅酸盐风化系列和磷酸盐风化系列,其共生次序分别为:沥青铀矿铀酰氢氧化物(氧化物)富硅铀酰相硅钙铀矿和沥青铀矿铀酰氢氧化物(氧化物)钙铀云母盈江铀矿。在该矿田中,铀酰氢氧化物是亚稳定相矿物,常常被铀酰硅酸盐或磷酸盐取代,因此,铀酰氢氧化物仅出现在少数样品中,而铀酰硅酸盐和铀酰磷酸盐矿物则非常普遍。矿田中的铀酰矿物在化学成分上以富钙为其显著特征,由于核废料地质处置库近场地下水中的Ca2+含量应该明显比下庄矿田地下水中的高,因此,我们预测含Ca的铀酰硅酸盐和铀酰磷酸盐矿物等热力学上的稳定物相很可能是地质处置系统中最主要的铀酰矿物,处置库内放射性核素的迁移主要是由这些矿物控制的。

Xiazhuang uranium ore field is located in wet-hot climatic regions. Uraninite in this ore field is hosted in fractured zone in granite. Abundant uranyl minerals （especially, uranyl silicates and uranyl phosphates） are found because of intensive weathering of uraninite. Thus, Xiazhuang uranium ore field is an ideal site for the study of natural analogy of UO2 oxidative corrosion with its special geochemical environment and abundant uranyl minerals. The high-level radioactive waste repository in China will be built in granite, therefore, the investigation of the behavior of uraninite weathering in this ore field is of an important role for the performance assessment of the disposal system of China. The combination of the uranyl minerals in Xiazhuang uranium ore field is comprised of uranyl oxide hydrates, uranyl silicates and uranyl phosphates, including schoepite, becquerelite, calciouranite, fourmarierite, Si-rich uranyl phase, uranophane, autunite and yingjiangite. In light of spatial relationship of these uranyl minerals, two paragensis series of uraninite weathering were identified： one is the silicate series with a mineral sequence of uraninite→uranyl oxide hydrates→Si-rich uranyl phase→uranophane, and the other is the phosphate series with a mineral sequence of uraninite→uranyl oxide hydrates→autunite→yingjiangite. Uranyl oxide hydrates are not stable and often substituted by stable uranyl silicates and phosphates. Consequently, uranyl silicates and phosphates are common minerals, and uranyl oxide hydrates are just found in a few samples, in this ore field. In this ore field, thermodynamic-stable uranyl silicates and phosphates are dominant uranyl minerals, and compositional characteristic of all uranyl minerals is Ca-riched, and Ca content of underground water in near-field of disposal systems is higher than in Xiazhuang ore field. These collectively foreshow that thermodynamic-stable Ca-rich uranyl silicates and phosphates should be the most common uranyl minerals and control the mobility of radionuclides, in disposal systems.