Abstract Fractured Crystalline Rocks (FCR) are being considered in several countries as hosts for radioactive waste repositories. In FCR, radionuclides may be transported relatively rapidly by bulk groundwater flow through open fractures, but much more slowly by diffusion through porewater in the rock matrices. Rock matrix diffusion (RMD) is the diffusion of radionuclides in the aqueous phase, between open fractures and rock matrices. Sorption or co-precipitation on the fracture surfaces and walls of the matrix pores causes further radionuclide retardation. RMD may be important in a repository’s safety case and has been investigated by many published short-term (to a few years) laboratory and in-situ experiments. To improve understanding over longer timescales, we investigated evidence for RMD of several natural radioelements, and radioelement analogues, in five exemplar fractured crystalline rock (FCR) samples aged between c. 70 Ma and c. 455 Ma. The sample suite consisted of two samples of Borrowdale Volcanic Group (BVG) meta-tuff from northwest England, a sample of Carnmenellis Granite from southwest England and two samples of Toki Granite from central Japan. Uptake or loss of the studied elements is limited to an altered damage zone in each sample, coupled to mineral alteration processes. These zones are most extensive (a few tens of millimetres) in the Toki Granite samples. We also found unstable primary igneous minerals to persist in the immediate wallrocks of fractures in studied granite samples, suggesting that pores were not permanently water saturated in these samples. Although only a small sample suite was studied, the results show that while RMD may be important in some kinds of FCR, in others it may be negligible. Site-specific information is therefore needed to determine how much reliance can be placed on RMD when developing a safety case.

中文翻译：

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控制破碎结晶岩吸收放射性核素的自然模拟证据

摘要 裂隙结晶岩 (FCR) 在几个国家被考虑作为放射性废物处置库的宿主。在 FCR 中，放射性核素可以通过大量地下水流通过开放裂缝而相对快速地传输，但通过岩石基质中的孔隙水扩散要慢得多。岩石基质扩散 (RMD) 是放射性核素在水相中、开放裂缝和岩石基质之间的扩散。基质孔的断裂表面和壁上的吸附或共沉淀导致放射性核素进一步延迟。RMD 在储存库的安全案例中可能很重要，并且已通过许多已发表的短期（至几年）实验室和原位实验进行了调查。为了提高对更长时间尺度的理解，我们调查了几种天然放射性元素的 RMD 证据，和放射性元素类似物，在 c. 之间老化的五个示例性断裂结晶岩 (FCR) 样品中。70 毫安和 c。455 马。样品组包括来自英格兰西北部的两个 Borrowdale Volcanic Group (BVG) 变质凝灰岩样品、来自英格兰西南部的 Carnmenellis 花岗岩样品和来自日本中部的两个 Toki 花岗岩样品。所研究元素的吸收或损失仅限于每个样品中的蚀变损伤区，与矿物蚀变过程相结合。这些区域在 Toki 花岗岩样本中最为广泛（几十毫米）。我们还发现不稳定的原生火成岩矿物持续存在于研究的花岗岩样品中的裂缝的直接围岩中，这表明这些样品中的孔隙不是永久水饱和的。虽然只研究了一个小样本套件，结果表明，虽然 RMD 在某些类型的 FCR 中可能很重要，但在其他类型中可能可以忽略不计。因此，需要特定地点的信息来确定在开发安全案例时可以在多大程度上依赖 RMD。