Cementitious materials will be used during the construction and operation of a geological repository for spent nuclear fuel at Olkiluoto in Finland. Upon contacting water, cement dissolution will generate high-pH leachates, which might eventually reach the proximity of a canister deposition hole through an interconnected network of fractures in the crystalline host rock. Highly-alkaline conditions near the bentonite buffer surrounding the canister could affect the performance of the buffer safety functions. In this study, we evaluate the potential impact such cement leachates might have on the chemical composition of the buffer porewater over time-frames relevant for the safety assessment of the repository.

Although a comprehensive mechanistic assessment of these interactions is not possible due to their complexity, we demonstrate that key processes and their impact on the chemical composition of the bentonite porewater can be bounded. To this end we apply a reactive transport modelling based on an analysis of processes and parameter values. The model considers a 3D geometry including: the canister, the bentonite buffer, and a discrete fracture in the rock intersecting the deposition hole. Cement leachates flow through the fracture around the deposition hole, while solutes exchange with the buffer porewater via advective and diffusive mass transfer.

Modelling results indicate that a combination of restricted water flow within the fracture, slow diffusive solute transport in the buffer, and chemical reactions will act together to minimise the extent of buffer porewater perturbation, should cement leachates reach the vicinity of a deposition hole. The model pessimistically estimates a maximum pH variation to be below 0.1 unit, and a maximum concentration change of about factor 3 (relative to initial) for reacting components within most of the buffer volume. This is only about double the perturbation values predicted for “natural” evolution of the buffer porewater, in the absence of cement leachates.

Model uncertainties are evaluated by a series of sensitivity cases. These calculations suggest that additional processes, not directly accounted for in the base model (such as leachate-groundwater mixing and dilution on transport through the fracture network, and porosity reduction in the buffer due to mineral precipitation), could significantly contribute to a further reduction of the magnitude of potential buffer porewater perturbation.

胶结材料将在芬兰奥尔基洛托乏核燃料地质处置库的建造和运行过程中使用。与水接触后，水泥溶解会产生高 pH 值的渗滤液，最终可能通过结晶主岩中的相互连接的裂缝网络到达罐沉积孔附近。罐周围膨润土缓冲区附近的高碱性条件可能会影响缓冲区安全功能的性能。在这项研究中，我们评估了这种水泥渗滤液在与储存库安全评估相关的时间范围内可能对缓冲孔隙水的化学成分产生的潜在影响。

尽管由于它们的复杂性，不可能对这些相互作用进行全面的机械评估，但我们证明了关键过程及其对膨润土孔隙水化学成分的影响是可以确定的。为此，我们应用基于过程和参数值分析的反应传输模型。该模型考虑了 3D 几何结构，包括：罐、膨润土缓冲层以及与沉积孔相交的岩石中的离散裂缝。水泥渗滤液流过沉积孔周围的裂缝，而溶质通过对流和扩散传质与缓冲孔隙水交换。

建模结果表明，如果水泥渗滤液到达沉积孔附近，裂缝内受限制的水流、缓冲液中缓慢的扩散溶质迁移和化学反应将共同作用以最小化缓冲液孔隙水扰动的程度。该模型悲观地估计最大 pH 变化低于 0.1 单位，并且大部分缓冲体积内的反应组分的最大浓度变化约为因子 3（相对于初始）。在没有水泥渗滤液的情况下，这只是缓冲孔隙水“自然”演化预测的扰动值的两倍左右。

模型不确定性通过一系列敏感性案例进行评估。这些计算表明，基础模型中未直接考虑的其他过程（例如渗滤液-地下水混合和通过裂缝网络传输时的稀释，以及由于矿物沉淀导致缓冲区中的孔隙率降低）可能显着有助于进一步减少潜在缓冲孔隙水扰动的幅度。