Abstract
In radioactive waste disposal facilities, low-permeability engineered barrier materials are important for inhibiting radionuclide migration. However, dissolution–precipitation reactions under alkaline conditions change the permeability of engineered barriers. To understand long-term dissolution–precipitation reactions under alkaline conditions in chemically complex systems, trenches and drill holes were excavated at Narra in Palawan, where alkaline fluids (pH > 11) have been naturally produced, seeping into clastic sediments derived from serpentinized ultramafic rocks and gabbro of Palawan ophiolite. Interaction between the alkaline seepage and clastic sediments, which have been deposited since 15,000 radiocarbon years before present (14C yr BP), led to dissolution of minerals and the precipitation of Si-bearing phases which were divided into two main categories: Fe-Mg-Si infillings and Ca-Si infillings. The former category was composed of iron-magnesium-silicate-hydrate (F-M-S-H) and a nontronite-like mineral and was widely recognized in the clastic sediments. The nontronite-like mineral likely formed by interaction between silicates and alkaline seepage mixed with infiltrated seawater, whereas F-M-S-H formed by the reaction of silicates with alkaline seepage in the absence of seawater infiltration. Ca-Si infillings included 14 Å tobermorite and were precipitated from alkaline seepage combined with the Ca and Si supplied by the dissolution of calcite and silicates in the clastic sediments.
Highlights
In radioactive waste disposal facilities, low-permeability engineered barriers are placed around radioactive waste to inhibit radionuclide migration
This study reveals the temporal-spatial distribution of primary and secondary minerals and their related depositional environments to understand long-term dissolution–precipitation reactions under alkaline conditions in a chemically complex system
Narra, derived from serpentinized ultramafic rocks and gabbro of the Palawan ophiolite, started 15,000 yr BP, as it has been documented by 14 C dates
Summary
In radioactive waste disposal facilities, low-permeability engineered barriers are placed around radioactive waste to inhibit radionuclide migration. These barriers often comprise bentonite and cementitious materials; alteration of cementitious materials can produce alkaline leachates [1]. It is important to analyze the dissolution of constituent minerals in bentonite, as well as the formation of secondary minerals, under alkaline conditions. For the safety assessment of radioactive waste disposal, it is important to consider the long-term thermodynamics and/or kinetics of geochemical reactions in chemically complex systems. In order to determine a formula which describes the effects of pH on the dissolution kinetics of minerals, laboratory experiments should be conducted under different pH conditions using alkaline solutions such as NaOH or Ca(OH). It is difficult to select a possible secondary mineral species because the composition of secondary mineral phases strongly depends on the experimental chemical system
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