Gaseous carbonation of cementitious backfill for geological disposal of radioactive waste: Nirex Reference Vault Backfill

The ability of Nirex Reference Vault Backfill (NRVB), a cement backfill material, to capture carbon dioxide from Intermediate Level Radioactive waste packages after repository backfilling, has been assessed. Large-scale trials assessed the physical and chemical reaction of carbon dioxide with the hardened backfill grout. A carbonation front, radial in nature, was observed extending into the grout and three distinct regions were identified in the hardened grouts. A carbonated region, a carbonation front, and a partially carbonated zone were discerned. Potassium, and to a lesser extent sodium, were concentrated in the carbonated region just behind of the main reaction front. The area just ahead of the carbonation front was enriched in both sulphur and aluminium, while sulphur was found to be depleted from the carbonated material behind the main reaction front. Within the main carbonated region, virtually all of the hydrated cement phases were found to be carbonated, and carbonation extended throughout the grout, even within material indicated by phenolphthalein solution to be uncarbonated. Importantly, carbonation was observed to impact both the mineral assemblage and porosity of the cement backfill; it is therefore important to understand these characteristics in terms of the long term evolution of NRVB and its groundwater buffering safety function within the geological disposal facility near-field

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

Fluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to ~1 km depth. Within such lithified successions, dissolution associated with the circulation of meteoric water results in increased permeability (K~10−1–100 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic conductivity is lower, decreasing from K~10−2 m/day at 150–400 m BGL to 10−3 m/day down-dip at ~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7–1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields

Electrical resistivity monitoring of a collapsing meta-stable soil

A model of electrical conduction through clay-coated, silt-sized quartz-grains inter-connected by clay-bridges (e.g. brickearth) is developed. Underpinned by SEM studies of brickearth, the model predicts resistivity to be proportional to the size of the quartz-grains, where the resistance afforded by clay grain-coatings and clay-bridges is comparable. The model accommodates resistivity that increases through bridge breakage and decreases through bridge compression. The resistivity of in-situ undisturbed brickearth was found to be in the range 15 to 35 ohm-m. At such low values we demonstrate that electrical flow is dominated by conduction within clay-coatings and their interconnecting clay-bridges, rather than in mobile pore-water. A small electrode array, buried at shallow depth beneath the load plate (1.0m by 1.0m) of a field collapse experiment, monitored resistivity to a depth of 1.5m over a 260 hour period. While the water level beneath the load plate remained below 1.0m depth, the resulting 3D inverted resistivity models detected water injected immediately beneath the plate; recording rapid increases, in stages over 90 minutes, in the depth interval 0.45 to 0.75m directly under the plate, during what appears to be collapse. These increases are attributed to breaking of clay-bridges weakened by injected water

Diagenetic and very low-grade metamorphic characteristics of the Paleozoic series of the Istanbul Terrane (NW Turkey)

The Istanbul Terrane along the Black Sea coast in NW Anatolia, is a Gondwana-derived continental microplate, comprising a well-developed Paleozoic succession. Petrographic and X-ray diffraction studies were performed on rock samples from measured sections throughout Ordovician-Carboniferous sedimentary units. Diagenetic-very low-grade metamorphic clastic (shale/mudstone, siltstone, sandstone) and calcareous rocks (limestone, dolomite) mainly contain phyllosilicates, quartz, feldspar, calcite, dolomite, hematite and goethite minerals. Phyllosilicates are primarily represented by illite, chlorite, mixed-layered chlorite-vermiculite (C-V), chlorite-smectite (C-S) and illite-chlorite (I-C). Feldspar is commonly present in the Ordovician and Carboniferous units, whereas calcite and dolomite are abundant in the Silurian and Devonian sediments. The most important phyllosilicate assemblage is illite + chlorite + I-C + C-V + C-S. Illite and chlorite-bearing mixed layer clays are found in all units. The amounts of illites increase in the upper parts of the Silurian series and the lower parts of the Devonian series, whereas chlorite and chlorite-bearing mixed-layers are dominant in the Ordovician and Carboniferous units. Kubler index values of illites reflect high-grade anchimetamorphism for the Early Ordovician rocks, low-grade metamorphism to high-grade diagenesis for the Middle Ordovician-Early Silurian rocks and high-grade diagenesis for the Late Silurian-Devonian units. The K-white micas b cell dimensions indicate intermediate pressure conditions in the Early Ordovician-Early Silurian units, but lower pressure conditions in the Middle Silurian-Devonian units. Illites are composed of 2M(1) +/- 1M(d) polytypes in all units, except for Upper Ordovician-Lower Silurian units which involve 1M polytype in addition to 2M(1) and 1M(d) polytypes. The 2M(1)/(2M(1) + 1M(d)) ratios rise from Devonian to Ordovician together with the increasing diagenetic-metamorphic grade. Chlorites have IIb polytype. In general, crystal-chemical data of clay minerals in the Istanbul Terrane show a gradual increase in the diagenetic/metamorphic grade together with increasing depth. The new data presented in this work indicate that the diagenetic/metamorphic grade of the Paleozoic of the Istanbul Terrane is higher than that of the neighboring Zonguldak Terrane and generated by a single metamorphic phase developed at the end of Carboniferous. This finding contrasts with the metamorphic history of the neighboring Zonguldak Terrane that displays a distinct Early Devonian unconformity and a thermal event

Permo-Triassic unconformity- related Au-Pd mineralisation, south Devon, UK : new insights and the European perspective

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (13C and 18O) and radiogenic (87Sr/86Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, 2)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl2)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity).="" the="" sodic="" brine="" is="" reasoned="" to="" be="" the="" parent="" metalliferous="" fluid="" and="" to="" have="" acquired="" its="" enrichment="" in="" au="" and="" pd="" by="" the="" leaching="" of="" immature="" sediments="" and="" intra-rift="" volcanic="" rocks="" within="" the="" local="" permo–triassic="" basins.="" metal="" precipitation="" is="" linked="" to="" the="" destabilisation="" of="" au="" and="" pd="" chloride="" complexes="" by="" either="" mixing="" with="" calcic="" brines,="" dilution="" by="" groundwaters="" or="" interaction="" with="" reduced="" lithologies.="" this="" explains="" the="" diversity="" of="" mineralised="" settings="" below="" and="" above="" the="" unconformity="" and="" their="" affinity="" with="" red="" bed="" brines.="" the="" paucity="" of="" sulphide="" minerals,="" the="" development="" of="" selenides="" (as="" ore="" minerals="" and="" as="" mineral="" inclusion="" in="" gold="" grains),="" the="" presence="" of="" rhodochrosite="" and="" manganoan="" calcites="" (up="" to="" 2.5 wt%="" mn="" in="" calcite)="" and="" the="" co-precipitation="" of="" hematite="" and="" manganese="" oxides="" are="" consistent="" with="" the="" overall="" high="" oxidation="" state="" of="" the="" ore="" fluids.="" a="" genetic="" model="" is="" proposed="" linking="" permo–triassic="" red="" beds,="" the="" mixing="" of="" oxidising="" and="" reducing="" brines,="" and="" the="" development="" of="" unconformity-related="" precious="" metal="" mineralisation.="" comparison="" with="" other="" european="" permo–triassic="" basins="" reveals="" striking="" similarities="" in="" geological="" setting,="" mineralogy="" and="" geochemistry="" with="" au,="" au-pd="" and="" selenide="" occurrences="" in="" germany="" (tilkerode,="" korbach-goldhausen),="" poland="" (lubin)="" and="" the="" czech="" republic="" (svoboda="" nad="" úpou="" and="" stupná).="" though="" the="" known="" au-pd="" occurrences="" are="" sub-economic,="" several="" predictive="" criteria="" are="" proposed="" for="" further=""