Abstract
The Forsmark site was recently proposed by the Svensk Kärnbränslehantering AB (SKB) to serve as the potential site for construction of a future geological repository for spent high-level nuclear fuel at about 470 m depth in fractured crystalline rock. The considerations included, among other things, distance from regionally significant deformation zones with highly strained rock, lithological homogeneity, low hydraulic conductivity, groundwater salinity with an acceptable range, and lack of potential mineral resources. This report describes the calculation of transmissivity of deduced deformation zones at Forsmark and the transmissivity model used in the regional groundwater flow modeling carried out in support of the integrated site description. Besides significant decrease with increasing depth (more than four orders of magnitude over a depth of about 1 km), the calculated transmissivity values also reveal considerable spatial variability along the strikes of the zones, i.e. lateral heterogeneity (more than two orders of magnitude). A hydro-mechanical coupling is discussed, based on presented models for the tectonic evolution and the principal stress tensor. Tentatively, laboratory-scale relationships developed from normal stress experiments on a single fracture in crystalline rock can be used to estimate the maximum values of transmissivity of deduced deformation zones at Forsmark.
Résumé
La compagnie de gestion du combustible et des déchets nucléaires suédois a récemment proposé le site de Forsmark pour servir d'emplacement potentiel à la construction d'un futur dépôt géologique de combustible nucléaire haute activité usé à environ 470 m de profondeur dans des roches cristallines fracturées. Les considérations ont inclus, entre autres, la distance des zones de déformation régionalement significatives comprenant des roches sous forte tension, l’homogénéité lithologique, la faible conductivité hydraulique, la salinité des eaux souterraines dans une gamme acceptable, et l’absence de ressources minérales potentielles. Cet article décrit le calcul de la transmissivité des zones de déformation déduites à Forsmark et le modèle de transmissivité utilisé dans un modèle régional d'écoulement d'eaux souterraines réalisé en appui à la description intégrée du site. En plus d’une diminution significative avec l'augmentation de la profondeur (plus de quatre ordres de grandeur pour une profondeur d'environ 1 km), les valeurs calculées de transmissivité indiquent également une variabilité spatiale considérable le long des failles de ces zones, c.-à-d. une hétérogénéité latérale (plus de deux ordres de grandeur). Un couplage hydromécanique est discuté, basé sur les modèles présentés pour l'évolution tectonique et le tenseur de contraintes principales. À titre d'essai, des relations à l’échelle du laboratoire développées à partir d’essais de contraintes normales sur une fracture unique dans une roche cristalline peuvent être utilisées pour estimer les valeurs maximales de transmissivité des zones de déformation déduites à Forsmark.
Resumen
El sitio de Forsmark fue propuesto recientemente por la Svensk Kärnbränslehantering AB para servir como un lugar potencial para la construcción de un futuro repositorio geológico para combustible nuclear usado de alta radiactividad a alrededor de 470 m en rocas cristalinas fracturadas. Las consideraciones incluían, entre otras cosas, la distancia de zonas de deformación regional significativas con altas deformaciones de la roca, la homogeneidad litológica, la baja conductividad hidráulica, la salinidad del agua subterránea dentro de un rango aceptable, y la carencia de potenciales recursos minerales. Este trabajo describe el cálculo de la transmisividad de las zonas de deformación deducidas en Forsmark y los modelos de transmisividad usados en el modelado del flujo regional del agua subterránea realizados en apoyo de una descripción integrada del sitio. Además de la disminución significativa con el incremento de la profundidad (más que cuatro órdenes de magnitud para una profundidad de alrededor de 1 km), los valores calculados de la transmisividad revelan una considerable variabilidad espacial a lo largo de los rumbos de las zonas, es decir la heterogeneidad lateral (más de dos órdenes de magnitud). Se discute un acoplamiento hidromecánico, basado en los modelos presentes para la evolución tectónica y el tensor principal de tensiones. Tentativamente, relaciones en escala de laboratorio desarrollado a partir de experimentos de tensión normal sobre una sola fractura en rocas cristalinas pueden ser usados para estimar los valores máximos de transmisividad de las zonas de deformación deducidas para Forsmark.
Resumo
O local de Forsmark foi recentemente proposto pela companhia Sueca de Combustível Nuclear e Gestão de Resíduos para servir como local potencial para a construção de um futuro repositório geológico para combustível nuclear usado de alto nível em rochas cristalinas fraturadas, a cerca de 470 m de profundidade. As considerações incluíram, entre outras coisas, a distância a zonas de deformação com significado regional, com rochas altamente deformadas, homogeneidade litológica, condutividade hidráulica baixa, salinidade da água subterrânea num intervalo aceitável e a inexistência potencial de recursos minerais. Este artigo descreve o cálculo das transmissividades em zonas de deformação inferidas em Forsmark e o modelo de transmissividade usado na modelação do fluxo regional da água subterrânea realizados como apoio à descrição integrada do local. Para além da diminuição significativa com o aumento da profundidade (mais de quatro ordens de magnitude ao longo de uma profundidade de cerca de 1 km) os valores de transmissividade calculados também revelam uma variabilidade espacial considerável ao longo das orientações das zonas, i.e. heterogeneidade lateral (mais de duas ordens de magnitude). É discutida uma ligação hidromecânica, com base nos modelos apresentados para a evolução tectónica, para o tensor principal das tensões. Experimentalmente, as relações à escala de laboratório, desenvolvidas a partir de experiências com tensões normais numa única fratura em rocha cristalina, podem ser usadas para estimar os valores máximos de transmissividade em zonas de deformação inferidas em Forsmark.
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Acknowledgements
This report was made possible due to the significant field work conducted on behalf of the Svensk Kärnbränslehantering AB (SKB) during the site investigation for a geological repository for spent high-level nuclear waste at the Forsmark site. We would like to thank those who contributed to the extensive data collection and/or analysis including: M. Stephens, A. Simeonov, J. Petersson, J. Levén, I. Rhén, L. Hartley, D. Martin and R. Christiansson. Finally, we would like to thank the reviewers for their helpful comments and suggestions, which improved the quality of the manuscript. A special thanks is given to M. Becker, who suggested focusing on the possible correlation between the calculated transmissivity values of deterministic deformation zones and in situ stress. All SKB reports referred to in this report, as well as the underlying data reports, can be downloaded from the SKB website—http://www.skb.se/; click on “Publications”, and then enter the report number, e.g. “R-07-45”.
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This article belongs to a series describing site-descriptive modeling of bedrock hydrogeology for the proposed high-level nuclear waste repository site at Forsmark, Sweden
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Follin, S., Stigsson, M. A transmissivity model for deformation zones in fractured crystalline rock and its possible correlation to in situ stress at the proposed high-level nuclear waste repository site at Forsmark, Sweden. Hydrogeol J 22, 299–311 (2014). https://doi.org/10.1007/s10040-013-1078-9
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DOI: https://doi.org/10.1007/s10040-013-1078-9