Abstract
The concept for the final disposal of high level nuclear waste (HLNW) developed by the Swedish Nuclear Waste Management Company (SKB) entails a multi-barrier system that surrounds the HLNW, which is also known as the near-field. In the near-field, the buffer is initially subject to a high thermal gradient induced by the heat generated by the radioactive decay of the HLNW. During this period, the buffer is also subject to a hydrodynamic pressure induced by the surrounding water saturated rock massif which progressively leads to the saturation of the buffer. After saturation and cooling of the near-field, the interaction of groundwater with the bentonite buffer may result in an evolving distribution of some aqueous species in the bentonite porewater, as well as the redistribution of accessory minerals and the cation exchanger composition in the montmorillonite interlayer. The distribution of aqueous and solid species in the buffer can affect, directly or indirectly, some of the relevant safety function indicators defined by. In this context, the work developed by Arcos et al is revisited in the present work and, based on new data from SKB, additional models are developed for the SR-Site Safety Assessment. The work presented here represents an update of
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Citation Formats
Sena, Clara, Salas, Joaquin, and Arcos, David.
Aspects of geochemical evolution of the SKB near field in the frame of SR-Site.
Sweden: N. p.,
2010.
Web.
Sena, Clara, Salas, Joaquin, & Arcos, David.
Aspects of geochemical evolution of the SKB near field in the frame of SR-Site.
Sweden.
Sena, Clara, Salas, Joaquin, and Arcos, David.
2010.
"Aspects of geochemical evolution of the SKB near field in the frame of SR-Site."
Sweden.
@misc{etde_1004318,
title = {Aspects of geochemical evolution of the SKB near field in the frame of SR-Site}
author = {Sena, Clara, Salas, Joaquin, and Arcos, David}
abstractNote = {The concept for the final disposal of high level nuclear waste (HLNW) developed by the Swedish Nuclear Waste Management Company (SKB) entails a multi-barrier system that surrounds the HLNW, which is also known as the near-field. In the near-field, the buffer is initially subject to a high thermal gradient induced by the heat generated by the radioactive decay of the HLNW. During this period, the buffer is also subject to a hydrodynamic pressure induced by the surrounding water saturated rock massif which progressively leads to the saturation of the buffer. After saturation and cooling of the near-field, the interaction of groundwater with the bentonite buffer may result in an evolving distribution of some aqueous species in the bentonite porewater, as well as the redistribution of accessory minerals and the cation exchanger composition in the montmorillonite interlayer. The distribution of aqueous and solid species in the buffer can affect, directly or indirectly, some of the relevant safety function indicators defined by. In this context, the work developed by Arcos et al is revisited in the present work and, based on new data from SKB, additional models are developed for the SR-Site Safety Assessment. The work presented here represents an update of the model conducted within the SR-Can exercise and, therefore, similar simulation cases are developed. Three aspects must be considered regarding the geochemical evolution of the near field: (1) the effect of the thermal period; (2) the processes during the saturation of bentonite; and, (3) the interaction of the water-saturated bentonite with the local groundwater. In this numerical exercise, two types of bentonite are analysed: the MX-80 and the Deponit CA-N. The effect of the thermal period and the water saturation are analysed in a series of one-dimensional radial-symmetric simulations performed using TOUGHREACT which is a reactive transport code that accounts for variably saturated multi-phase flow under non-isothermal conditions. These simulations are based on the outcomes of a previous numerical model built with the same code for the simulation of the LOT-A2 experiment. The interaction of the fully water-saturated bentonite with groundwater is assessed through a series of tree-dimensional (3D) numerical models performed in PHAST. In order to assess the thermo-hydraulic and geochemical evolution of the near-field, a fracture in the granitic host rock is considered to intersect the deposition hole in which bentonite buffer surrounds the copper canister containing the spent fuel. The main achievements obtained in the numerical models developed for the saturation of the bentonite buffer during the thermal period indicate that although the model predicts a temporary evaporation of water near the copper canister, no chloride salt reaches saturation, and therefore, chloride is considered to behave conservatively. In addition, if the advective flow rate through the fracture is relatively low, the chemical composition of the groundwater in the fracture around the buffer could be influenced by the bentonite chemistry. Anhydrite, the high-temperature stable Ca-sulphate mineral, is predicted to precipitate around the copper canister when the higher temperatures are established (above 56 deg C). Nevertheless, as the temperature in the near-filed tends to decrease towards the regional value of 15 deg C, anhydrite previously precipitated progressively dissolves, and in turn, gypsum tends to replace the higher temperature Ca-sulphate mineral. During the thermal period, the primary quartz of the bentonite tends to dissolve close to the copper canister and re-precipitate close to the contact between the bentonite buffer and the granite. The evolution of this mineral is a function of the thermal conditions as its solubility increases with temperature. Computed results for the water-saturated period of the near-field indicate that the interaction between the minerals that constitute the MX-80 bentonite and the Forsmark groundwater that occurs nowadays at the depth of the future repository leads to an overall depletion in gypsum in favour of calcite precipitation. In addition, the initially Na-rich montmorillonite interlayer is predicted to enrich in calcium. In the case that the bentonite buffer is composed of Deponit CA-N bentonite, the inflow of Forsmark groundwater leads to the dissolution of gypsum and dolomite in favour of more calcite precipitated. In addition, the initially Mg and Ca-rich montmorillonite interlayer becomes depleted in these cations and enriched in sodium. In both bentonite types considered, the inflow of Forsmark groundwater leads to relatively small changes on the pH of the near-field with respect to the values expected for the initial porewater of the engineered barriers}
place = {Sweden}
year = {2010}
month = {Sep}
}
title = {Aspects of geochemical evolution of the SKB near field in the frame of SR-Site}
author = {Sena, Clara, Salas, Joaquin, and Arcos, David}
abstractNote = {The concept for the final disposal of high level nuclear waste (HLNW) developed by the Swedish Nuclear Waste Management Company (SKB) entails a multi-barrier system that surrounds the HLNW, which is also known as the near-field. In the near-field, the buffer is initially subject to a high thermal gradient induced by the heat generated by the radioactive decay of the HLNW. During this period, the buffer is also subject to a hydrodynamic pressure induced by the surrounding water saturated rock massif which progressively leads to the saturation of the buffer. After saturation and cooling of the near-field, the interaction of groundwater with the bentonite buffer may result in an evolving distribution of some aqueous species in the bentonite porewater, as well as the redistribution of accessory minerals and the cation exchanger composition in the montmorillonite interlayer. The distribution of aqueous and solid species in the buffer can affect, directly or indirectly, some of the relevant safety function indicators defined by. In this context, the work developed by Arcos et al is revisited in the present work and, based on new data from SKB, additional models are developed for the SR-Site Safety Assessment. The work presented here represents an update of the model conducted within the SR-Can exercise and, therefore, similar simulation cases are developed. Three aspects must be considered regarding the geochemical evolution of the near field: (1) the effect of the thermal period; (2) the processes during the saturation of bentonite; and, (3) the interaction of the water-saturated bentonite with the local groundwater. In this numerical exercise, two types of bentonite are analysed: the MX-80 and the Deponit CA-N. The effect of the thermal period and the water saturation are analysed in a series of one-dimensional radial-symmetric simulations performed using TOUGHREACT which is a reactive transport code that accounts for variably saturated multi-phase flow under non-isothermal conditions. These simulations are based on the outcomes of a previous numerical model built with the same code for the simulation of the LOT-A2 experiment. The interaction of the fully water-saturated bentonite with groundwater is assessed through a series of tree-dimensional (3D) numerical models performed in PHAST. In order to assess the thermo-hydraulic and geochemical evolution of the near-field, a fracture in the granitic host rock is considered to intersect the deposition hole in which bentonite buffer surrounds the copper canister containing the spent fuel. The main achievements obtained in the numerical models developed for the saturation of the bentonite buffer during the thermal period indicate that although the model predicts a temporary evaporation of water near the copper canister, no chloride salt reaches saturation, and therefore, chloride is considered to behave conservatively. In addition, if the advective flow rate through the fracture is relatively low, the chemical composition of the groundwater in the fracture around the buffer could be influenced by the bentonite chemistry. Anhydrite, the high-temperature stable Ca-sulphate mineral, is predicted to precipitate around the copper canister when the higher temperatures are established (above 56 deg C). Nevertheless, as the temperature in the near-filed tends to decrease towards the regional value of 15 deg C, anhydrite previously precipitated progressively dissolves, and in turn, gypsum tends to replace the higher temperature Ca-sulphate mineral. During the thermal period, the primary quartz of the bentonite tends to dissolve close to the copper canister and re-precipitate close to the contact between the bentonite buffer and the granite. The evolution of this mineral is a function of the thermal conditions as its solubility increases with temperature. Computed results for the water-saturated period of the near-field indicate that the interaction between the minerals that constitute the MX-80 bentonite and the Forsmark groundwater that occurs nowadays at the depth of the future repository leads to an overall depletion in gypsum in favour of calcite precipitation. In addition, the initially Na-rich montmorillonite interlayer is predicted to enrich in calcium. In the case that the bentonite buffer is composed of Deponit CA-N bentonite, the inflow of Forsmark groundwater leads to the dissolution of gypsum and dolomite in favour of more calcite precipitated. In addition, the initially Mg and Ca-rich montmorillonite interlayer becomes depleted in these cations and enriched in sodium. In both bentonite types considered, the inflow of Forsmark groundwater leads to relatively small changes on the pH of the near-field with respect to the values expected for the initial porewater of the engineered barriers}
place = {Sweden}
year = {2010}
month = {Sep}
}