Water, vapour and heat transport in concrete cells for storing radioactive waste

Water is collected from a drain situated at the centre of a concrete cell that stores radioactive waste at ‘El Cabril’, which is the low and intermediate level radioactive waste disposal facility of Spain. This indicates flow of water within the cell. 2D numerical models have been made in order to reproduce and understand the processes that take place inside the cell. Temperature and relative humidity measured by sensors in the cells and thermo-hydraulic parameters from laboratory test have been used. Results show that this phenomenon is caused by capillary rise from the phreatic level, evaporation and condensation within the cell produced by temperature gradients caused by seasonal temperature fluctuations outside. At the centre of the cell, flow of gas and convection also play a role. Three remedial actions have been studied that may avoid the leakage of water from the drain.Peer ReviewedPostprint (author's final draft

Modeling the organic carbon oxidation and redox sequence under the partial-equilibrium approach: a discussion by means of a semi-analytical solution

In this work, we have developed a semi-analytical solution for organic carbon oxidation coupled to the reduction-oxidation sequence assuming the Partial Equilibrium Approach (PEA) and using the decoupling procedure of De Simoni et al. (2005), https://doi.org/10.1029/2005WR004056. Our solution was applied to two very simple cases. The first assumes only advective transport and the second only diffusive transport. Comparison with a numerical solution showed the adequacy of our analytical solution to be implemented in several scenarios, for example, in organic carbon oxidation in the unsaturated zone or in highly heterogeneous advective domains. We found that for the diffusion case the PEA produced spurious reactions, such as oxidation of N2 by O2 when compared with an approach using full kinetics. These reactions do not occur in the advection case. An analysis with the semi-analytical solution revealed that they are the result of a combination of diffusive fluxes and the fact that the PEA assumes the electron acceptors to react with each other in equilibrium. Our analytical solutions are capable to quantify this shortcoming, becoming a tool to validate numerical models using PEA to describe organic carbon oxidation.This work was financially supported by MONOPOLIOS (RTI2018-101990-B-100, MINECO/FEDER), MEDISTRAESIII (Pid2019-110212RB-C22, MICINN), as well as the EU project MARADENTRO (PCI2019-103425-WW2017) and the Catalan Research Project RESTORA (ACA210/18/00,040). Additional funding was obtained from the Generalitat de Catalunya (2017 SGR1485). We also would like to thank Carlos Ayora for his constructive comments on the manuscript.Peer ReviewedPostprint (published version

Modeling the hydrogeochemical evolution of brine in saline systems: case study of the Sabkha of Oum El Khialate in South East Tunisia

We studied the effects of evaporation and groundwater flow on the formation of salt minerals in the Sabkha of Oum El Khialate in South East Tunisia, which contains large amounts of sulfate sodium mineral deposits. Due to the fact that there are no important surface water bodies present in this sabkha, transport of solutes is dominated by advection rather than mixing in lakes. For our study we used both analytical conservative and numerical reactive transport models. Results showed that salinity varies with distance and may reach very high levels near a watershed where the groundwater flux is zero. As a consequence, reactive transport simulations results showed that more minerals precipitate and water activity decreases values near this watershed. Model results also showed that a sequence of precipitating minerals could be deduced after 140,000 years. From the boundary of the sabkha towards the watershed the mineral sequence was dolomite, gypsum, magnesite, bloedite, halite and mirabilite. It was found that the amounts as well as the mineral precipitation distribution strongly depend on salinity and rates of inflowing water. (C) 2014 Elsevier Ltd. All rights reserved.Peer ReviewedPostprint (author’s final draft

Modelling of multiphase flow in evaporation tests in concrete columns

Postprint (published version

Reactive transport modelling of cement-groundwater-rock interaction at the Grimsel Test Site

An in situ experiment at the Grimsel Test Site (Switzerland) to study water-cement-rock interaction in fractured granite was modelled. It consisted of a hardened cement source in a borehole intersecting a water conducting fracture. Grimsel groundwater was injected into this borehole. Two other boreholes at about 0.56 m and 1.12 m from the emplacement borehole were used to monitor the evolution of water composition for 5 years. The modelling approach was based on a 1D radial model for the emplacement borehole and a small volume of rock (fault gouge) around it, and a 2D model for the rest of the domain. The results of the 1D model were used as input for the 2D model. Both models showed dissolution of the fault gouge minerals. Results from the 1D model showed dissolution of portlandite in the cement with an increase in porosity. The 2D model showed a reduction in porosity in the fault gouge due to mineral precipitation. Near the emplacement borehole ettringite precipitated. At the centre of the plume there was precipitation of C-A-S-H and hydrotalcite. At the edge of the hyperalkaline plume calcite, hydrotalcite and illite precipitated.Peer ReviewedPostprint (author's final draft

Multiphase flow models of concrete cells of the radioactive waste disposal facility at El Cabril (Spain)

El Cabril is the low and intermediate level radioactive waste disposal facility for Spain. From the start of the filling (1992) until 2003, no water was collected from the drain situated at the centre of the cell. From 2003 onwards small amounts of water were collected from the drain, indicating flow of water within the cell. This occurred in summer and winter. A hypothesis had been proposed to explain this phenomenon based on multiphase flow and heat transport. We corroborate this hypothesis by means of 2D numerical models, using data measured by sensors in the cells and data from laboratory test. There is a good agreement between the data measured and the ones calculated by the models.Postprint (published version

Temperature driven vapor fluxes in soils cause a net recharge

Temperature gradients can drive vapor diffusion by controlling vapor pressure in the soil. We studied vapor diffusion for soils in two different climates: A semiarid climate at El Cabril (Córdoba, Spain) and a subarctic climate in the Upper Tuul River basin (Mongolia). For El Cabril vapor diffusive fluxes were studied by means of the measured temperatures and an analytical model. For the second site (Upper Tuul) a physically based soil water and energy balance model was developed accounting for relevant processes such as melting-freezing of water and vapor diffusion in the soil. Results of both sites show that vapor diffuses downwards during summer and upwards during winter, while yearly averaged fluxes diffuse downwards. The overall amount is small for El Cabril, but significant for the Upper Tuul. The latter large values can be explained by the large temperature oscillations of the Mongolian climate and the freezing/thawing of subsoil layer.Postprint (published version

Taller de pràctiques de laboratori d'hidrologia subterrània mitjançant models físics experimentals

En cap de les assignatures de grau d'hidrologia subterrània s'imparteixen pràctiques de laboratori. Això fa que l'estudiant rebi una educació purament teórica sense aaribar a assolir correctament els conceptes bàsics d'aquesta assignatura. Aquest projecte tracta de solucionar aquest problema. Per aconseguir-ho, s’ha desenvolupat un taller de pràctiques de laboratori en el qual s'utilitza un model físic experimental. El model físic és un aqüífer sintètic reconstruït al laboratori dins d'un tanc de metacrilat transparent a on es realitzen assaigs de bombeigs i de traçadors.Peer Reviewe

Reactive transport modelling of a high-pH infiltration test in concrete

A laboratory-scale tracer test was carried out to characterize the transport properties of concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). A hyperalkaline solution (K-Ca-OH, pH = 13.2) was injected into a concrete sample under a high entry pressure in order to perform the experiment within a reasonable time span, obtaining a decrease of permeability by a factor of 1000. The concentrations of the tracers, major elements (Ca2+, SO42-, K+ and Na+) and pH were measured at the outlet of the concrete sample. A reactive transport model was built based on a double porosity conceptual model, which considers diffusion between a mobile zone, where water can flow, and an immobile zone without any advective transport. The numerical model assumed that all reactions took place in the immobile zone. The cement paste consists of C-S-H gel, portlandite, ettringite, calcite and gypsum, together with residual alite and belite. Two different models were compared, one with portlandite in equilibrium (high initial surface area) and another one with portlandite reaction controlled by kinetics (low initial surface area). Overall the results show dissolution of alite, belite, gypsum, quartz, C-S-H gel and ettringite and precipitation of portlandite and calcite. Permeability could have decreased due to mineral precipitation.Peer ReviewedPostprint (author's final draft

Heat storage efficiency, ground surface uplift and thermo-hydro-mechanical phenomena for high-temperature aquifer thermal energy storage

High-temperature aquifer thermal energy storage (HT-ATES) systems can help in balancing energy demand and supply for better use of infrastructures and resources. The aim of these systems is to store high amounts of heat to be reused later. HT-ATES requires addressing problems such as variations of the properties of the aquifer, thermal losses and the uplift of the surface. Coupled thermo-hydro-mechanical (THM) modelling is a good tool to analyse the viability and cost effectiveness of HT-ATES systems and to understand the interaction of processes, such as heat flux, groundwater flow and ground deformation. The main problem of this modelling is its high computational cost. We propose a dimensional and numerical analysis of the thermo-hydro-mechanical behaviour of a pilot HT-ATES. The results of this study have provided information about the dominant thermo-hydraulic fluxes, evolution of the energy efficiency of the system and the role of the hydraulic and thermal loads generated by the injection and extraction of hot water.We acknowledge the financial support received from the ERANET project HEATSTORE (170153–4401). This project has been subsidized through the ERANET cofund GEOTHERMICA (Project n. 731117), from the European Commission, RVO (the Netherlands), DETEC (Switzerland), FZJ-PTJ (Germany), ADEME (France), EUDP (Denmark), Rannis (Iceland), VEA (Belgium), FRCT (Portugal), and MINECO (Spain). We wish to thank the Department of Research and Universities of the Generalitat de Catalunya by supporting RV with a grant (2021 FI_B 00940).Peer ReviewedPostprint (published version