María Victoria Villar

Centro Investigaciones Energéticas, Medioambientales y Tecnológicas | CIEMAT · Departamento de Medio Ambiente

A common design of a high-level radioactive waste (HLW) disposal system consists of the waste canisters emplaced in tunnels or shafts, with the space between the heat-emitting canisters and the surrounding rock filled with a bentonite-based material. Understanding the behavior of this barrier, in particular the effect of prolonged heating on its pr...

To simulate in the laboratory the conditions of the buffer material in an underground repository for radioactive waste, three tests were performed in instrumented stainless steel cylindrical cells using compacted Wyoming-type bentonite. One of the cells was laid in vertical position and the other two stayed horizontal during operation, with the aim...

Bentonite barriers are a key component in may designs of deep geological repositories for high level nuclear waste. During the hydration stage, the bentonite undergoes non-uniform changes in dry density that may persist even after reaching a fully saturated state. Since dry density controls the properties of the bentonite that ensure the functions...

A controlled re-saturation of a heated 50 cm long column made of MX-80 bentonite pellets in the CIEMAT laboratory facilities (Madrid, Spain) has been running for 10 years (2011-2021). The experimental data on the saturation state and the swelling response of the bentonite material at high temperatures provided useful information for the verificatio...

Bentonite-based materials are employed in some nuclear waste disposal concept designs to seal underground tunnels and shafts. In some cases, these barriers consist of two parts: highly compacted blocks and granular buffer material consisting of pellets. These two components have highly distinct initial properties in terms of dry density, water cont...

In the context of the deep geological disposal of nuclear waste and to improve the understanding of the homogenisation process of bentonite barriers, in particular with regard to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling conditions. The samples were saturate...

Some deep geological nuclear waste disposal concepts are considering the installation around the waste canisters of bentonite barriers made up of two components: highly compacted blocks to support the canister and a granular buffer material consisting of bentonite pellets to fill the rest of the disposal gallery. The combined use of pellets and blo...

To improve the understanding of the homogenisation process of bentonite barriers for geological disposal of nuclear waste, in particular with regards to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling. The samples were saturated from the top surface using the vapo...

To simulate in the laboratory the conditions in the HE-E in situ test, a 50-cm long column of MX-80 bentonite pellets was heated on its base to 140 °C while Pearson water was supplied through its upper surface. The test consisted of a 7-month heating phase and a heating+hydration phase that went on for more than nine years. The initial dry density...

This report summarises the results from the FEBEX in-situ experiment at the GTS. The full-scale heater test ran for over 18 years from the start of heating in 1997 to final dismantling ending in 2015. This report is one of the outputs from the FEBEX-DP project and culminates a series of Nagra work reports (NAB) covering different aspects of the FEB...

Las funciones de seguridad de las barreras de bentonita para residuos nucleares dependen principalmente del valor de su densidad seca (o porosidad). La evolución de esta variable durante la fase transitoria de la barrera es compleja. El artículo presenta una formulación teórica para calcular, mediante análisis numéricos acoplados, los cambios de de...

Some deep geological nuclear waste disposal concepts are considering the installation around the waste canisters of bentonite barriers made up of two components: highly compacted blocks to support the canister and a granular buffer material consisting of bentonite pellets to fill the rest of the disposal gallery. The initial characteristics of thes...

In a deep geological nuclear waste repository gas can be generated by different processes. Understandingthe gas transport mechanisms across the engineered and natural barriers in a repository is relevant for itssecurity assessment, both in terms of mechanical stability and of radionuclide transport. The engineeredbarrier may be composed of compacte...

The FEBEX in situ test provided bentonite samples that had been submitted to the conditions of the engineered barrier of a nuclear waste repository for 18 years. These samples can be considered quite evolved from the microstructural point of view (aged, matured) when compared with samples prepared in the laboratory under shorter and more usual time...

The geo-science and engineering fields have critical roles to play towards a sustainable energy future. This state-of-the-art review focusses on five areas where the geotechnical community has been involved the most: the oil and gas sector with emphasis on methane hydrates, carbon geological storage, geothermal, energy geo-storage, and nuclear wast...

The safe isolation of high-level radioactive waste from the human environment is a key requisite in the nuclear fuel cycle. Deep geological repositories (i.e. >500m) in a suitable rock mass, involving both engineered and natural barriers is currently the most favoured option for the disposal of high-level nuclear waste. Complex Thermal-Hydro-Mechan...

Compacted bentonites are one of the best sealing and backfilling clays considered for use in Deep Geological Repositories of radioactive wastes. However, an in-depth understanding of their behavior after placement in the repository is required, including if the activity of indigenous microorganisms affects safety conditions. Here we provide an opti...

The study of the hydro-mechanical properties of compacted bentonite is relevant in the context of deep geological radioactive waste repositories, where bentonite will be used as buffer material between the waste canisters and the host rock and will be subjected to high temperatures and hydraulic gradients. This research aimed at determining the wat...

https://authors.elsevier.com/a/1a5e538lp9POt The FEBEX in situ experiment was a full-scale test reproducing the near-field of an underground nuclear waste repository. It was performed in a gallery excavated in granite, two heaters simulated the thermal effect of the waste canisters and a bentonite barrier composed of highly-compacted blocks surrou...

The hydration of a bentonite barrier in the early stage of a geologic nuclear waste repository with a bentonite buffer is a critical issue for its long-term performance and safety because bentonite might be permanently altered and subsequently affect the function of bentonite barrier. Large scale in situ testing integrated with modeling analysis is...

The FEBEX (Full-scale Engineered Barrier Experiment) in situ test simulated the engineered barrier of a nuclear waste repository and was in operation for 18 years under natural conditions, with a heater simulating the waste container. The water retention curves of bentonite samples retrieved during dismantling of the in situ test (FEBEX-DP samples)...

Available at: https://www.skb.com/publication/2493262/P-19-10.pdf Concrete will be used as the main construction material in SKB planned repository concepts for shortand long-lived low and intermediate level waste in Sweden. Concrete will also be one of the key engineered barriers to limit the release of radionuclides. Based on the experience acqui...

An experimental setup was designed to measure gas permeability and gas breakthrough pressures in Opalinus clay samples. The confining pressures applied were higher than the maximum in situ stress, and the tests were performed by slowly increasing the injection pressure whereas backpressure was kept atmospheric and the outflow was measured. The brea...

The FEBEX in situ test simulated the engineered barrier of a nuclear waste repository and was in operation for 18 years under natural conditions. The barrier was composed of blocks of compacted FEBEX bentonite and a heater simulated the waste container. The gas permeability of bentonite samples taken at different positions around the heater during...

The FEBEX in situ test simulated the engineered barrier of a nuclear waste repository and was in operation for 18 years under natural conditions. The water retention curves of bentonite samples retrieved during dismantling of the in situ test were determined in wetting paths under isochoric conditions with the vapour transfer technique. The water r...

The in situ Full-scale Engineered Barriers Experiment or Febex was a full-scale test reproducing the near field of a nuclear waste repository. It was performed in a gallery excavated in granite at the Grimsel Test Site in Switzerland, with a heater whose surface temperature was set to 100°C, simulating the waste canister and a bentonite barrier com...

The FEBEX in situ test was a full-scale experiment performed at the Grimsel Test Site (Switzerland) under natural conditions to reproduce the Engineered Barrier System of an underground repository for nuclear waste. After 18 years of operation under natural hydration from the granitic host rock and heating from a heater simulating the waste caniste...

The FEBEX “in situ” test was performed at an underground laboratory in Grimsel (Switzerland) with the aim of studying the behaviour of components in the near-field of a nuclear waste repository. A gallery of 2.3 m in diameter was excavated through the granite and two heaters, simulating the thermal effect of the wastes, were placed inside, surround...

Worldwide tank spills represent 10% of the average annual input of oil in the sea. When such spills arise from wrecks at depth, neutralisation of environmental impacts is difficult to achieve. Extracting oil from sunken tankers is expensive, and, unfortunately, all of the oil cannot be extracted, as the Prestige case demonstrates. We propose an env...

The FEBEX large-scale test simulating an underground repository of nuclear waste excavated in granite was dismantled after 18 years of operation. The engineered barrier between the heaters and the host rock consisted of compacted blocks of FEBEX bentonite which were retrieved during dismantling. The water content, dry density and suction of some of...

Two different sealing materials–a granulate of MX-80 bentonite pellets (B) and a 65/35 sand/MX-80 bentonite mixture (S/B)–were tested in cylindrical cells to analyse the effect of heating and hydration on the spatial distribution of temperature and humidity inside them. The 50-cm long columns were heated on their base to 140°C while Pearson water w...

Two twin 40-cm long columns of compacted FEBEX bentonite were tested in Teflon cells; water was supplied through the top surface of the columns and in one of them a heater was placed at the base and set to 100°C. The purpose of these tests was to simulate the behaviour of an engineered barrier in a radioactive waste repository and investigate the e...

This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed framework assumes the presence of two porous media linked through a mass-transfer term between them. In its more general form, the proposed approach allows the consideration...

Two different sealing materials–a granulate of MX-80 bentonite pellets (B) and a 65/35 sand/MX-80 bentonite mixture (S/B)–were tested in cylindrical cells to analyse the effect of heating and hydration on the spatial distribution of temperature and humidity inside them. The 50-cm long columns were heated on their base to 140 C while Pearson water w...

Two 20-cm long columns of MX-80 bentonite compacted at a nominal dry density of 1.7 g/cm³ with a water content of 17% were tested in thermo-hydraulic (TH) cells with the aim of simulating the conditions of a sealing material in a nuclear waste repository. On top of the columns a hydration surface simulated the host rock supplying groundwater and at...

To simulate the conditions of a sealing material in a nuclear waste repository a 20-cm long column of MX-80 bentonite compacted at a nominal dry density of 1.7 g/cm3 with a water content of 17% was tested in a thermo-hydraulic cell. On top of the column a hydration surface simulated the groundwater coming from the host rock and at the bottom a heat...

Evaporation tests in concrete columns have been analysed by numerical models to characterize the thermo-hydraulic properties and the processes in concrete. Two evaporation tests were performed: a column heated by a lamp and a column kept in room conditions. The conceptual model considers unsaturated liquid flow and transport of vapour and energy. W...

The EB experiment was a large-scale test performed in the Underground Research Laboratory of Mont Terri (Switzerland) for the demonstration of an engineered barrier concept for nuclear waste disposal consisting of the simultaneous use of high-density bentonite blocks and a lower-density bentonite pellets mixture (the granular buffer material, GBM)....

Concrete is used as a barrier on surface or near-surface facilities for the final disposal of low- and intermediate-level radioactive waste, where gas can be generated and affect the hydraulic properties and the processes taking place in concrete. In this framework, gas-transport properties of concrete samples were investigated using two different...

The gas-breakthrough pressure values in saturated FEBEX bentonite were determined for different dry densities and sample sizes. They increased clearly with dry density and were always higher than the swelling pressure of the bentonite. In high-density samples, gas flow tended to stop abruptly after breakthrough, whereas, in lower density samples, g...

The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests performed in cylindrical cells (length 60 cm, diameter 7 cm). Inside the cells, six blocks of FEBEX bentonite compacted to dry density 1.65 g/cm3 were piled up, giving rise to a total length similar to the thickness...

Laboratory tests in thermo-hydraulic cells that simulate the conditions of the sealing material in a radioactive waste repository are very useful to identify and quantify processes taking place in the engineered barrier. This kind of tests have gone on at CIEMAT for the last 20 years, evolving from simple designs in which just temperatures inside t...

This paper summarises the information gathered in the last 15 years on the saturation of compacted bentonite obtained from different laboratory-scale tests, a large-scale mock-up test, and a real-scale in situ test, that were performed to simulate the conditions of the bentonite barrier in a high-level radioactive waste repository and to better und...

The smectite basal spacings measured in samples with different water contents of bentonite compacted at various dry densities were reported and analysed. The materials used were two natural bentonites composed mainly of montmorillonite, one of them with a predominance of divalent cations in the exchange complex (FEBEX bentonite), and the other one...

There is increasing evidence from researchers on clay mineralogy that the density of water attached to clay minerals may be greater than 1.0 Mg/m(3). This fact becomes especially evident in compacted highly expansive clays close to water saturation. This kind of material is being considered in the design of engineered barriers for radioactive waste...

As a contribution to the understanding of bentonite hydration, the porosity of two different compacted bentonites with different degrees of saturation was analysed by mercury intrusion porosimetry and X-ray diffraction. The percentage of macropores, as well as the macropore mode, decreased with dry density both in wet and dry samples. Wet samples s...

The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended as sealing material in underground repositories for nuclear waste. One of the main advantages of the use of powder/pellets mixtures is the reduction of the compaction effort requ...

Water potential is the energy of water in a soil resulting from the different forces acting upon it. Water potential is a measure of the capacity of the soil to hold water in its pores. Experimental results show that the soil water retention capacity tends to reduce with increasing temperature. The paper presents a formulation developed using conce...

This paper presents an experimental study on a bentonite/silt mixture using odometers with suction controlled either by the osmotic technique or by air overpressure at two different temperatures of 20° and 60°C in a suction range between 0 and 8 M Pa. The compression curves permitted us to interpret the influence of the temperature on the different...

This work presents experimental results about the saturation of bentonite under constant volume and the simultaneous development of swelling pressure. The tests have been performed at room temperature under conditions of free availability of liquid water. Water intake and swelling pressure development were measured. The tests have been performed wi...

This paper presents results of an experimental study on the effect of suction on the shear behaviour of sand/silt mixtures as determined with direct shear tests performed under suction control. The device used to perform these tests is a direct shear apparatus in which the shear box has been replaced by an air pressure chamber where matric suction...

The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests. Cylindrical cells with an inner length of 60 cm and a diameter of 7 cm were constructed. Inside the cells, 6 blocks of FEBEX bentonite compacted to a dry density of 1.65 g/cm³ were stacked, resulting in a total len...

The storage of nuclear waste is still an unresolved problem of the nuclear industry, geological disposal being the most favored option. Most conceptual designs for the deep geological disposal of nuclear waste envisage placing the canisters containing the waste in horizontal drifts or vertical boreholes. The empty space surrounding the canisters is...

A 20-cm high column of MX-80 bentonite compacted at dry density 1.72g/cm3 with an initial water content of 16% was submitted to heating and hydration by opposite ends for 496days (TH test). The temperature at the bottom of the column was set at 140°C and on top at 30°C, and deionised water was injected on top at a pressure of 0.01MPa. Upon dismantl...

The bentonite barrier of underground repositories for high-level radioactive waste will be hydrated by the groundwater while it is subjected to high temperatures due to the radioactive decay of the wastes. These changes of temperature affect the hydraulic and mechanical responses of bentonite, which has important effects on design and performance o...

The main objective of this work is to highlight the influence of variables like temperature and porosity (dry density) on the water retention curve of expansive clays for the analysis of thermo-hydro-mechanical coupled processes (THM problem).The paper presents retention curves of MX-80 bentonite measured under isochoric conditions for different dr...

The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level nuclear waste. In the experiment, heaters are emplaced in the axis of a tunnel excavated in granite to simulate the heat production of radioactive waste. The test is fully instrum...

Final Report Preprint

The effect of the salinity of the saturating fluid on the hydro-mechanical properties of the FEBEX compacted bentonite was investigated by means of swelling, compressibility and permeability tests in which deionised water and solutions of different concentrations and compositions were used as saturating fluids. The solutions were chosen to simulate...

The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of...

The aim of this work is to extend an existent double structure model for expansive clays (Sanchez et al., 2005) to include the thermal effects in the analysis. Experimental results obtained in the context of the NF-PRO project have been used to extend the constitutive law. A fundamental characteristic of the double structure framework is the explic...

The retention curves of the FEBEX and MX-80 bentonites compacted at dry densities from 1.3 to 1.8 g/cm(3) have been determined using methods that allow the volume of the samples to remain constant during the determination. The methods have been upgraded to use them at high temperatures, and thus the range of temperatures between 20 and 120 degrees...

In the context of a project for the study of the behaviour of the clay barrier in a nuclear waste repository, the swelling properties of compacted bentonite have been investigated. Results on the influence of initial dry density and water content on the swelling pressure and swelling deformation of a compacted bentonite are presented. Swelling pres...

Concrete could be part of the barrier system and/or be needed during repository construction for the management of the High Level Radioactive Waste (HLW) in a Deep Geological Repository (DGR). Depending upon the design, concrete could be physically in contact with the bentonite, or be sufficiently close that alkaline pore fluid from the cement may...

This work has been focused on characterization of geochemical processes occurring in the bentonite barrier and their influence on corrosion of the carbon steel container. In the cells used for these tests, a block of compacted bentonite was subjected simultaneously to constant hydration and heating, in opposite directions, in order to simulate the...

A review of the more relevant laboratory results concerning the thermo-hydro-mechanical (THM) behaviour of the FEBEX bentonite is presented. Most of the THM features of compacted bentonite have been experimentally studied during the FEBEX project. The results about fabric characterisation, water retention, permeability, swelling capacity and mechan...

The FEBEX in situ experiment at the Grimsel Test Site (Switzerland) reproduces a gallery for high-level radioactive waste disposal according to the Spanish concept, with two heaters simulating the waste containers and a bentonite barrier around them. After five years of operation, heater 1 was switched off in February 2002. Following cooling of the...

This work presents the water retention curves obtained for two natural bentonites compacted at different dry densities. The density of the bentonite was kept constant during the determination, for which specific methodologies were developed. The materials tested are the FEBEX and the MX-80 bentonites, in the first of which divalent cations (Ca and...

Engineered barriers made up of compacted expansive clays are a basic component of the systems for the isolation of high-level radioactive waste (HLW). The barrier is initially unsaturated and during hydration the hydraulic gradient varies from high values (i.e. higher than 50000) at the beginning of the wetting to very low values (practically zero...

Two methods were developed to determine the retention curve for a fixed dry density and at high temperature. The materials used are the MX–80 and the FEBEX bentonites. The retention capacity of the FEBEX bentonite is higher than that of MX–80, i.e. for a given suction and dry density, the water content of the FEBEX bentonite is higher. However, the...