Early-age thermo-mechanical behaviour of concrete Supercontainers for radwaste disposal

The Belgian concept for the disposal of vitrified high-level waste and spent fuel assemblies is founded on the use of cylindrical concrete Supercontainers. This concept is based on a multiple barrier system where every component has its own specific safety function requirements. It consists of encapsulating the heat-emitting waste in a watertight carbon steel overpack and surrounding it by a cylindrical concrete buffer that will then be disposed in a deep clay layer. Finally, the buffer is sealed with a concrete lid. Th concrete materials provide favourable chemical conditions for the overpack allowing it to confine the radionuclides during the thermal phase. In addition, they also ensure shielding protection during construction and transport. A self-compacting concrete (SCC) and a Traditional Vibrated Concrete (TVC) are bein considered for the choice of the cementious buffer, enclosing the radwaste. The use of SCC will ease considerably the precast process and complies with all other requirements regarding strength, durability, chemical interactions,... A laboratory characterization program, in order to obtain the mechanical and thermal properties of the SCC, and 2.5 D thermal and crack modelling simulations, has been conducted to predict the early-age thermo-mechanical behaviour of the concrete buffer during the different construction stages, i.e. the casing of the buffer, insertion of the high-level waste and closure of the Supercontainer. Also the effect of gamma radiation and elevated temperatures on hardening SCC based mortar and hardened SCC was investigated. Finally, the prediction of the avoidance of through-going cracks in the concrete buffer is ensured by means of simulations, after implementation of the obtained test results, and by means of large scale tests, with temperature measurements, displacement and deformation registrations, for the validation of the simulation results of the early-age behaviour of the Supercontainer

Early age behaviour of concrete supercontainers for radioactive waste disposal

A

Hardening self-compacing mortar expsoed to gamma radiation

For the disposal of high level radioactive waste, cementitious barriers are considered worldwide and for various purposes. The Belgian supercontainer concept, for example, considers the use of cylindrical concrete containers: the radwaste is emplaced inside a hardened self-compacting concrete buffer, and for closure of the supercontainer the remaining gap is filled by casting a selfcompacting mortar. As a consequence, this cementitious layer is exposed to the radioactive waste and gamma radiation during hardening. In this research study, small self-compacting mortar samples are irradiated by gamma rays during hardening, and exposed to different doses (Gy) and different dose rates (Gy/h) at different hardening times at first exposure to investigate the cement-waste interactions that might occur during hardening of the cementitious barrier. The effect on the strength and the microstructure is investigated, by means of compressive strength tests, scanning electron microscopy, and nitrogen adsorption tests. It was found that the observed strength loss due to gamma irradiation increases with an increasing total received dose. Furthermore, the age at which irradiation starts, plays a role in the effect of the gamma irradiation. A link between the strength of the mortar samples and its porosity is found by means of the nitrogen adsorption tests. A higher received dose increases the porosity which leads to a decrease in compressive strength. BET-analysis shows that the specific surface of the pores also increase due to gamma irradiation. Finally, SEM-analysis revealed that gamma irradiation during hardening of cementitious samples affects the microstructure

Evaluation of the migration coefficient, diffusion coefficient and the ageing factor according to the chloride ingress model described in the fib bulletin 34 applied to Belgian concrete mixtures

In this contribution the chloride diffusion model according to fib bulletin 34 is applied and evaluated on Belgian concrete [1]. The service life estimation according to chloride-induced corrosion is, inter alia, affected by the chloride migration coefficient or chloride diffusion coefficient. In addition, the time dependency of these parameters, reflected in the ageing exponent, has an even greater influence on the model. In a first part the different diffusion coefficients will be discussed. In a second part the parameters are calibrated to the Belgian concrete compositions. The chloride migration coefficient and chloride diffusion coefficient depend on the type of cement and vary according to different types of cement and w/c ratios. The ageing exponent is also dependent on the exposure class. Such a calibration is performed on the most commonly used types of cement in Belgium, which are Portland cement (CEM I) and blended cements (CEM III/A and CEM III/B). Distributions are proposed and evaluated based on available Belgian data and results from laboratory experiments. Finally the time dependency of the instantaneous diffusion coefficients and the time dependency of the apparent diffusivities are investigated

Influence of concrete composition on chloride ingress and carbonation : analysis by means of an extended data-set

In 2015 an IWT-TETRA project, called DurOBet, was initiated focusing on service life design assessment according to different chloride diffusion and carbonation models applied on Belgian concrete mixtures. The main purpose of this research project is to develop a quantitative method for a service-life based design of concrete structures, more particular applicable for the Belgian concrete industry. In this way an improvement of the deemed-to-satisfy approach of the EN206-1 code can be established which is more reliable with regards to service life predictions of concrete structures. In the framework of this DurOBet project it was decided to develop an extensive database incorporating concrete related results on i) fresh properties, ii) hardened properties and iii) durability related properties such as porosity, permeability and more specifically on chloride ingress and carbonation. The data originate from numerous journal articles and conference papers, doctoral research projects and master thesis studies. At this time more than 100 papers or studies were investigated, reported between 1992 and 2016, generating a dataset of over a thousand unique concrete recipes, geographically spread but with focus on the concrete mixes applicable for the Belgian industry. Both traditional and self-compacting concrete mixes are incorporated into the database. This database is being used for the analysis of the durability related properties, such as the chloride diffusion and carbonation coefficient, and their relation with mix proportioning parameters of the concrete mixtures (cement or binder content, type of binder, water-to-binder ratio,…). The main focus of this paper is to highlight the framework of the database: the mix proportioning of the concrete mixes is being discussed and the origin of the concrete data (country, reference info, etc.). By means of data mining and some known relations with respect to the durability related properties, e.g. correlation between w/b-ratio and chloride diffusion coefficient, are being evaluated

Cement-waste interactions: effect of gamma radiation on cementitious materials during hardening

edition: SCK°CEN-30091909status: publishe