Description of work
The PRACLAY experiments to be conducted at the HADES Underground Research Laboratory, Mol is designed to simulate the construction and operation (mainly heating) of a waste disposal gallery in a clay host rock.
The effect of a large scale thermal load on the behaviour of Boom Clay is one important remaining issue in the feasibility study of the disposal. Indeed, the impact of the thermal load generated by the waste is particularly important since it will significantly affect the temperature and the stress profiles on the whole thickness of Boom Clay in the short term after the disposal. The early transient THM perturbation might be the most severe impact that the repository system will undergo on a large spatial scale and in a relatively short period of time.
The PRACLAY in-situ experiment is developed to be design-independent to overcome possible future changes in the reference disposal design. The PRACLAY experiments are performed within “The PRACLAY Gallery”, which will be 45 m long with an internal diameter about 2 m, lined with concrete segments and perpendicular to the connecting gallery. The Boom Clay will be instrumented to characterise the THM behaviour around the gallery. The heater length will be about 30 m. The PRACLAY in-situ experiments regroups a set of three tests (see figure 1): the gallery and crossing test, the heater test, the plug test. The gallery will be constructed in 2007 and the heating phase will start at the end of 2008.
Figure 1. The PRACLAY experiments
EURIDICE will provide all the necessary information and results for the project in deliverable D11. This concerns the construction and the installation of the PRACLAY experiment and the measurements of pore water pressures, total pressures, displacements and the chemistry evolution around the PRACLAY gallery.
ASC will build upon advanced seismic processing techniques successfully developed in the OMNIBUS project, and apply these techniques to the interpretation of measurements from the PRACLAY heater test. microseismic, acoustic emission (AE) and ultrasonic-survey technologies are scaled seismic investigations that provide remote methods of examining the disturbance and damage evolution induced in the rock mass around an underground excavation, potentially caused by the operating conditions of a high-level radioactive waste repository (through mechanical, thermal and hydraulic stresses, and chemical and biological processes). These technologies provide unique data for remotely characterizing the host rock, allowing volumes of disturbance and damage to be delineated and quantified, and providing measurements for the calibration and validation of numerical models that predict the effects of the repositories operating conditions. These technologies will be used in the framework of the large scale PRACLAY heater test. The equipment configuration utilizes 18 ultrasonic transmitters and receivers installed in three boreholes, quasi-parallel to the future PRACLAY gallery, and based on the equipment package utilized in the SELFRAC project also performed at HADES. The equipment package and seismic data acquisition will be supplied within the framework of the PRACLAY heater test and will not be charged to the TIMODAZ project. Data and results from the interpretations performed here will be made available to the consortium in electronic data files and project reports, and disseminated to a wider audience through appropriate scientific journals in consultation with the project coordinator.
-Development of techniques for processing ultrasonic survey data and performing waveform propagation in numerical models
The advanced processing of ultrasonic surveys and the full-waveform modelling process will be further refined and optimised, following the work performed in the OMNIBUS project. Crack parameters (density, size and geometries), resulting from the processing of ultrasonic surveys and correlation with wave-propagation models, will be linked to rock permeability and porosity parameters. Existing data from past in situ and laboratory tests will be used to verify the processed parameters. A thermal response will be added to the wave-propagation codes in order to model the behaviour of the PRACLAY heating experiment on Boom clay. Existing data from laboratory and in situ tests will be used to verify the thermal response simulated by the codes.
-Construction of waveform propagation models for ultrasonic surveys conducted during the PRACLAY experiment in Boom clay
Wave propagation models will be constructed for the specific case of the PRACLAY experiment performed in Boom clay, following the strategy developed in the OMNIBUS project. In this strategy, many different models will be performed using slightly different crack parameters in simulated EDZ volumes. Available laboratory data from Boom clay, using experiments performed in the TIMODAZ project and elsewhere, will be used to calibrate the material properties used in the numerical models. A review of available experiments will be conducted at the start of the project.
-Processing and interpretation of ultrasonic data from the PRACLAY experiment
The long-term evolution of the rock fracturing and self-healing processes around the PRACLAY gallery will be analysed. Ultrasonic survey data obtained in the PRACLAY experiment, and within the timeframe of the TIMODAZ project, will be processed for rock parameters following the strategies developed here and in the OMNIBUS project. Results from waveform propagation models will be correlated in the time and frequency domains with these survey results in order to provide an interpretation of the PRACLAY excavation and heating on the Boom clay host rock.