Studying the thermal conductivity of a deep Eocene clay formation: direct measurements vs back-analysis results

An experimental study on Ypresian clays–one of the potential deep and sedimentary clay formations in Belgium for the geological disposal of heat-emitting radioactive waste–has been undertaken to systematically study its thermal conductivity using different experimental techniques. As a first step, a new experimental setup with heat flux measurement has been used and careful pre-conditioning protocols have been followed to directly measure this thermal property. The aim of these pre-conditioning tests has been ensuring a very high degree of saturation and the closure of fissures / gaps along bedding planes before the thermal tests are run under low stress conditions. Thermal tests have shown to be particularly sensitive when the thermal conductivity is determined along a direction orthogonal to these bedding planes. The study is then complemented by using a constant volume heating cell, in which heating pulse tests have been carried out under fully saturated conditions that have been ensured with a high water back-pressure. Numerical models have been used to interpret this pulse test, to exploit all the information provided by temperature measurements and to back-analyse the thermal conductivity. Direct thermal conductivity data with the improved pre-conditioning protocol allowed obtaining results consistent with the values reported when using back-analysis in the constant volume cell. The article discusses the importance of restoring full saturation conditions, particularly on retrieval of deep sedimentary clays, which may undergo opening of fissures along bedding planes that may affect the correct determination of the thermal conductivity.Peer ReviewedPostprint (author's final draft

Hydro-chemical modelling of in situ behaviour of bituminized radioactive waste in Boom Clay

The hydro-chemical (CH) interaction between swelling Eurobitum bituminized radioactive waste (BW) and Boom Clay was investigated to assess the feasibility of geological disposal for the long-term management of this waste. First, the long-term behaviour of BW in contact with water was studied. A CH formulation of chemically and hydraulically coupled flow processes in porous materials containing salt crystals is discussed. The formulation incorporates the strong dependence of the osmotic efficiency of the bitumen membrane on porosity and assumes the existence of high salt concentration gradients that are maintained for a long time and that influence the density and motion of the fluid. The impacts of temporal and spatial variations of key transport parameters (i.e. osmotic efficiency (s), intrinsic permeability (k), diffusion, etc.) were investigated. Porosity was considered the basic variable. For BW porosity varies in time because of the water uptake and subsequent processes (i.e. dissolution of salt crystals, swelling of hydrating layers, compression of highly leached layers). New expressions of s and k describing the dependence of these parameters on porosity are proposed. Several cases were analysed. The numerical analysis was proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour in terms of osmotic-induced swelling, leached mass of NaNO3 and progression of the hydration front when heterogeneous porosity and crystal distributions have been assumed. Second, the long-term behaviour of real Eurobitum drums in disposal conditions, and in particular its interaction with the surrounding clay, was investigated. Results of a CH analysis are presented.Peer ReviewedPostprint (published version

Gas migration in a Cenozoic clay: experimental results and numerical modelling

Gas migration through a potential host clay formation for the geological disposal of radioactive waste in Belgium is experimentally investigated in the laboratory, and numerical modelling is performed to help in the interpretation of the results. Selected air injection tests under oedometer conditions on initially saturated Boom Clay samples with oriented bedding planes are presented in the paper. Priority in the experimental programme was given to the study of the deformation response along the injection and dissipation stages, as well as to the analysis of the pore network changes, which detect the opening of fissures that can act as preferential air pathways. The experimental results were simulated using a fully coupled hydro-mechanical finite element code, which incorporates an embedded fracture permeability model to account for the simulation of the gas flow along preferential pathways. Clay intrinsic permeability and its retention curve were assumed to be dependent on strains through fracture aperture changes. The numerical results could reproduce upstream/downstream pressures, outflow volume and soil volume change accurately. The experimental results, combined with the numerical simulation, provide good insight into the role of the volumetric response and of the bedding planes on the air transport properties of Boom Clay samples, confirming that fracture aperture occurs during gas injection, which eventually dominates further injection and pressure release stages.Peer ReviewedPostprint (author's final draft

Efficiency of a borehole seal by means of pre-compacted bentonite blocks

The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded project RESEAL studied the feasibility of sealing off a borehole in plastic Boom Clay by means of pre-compacted bentonite blocks. Two bentonites, namely the FoCa and Serrata clay, have been used. Based on laboratory tests, the bentonite blocks had an initial dry density of about 1.8 g/cm3 to obtain a swelling pressure of about 4.4 MPa, corresponding to the in situ lithostatic stress, at full saturation. The set-up was equipped with several sensors to follow-up the behaviour of the seal and the surrounding host rock during hydration. Full saturation was reached after five months and was mainly reached by natural hydration. Swelling pressure was lower than originally foreseen due to the slow reconsolidation of the host rock. Later on, the efficiency of the seal with respect to water, gas and radionuclide migration was tested. The in situ measured permeability of the seals was about 5 × 10-13 m/s. A gas breakthrough experiment did not show any preferential gas migration through the seal. No evidences of a preferential pathway could be detected from 125I tracer test result

RESEAL II: a large-scale in situ demonstration for repository sealing in an argillaceous host rock: phase II

Final ReportPreprin

Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration

Instability of most prototypical metal organic frameworks (MOFs) in the presence of moisture is always a limita- tion for industrial scale development. In this work, we examine the dissociation mechanism of microporous paddle wheel frameworks M(bdc)(ted)0.5 [M=Cu, Zn, Ni, Co; bdc= 1,4-benzenedicarboxylate; ted= triethylenediamine] in controlled humidity environments. Combined in-situ IR spectroscopy, Raman, and Powder x-ray diffraction measurements show that the stability and modification of isostructual M(bdc)(ted)0.5 compounds upon exposure to water vapor critically depend on the central metal ion. A hydrolysis reaction of water molecules with Cu-O-C is observed in the case of Cu(bdc)(ted)0.5. Displacement reactions of ted linkers by water molecules are identified with Zn(bdc)(ted)0.5 and Co(bdc)(ted)0.5. In contrast,. Ni(bdc)(ted)0.5 is less suscept- ible to reaction with water vapors than the other three compounds. In addition, the condensation of water vapors into the framework is necessary to initiate the dissociation reaction. These findings, supported by supported by first principles theoretical van der Waals density functional (vdW-DF) calculations of overall reaction enthalpies, provide the necessary information for de- termining operation conditions of this class of MOFs with paddle wheel secondary building units and guidance for developing more robust units

Numerical and experimental water transients in sewer pipes

The paper deals with transient transcritical flow in closed sewer pipe. Among the various shock-capturing schemes used for solving hyperbolic systems of conservation laws, an upwind scheme is adopted in such a way that automatic description of hydraulic jumps and bores becomes possible by a steep variation of hydraulic variables over a few grid points. The Pavia Flux Predictor scheme (P.F.P.) was selected, because of its simplicity, robustness and physical consistency. To validate the numerical model, experiments were carried out for a steep slope circular pipe. An application to flush flows at sewer heads is also presented in a context of parsimonious water consumption

Analysis of Human Intrusion Scenarios for the Deep Disposal of Fusion Wastes

During the development of the International Thermonuclear Experimental Reactor (ITER) fusion reactor, many efforts are done to minimise the amount of radioactive waste that will arise from the decommissioning of a nuclear fusion power plant. Nevertheless, the most active waste types will have to be disposed in a repository. The impact of possible future human actions on the performance of the repository has to be evaluated. The most active waste types are selected from the inventory of fusion plant model PM-2 with low-activation martensitic steel as main material. After a cooling period of 100 years, the fusion waste is assumed to be disposed off in a repository located in the boom clay layer at the Mol site (Belgium). A systematic approach for the identification of the relevant intrusion scenarios is applied. Three scenarios resulting from borehole drilling are identified as relevant, core inspection, residence and unsealed borehole. This paper will focus on the analysis of the most drastic human intrusion scenario, i.e. the core inspection scenario. The maximum dose is calculated for waste arising from Be-coatings. In the case of a routine inspection, the dose to a geological worker is always under 0.5 Sv. This value, under which serious deterministic health effects are unlikely, can be considered as a reference level for acute exposure. In the case of the very pessimistic close inspection variant a maximum dose of 0.4 Sv is calculated if the intrusion occurs immediately after the disposal. After 60 000 years, the dose becomes lower than 3.5 mSv, which is the average annual background dose. The results of an alternative approach, considering a fusion-specific repository and wells drilling as the main intrusion scenario, are also included in this paper. Finally, a comparison of the consequences due to intrusion scenarios calculated for fusion and fission waste is presented. For the most active fission waste types it lasts 1 million years before the close inspection dose drops under 0.5 Sv