Oral Session 6A : THM and chemical (THM-C) processes ... session 6A.pdf · During the mock-up test, some unexpected phenomenons were observed. For example, the swelling pressure built

Oral Session 6A

THM and chemical (THM-C)processes: mock up and in-situ experimentsChair: Patrik Sellin - Jean-Claude Robinet

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste Confinement

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THE MOCK-UP OPHELIE: A LARGE SCALEBACKFILL TEST FOR HLW DISPOSAL

Hughes Van Humbeeck1, Jan Verstricht1, Claude Gatabin2

1. EIG EURIDICE, Boeretang 200, 2400 Mol, Belgium2. CEA Saclay, DEN/DPC/SCCME/LECBA, 91191 Gif-sur-Yvette, France

Prior to the in-situ PRACLAY experiment (Preliminary demonstration test for clay disposal of high levelradioactive waste), which aims at simulating a section of a disposal gallery for vitrified HLW, the EIGEURIDICE (Economic Interest Grouping between ONDRAF/NIRAS and SCK-CEN) has performed a largescale surface test called OPHELIE mock-up (On surface Preliminary Heating simulation ExperimentingLater Instruments and Equipment, fig. 1). The objectives of the experiment were to review several aspects

not yet worked out in detail of the HLW disposaldesign (developed in the early 1990's) in a clayeyhost rock. This mock-up focused in particular on thebackfill material (specifications, manufacturing,installation and hydration), the steel disposal tubeand the monitoring equipments. The mock-up hadan internal diameter of 2 m and a length of about 5 m.The gallery lining was replaced by a steel liner withon its intrados 16 hydration tubes. A central tubewith dimensions similar to the waste disposal tubecontained the heating elements (450 W/m linearheat dissipation). The mock-up was backfilled withpre-compacted blocks of clay-based material. Thesehad been developed by the French Atomic EnergyCommission (CEA) taking into account designspecifications dealing with swelling pressure, thermal

conductivity and handling. The backfill material resulted in a mixture of FoCa1 clay (60 % of mass) givingthe swelling properties and low permeability to the mixture, sand (35 %) to limit the maximum swellingpressure, and graphite (5 %) to obtain a thermal conductivity almost independent of the saturation degreeof the mixture (around 2.5 W/mK). The mock-up was instrumented with about 150 sensors, mainly tomonitor the thermo-hydro-mechanical (THM) behaviour of the backfill material. All steel surfaces incontact with the backfill were made from stainless steel (AISI 304 or similar).The hydration of the mock-up started on 2 December 1997. Six months later, the heating was switchedon for four years.The measurements and observations during the experiment pointed the attention to some unexpectedphenomena, such as the high apparent thermal conductivity of the backfill material and the low anddecreasing swelling pressure exerted by the backfill material. Water samples indicated an unexpectedlyhigh content of chloride and other solutes close to the central tube, these species having an impact onthe resistance of the metallic parts against corrosion. Presence of free sulphides in the water indicatessulphate reducing mechanisms in the backfill.After a cooling period of 2 months, the mock-up was dismantled in October 2002. The dismantling wasaccompanied by a large scientific programme to better understand the unexpected phenomena. The backfill has swelled and filled all technological voids. By the way, the microbiological activity andthe corrosion have been limited inside the mock-up. The water saturation degree increased from the centraltube to the external liner (96%→ 100%). However, the backfill material was not fully saturated. The jointsbetween blocks were still visible and the water content of these joints was higher than inside a block.The swelling was therefore not uniform inside the mock-up.

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Figure 1: Mock-up during assembly, with thebackfill blocks, the hydration tubes, the disposaltube and the sensor cabling

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste ConfinementPage 82

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The results of the THM post-dismantling analyses showed an increase of the saturated hydraulic conductivity from values around 2 10-14 – 2 10-13 m/s (depending on the dry density) for the initial materialto values three time higher for the exposed material (fig. 2). No effect of the thermal gradient on thehydraulic conductivity was observed. We also detected a slight decrease of the swelling pressure and aslight increase of the thermal conductivity (2,7 – 3,1 W/mK for blocks from the middle and externalrings and around 2,5 W/mK – like the non exposed blocks – for the internal rings)

Compared with the non exposed material, no significant changes were noticed regarding waterretention. Exposed and initial backfill materials havethe same behaviour for high suctions (> 80 MPa).For low suctions, however, a slight decrease of waterretention was observed for the exposed blocks(may be due to the carbonates).

Apart from some cristallization phenomena (gyp-sum and silica grains), no evidence of mineralogicalchanges could be observed. This is consistent withthe small reduction of the swelling pressure.

The chemical analyses of the pore water and solid phase showed no significant changes for pH and CEC.An increase in Cl, dissolved silica and DOC towards the heater tube confirmed the results of the analysesmade during the operation phase. The enrichment of salts towards the heater tube can be explained throughadvective transport by the water front moving in the unsaturated backfill material during the hydrationprocess. The high concentration of dissolved silica can also be explained by the increased solubility ofsilicate and clay minerals at high temperature. The increase of DOC is more difficult to understand andcould also be affected by different artefacts occuring in the mock-up.

The paper will give more results about THM, mineralogical, chemical, microbiological and corrosionanalyses.The impact of the OPHELIE mock-up on the change of disposal design for vitrified HLW will be alsoexplained.

The OPHELIE mock-up experiment is financed by ONDRAF/NIRAS. The mock-up design and construction has also been supported by the EC.

The authors thank X. Li and F. Bernier from EURIDICE, P. De Cannière, P. Deboever, B. Kursten andL. Ortiz from SCK•CEN, M. Descostes, M. Jullien and J. Raynal from CEA, S. Daumas from BRGMand S. Reeder from BGS for their participation in the post-dismantling analyses.

Figure 2: Hydraulic conductivity


O/06A/2

EBS RESEARCHES IN BELGIUM:FROM LABORATORY TESTS TO ON-SURFACE

MOCK-UP TOWARDS UNDERGROUND IN-SITU REAL SCALE EXPERIMENT

Xiangling Li1, E. Romero2, C.Gatabin3, C. Schroeder4

1. EIG, EURIDICE, Boeretang 200, B-2400 Mol, Belgium2. UPC, Jordi Girona 1-3, Building D2, 08034, Barcelona, Spain3. CEA Saclay, Bâtiment 158, F-91191 Gif-sur-Yvette Cedex, France4. GéomaC, l’ULG, Chemin des Chevreuils 1, 4000 Liège - Sart Tilman, Belgium

Since 1974, Belgium investigates the design for disposal of its High Level Radioactive Waste (HLW) in adeep clay formation, the “Boom Clay”. Although the clay formation is the main (natural) barrier against thetransport of the radionuclides towards the biosphere, the design also involves several engineered barriers(EBS) to seal effectively the access shafts and galleries. In the design developed in the 1990’s, a non-saturated bentonite based material was chosen as part of thebarrier system. Prior to demonstrating this design in in-situ conditions, a surface mock-up test (OPHELIE)has been operated between 1997 and 2002. The OPHELIE backfill material is a compacted mixture of60 % FoCa smectite clay, 35 % sand, and 5 % graphite (ref. Hughes, et al. 2004, for the detail designof the OPHELIE mock up). During the mock-up test, some unexpected phenomenons were observed.For example, the swelling pressure built in the mock up is much smaller than expected, moreover, after twoand half years of test, it displayed a tendency to decrease by all total stress sensors (see Fig. 1). The lowerthan expected swelling pressure may result from different causes: the high temperature (up to 140°C)and the lack of saturation are possible explanations. The swelling pressure decreasing in the mock upafterwards can be attributed to different processes: the collapse (volume reduction upon wetting, whichcan be enhanced upon heating, this is indeed a crucial aspect to study in terms of the performance ofthe backfill/sealing materials); the thermal plasticity; the time related behaviour (stress relaxation, etc.).Consequently, in order to investigate in detail the Thermo-Hydro-Mechanical behaviour of this OPHLIEbackfill (constitutive law building), also to support the interpretation of the measurements and observationsduring OPHELIE mock up test, a systematic laboratory testing program under odometer and triaxialconditions has been performed under simultaneous control of suction and temperature. Basic thermo-hydraulic characteristics of the mixture (thermal conductivity, water retention properties, relative permeability, etc.) have been determined on both reference samples (prior to mock-up test) and exposedsamples (after dismantling of the mock-up). Thermo-mechanical tests were carried out at two contrastingtemperatures (room temperature and 80°C) to identify thermal and suction effects on compressibilityand shear strength properties. It is the first time that the OPHELIE mixture was characterised deeply inlaboratories. A preliminary numerical simulation of the mock up is ongoing based on the parameters drawnfrom the laboratory tests. Even the OPHELIE concept is no more considered by NIRAS/ONDRAF(Belgian agency for radioactive waste and enriched fissile materials), the experience gained, the lessonslearned from OPHELIE mock up, the deep knowledge got from the laboratory THM characterizationtests furnish an important asset for the development of the new design for the HLW disposal.

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Recently, the EIG EURIDICE (European Underground Research Infrastructure for Disposal of radioactivewaste In a Clay Environment) is planning to conduct an in-situ large scale PRACLAY heater test in theunderground research laboratory at MOL, in which a horizontal plug (namely PRACLAY plug test) willbe incorporated. The objective of the PRACLAY plug test is to demonstrate that it is possible to cut-offhydraulically the Excavation Damaged Zone (if presents) and the engineered barriers of the disposalgalleries with a horizontal plug. The plug also avoids the hydraulic shortcut to the access gallery (ref.Bernier et al. 2004, for detail design of the PRACLAY heater test). The material that will be used as plugmaterial is not yet selected. The benefits from OPHELIE mock up will be taken to the development ofthe design of the near future PRACLAY plug test.This paper tries to study/analyse in a comprehensive way the THM behaviour of the OPHELIE mockup. This study reposed on the detail analysis of the previously described laboratory tests performed onthe OPHELIE mixture and on the measurements during/after dismantling the OPHELIE mock – up test.Its swelling capacity at different temperature, its collapse potential at different states, its re-saturationprocess (liquid transfer or vapour transfer, the necessary time to be re-saturated) under different conditions,its permeability evolution at changing water content are the most important factors that are considered.As a result, the main observations during/after dismantling the OPHELIE mock up test will be interpretedin parallel in this paper. The preliminary simulation results (sensibility analysis) will provide the supportfor the understanding of the THM behaviour of the mock-up. Finally, the lessons learned from the OPHELIE mock up test for the future in-situ PRACLAY experienceand the development for the development of the design for the HLW disposal will be detailed.

References:Hughes Van Humbeeck, Jan Verstricht and Xiangling Li, 2004, “THE MOCK-UP OPHELIE: A LARGESCALE BACKFILL TEST FOR HLW DISPOSAL”, abstract submitted to 2nd international meeting on Claysin Natural & Engineering Barriers for Radioactive Waste Confinement, March 14-18, 2005, TOURS

F. Bernier, X.L. Li, J. Verstricht and W. Bastiaens, 2004 “THE DESIGN OF A LARGE SCALE HEATERTEST IN BOOM CLAY - PRACLAY EXPERIMENT” abstract submitted to 2nd international meeting onClays in Natural & Engineering Barriers for Radioactive Waste Confinement, March 14-18, 2005, TOURS

Figure 1: swelling pressure evolution during the OPHELIE mock up test


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ENGINEERED BARRIER EXPERIMENTMONT TERRI UNDERGROUND LABORATORY

J.C. Mayor1, E. Alonso2, J.L. García-Siñeriz3

1. ENRESA, Emilio Vargas 7, 28043 Madrid, Spain2. UPC-CIMNE, Jordi Girona 1-3, Building D2, 0803 Madrid, Spain3. AITEMIN, Alenza 1, 28003 Madrid, Spain

ABSTRACTThe Engineered Barrier (EB) experiment is being carried out at the Mont Terri underground laboratory(Switzerland). The aim of the EB experiment is the demonstration of a new concept for the buffer construction of HLW repositories in horizontal drifts, in competent clay formations. The principle ofthis new buffer construction method is based on the combined use of a lower bed made of compactedbentonite blocks, and an upper backfill made with a bentonite pellets based material.

The emplacement layout proposed in this project represents an important innovation for repositories inhorizontal drifts. The fact of filling the upper part of the gap between the canister and the rock with apellets-based type of material makes the emplacement operation much simpler, eliminating some of themost critical aspects of such operation.

The experiment is carried out in a gallery excavated in the shaly facies of the Opalinus clay of Mont Terri.The geometry of the test site is a horseshoe section, 2,55 m high, 3 m wide and 15 m long. A dummycanister of the same dimensions and weight than the reference one was installed on the top of a com-pacted bentonite blocks bed, and the gap canister-rock was backfilled with compacted bentonite pellets.The experimental area was isolated by a concrete plug. An artificial hydration system was installed toaccelerate the hydration process. In order to monitor the evolution of the system and record the valuesof different parameters, a data acquisition system was installed.

This project was co-financed by the European Commission (contract nº FIKW-CT-2000-00017) to becarried out in the framework of the research and training programme (EURATOM) in the field of nuclearenergy. ENRESA (Spain), BGR (Germany) and NAGRA (Switzerland) were the principal contractors,and AITEMIN (Spain) and CIMNE (Spain) the assistant contractors. ENRESA acted as coordinator andMr. J.C. Mayor was the Project Coordinator.

The hydration of the buffer started in May 2002, after the installation phase had finished. After morethan two years of continuous hydration, the main results are the following:• Swelling pressures showed a gradual increase from the starting of buffer hydration. For the pellets mixture,

which is now close to saturation, these pressures range from 1 to 1,5 Mpa which is consistent with thereported value of the in-situ dry density (1,36 g/cm3).

• Relative Humidity: sensors installed in rock boreholes reached saturation after one year of hydration;sensors installed in bentonite buffer also reached saturation in one year, although it is yet not possibleto assure the complete saturation of the buffer.

• Rock pore pressure: values were not homogeneous in the different boreholes, but the initial drying ofthe rock due to the excavation phase and due to the buffer emplacement was detected. A maximumpressure value of 12 bara was registered.

Performance calculations were carried out with Enresa/UPC finite element code CODE_BRIGHT.Parameters were derived from a comprehensive laboratory testing programme conducted at UPC (Spain).One significant feature of the analysis was the law adopted for permeability changes during saturation,

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because the fill expansion leads to a reduction of intrinsic permeability. The Opalinus rock was representedby an elastoplastic model capable of simulating the development of damage. Model calculations have beencompared with field measurements. A good estimation of the rock EDZ is derived from calculations.Buffer response was compared with measurements at the position of the monitoring points. A reasonablygood agreement was found for suction evolution and swelling pressure development. However, fieldmeasurements indicate a marked heterogeneous behaviour which cannot be reproduced by the model.The heterogeneous response of the buffer is explained by the irregular hydration of the buffer which is aconsequence of the emplacement conditions and the nature of the evolving permeability of the pellet´smixture.


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HYDRO-MECHANICAL, GEOCHEMICALAND MINERALOGICAL CHARACTERISTICSOF THE BENTONITE BUFFER IN A HEATEREXPERIMENT THE HE-B PROJECT AT THE

MONT TERRI ROCK LABORATORYMichael Plötze1, Günter Kahr1, Reiner Dohrmann2, Hanspeter Weber3

1. Swiss Federal Institute of Technology Zurich (ETHZ), Division for Geotechnical Engineering,ClayLab, Zurich, Switzerland

2. Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany3. Nagra, National Cooperative for the Disposal of Radioactive Waste, Wettingen, Switzerland

The long-term safety of underground repositories for radioactive waste relies on a combination of severalengineered and geological barriers. The performance of the engineered barriers is a result of their design,construction, and changes of the material properties in the response of conditions expected in a high levelwaste repository. In a heating experiment at the Mont Terri underground laboratory a heater was installedin a borehole in the Opalinus clay. The gap between the heater and the host rock was backfilled withcompacted bentonite blocks. A constant temperature of 100°C was maintained at the surface of the heaterin contact with the artificially saturated bentonite buffer for about 18 months. The basic objective of theexperiment was to gain knowledge about the coupled THM processes developing in the host rock andin the bentonite buffer to validate the existing tools for modeling THMC processes.

This test is an international program under the leadership of BGR (Germany) together with the ENRESA(Spain), GRS (Germany), and NAGRA (Switzerland) and with participation of AITEMIN and CIMNE(Spain), and ETHZ, COLENCO and RL (Switzerland). It is co-financed by the EC under the contractNo: FIKW-CT-2001-00132.

ETH studied the thermo-hydro-geochemical processes in the bentonite buffer. Extensive on-site andlaboratory tests were performed to determine the hydro-mechanical, geochemical and mineralogicalcharacteristics of the bentonite buffer to validate model predictions and to identify variations in thematerial properties in response of the heating.

The bentonite showed an in-situ volume increase due to swelling up to 10 %. The distribution of the watercontent and dry density in the buffer show clear relationship to the heater position. The water content ison average 30 % with higher values at the top, bottom and inner side of the buffer. The bulk density ison average 1.9 g/cm3 with lower values at top and bottom. The porosity was calculated with 41 %. Thereis a relative increase of the amount of small pores (< 100 nm) in samples near the heater. A more grainytexture of samples from the heater region was found in electron microscopic studies. The pore solutionis sodium rich because of the high sodium content of the so-called Pearson water that was used forhydration. The free water uptake of bentonite pieces is about 60 % within 24 h. A moderate swellingpressure of the bentonite up 2.8 MPa was determined. The heat conductivity for moist bentonite samples(water content about 20 %) is up to 1.3 W/m.K and the heat capacity is 1.3 J/g.K. The bentonite containsabout 80 % smectite and up to 10 % illite as well as minor amounts of quartz, feldspar and carbonate.The smectite is a Ca-Mg smectite with a CEC of 103 meq/100 g. The layer charge was determined byBGR being 0.35 C/FU, with slightly lower values near the heater. A relative increase of magnesium asexchangeable cation in the interlayer of these samples was found.

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The results indicate that the bentonite besides the macroscopic swelling and hydration shows only veryweak modifications in the geochemical and mineralogical properties during the heater experiment.There are cementing processes, which slightly affect some THM characteristics like porosity and thermalproperties. The release of magnesium from the octahedral position of the montmorillonite in responseof heating causes very weak changes in layer charge and interlayer chemical composition.

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Oral Session 6A : THM and chemical (THM-C) processes ... session 6A.pdf · During the mock-up test, some unexpected phenomenons were observed. For example, the swelling pressure built