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MEGAS
 
EXPERIMENT'S NAME
MEGAS
LAST MODIFIED
2004-04-15
EXPERIMENT'S ACRONYM
MEGAS 		TYPE OF TEST
Clay characterization
RN’s migration
Overpack corrosion

PERSON RESPONSIBLE
Name : L. Ortiz
Email : lortiz@sckcen.be

COLLABORATIONS
BGS, INTERA (QuantiSci), ISMES

FINANCING ORGANISM(S)
ONDRAF/NIRAS, EC

GLOBAL BUDGET (k€)
NC
Phenomena
Components
Safety (sub) functions concerned (if relevant)

i
Gas generationand transport
Host rock
SF
D
L
SSF
C1
R1
ii
Migration of radionuclides
Host rock
SF
C
D
L
SSF
C1
C2
R1
iii
Hydro-mechanical
Host rock
SF
R
D
L
SSF
C1
R1
R2
iv
Chemical
Host rock
SF
C
D
L
SSF
C1
C2
R1
 
 
SF
SSF
R1
vi
Corrosion
Overpack
SF
R
D
L
SSF
C2
R1
R2
 
Background
The generation of gas in a radioactive waste repository raises a number of questions concerning repository safety and integrity. A review of these questions of importance to safety assessments includes the build-up of gas pressure in a saturated repository, leading to fracturing of the repository materials and the expulsion of pore-water containing released radionuclides. The impact of gas on repository safety is determined firstly by how much gas is generated, and secondly by how the gas interacts with pore-water as it migrates from the repository. The focus of the MEGAS project has been gaining a greater understanding of the gas migration issues associated with repository safety.
 
Objective of the experiment
The primary objective of the MEGAS project was to understand the consequences of gas generation on a radioactive waste repository located in a deep clay layer. The final objective of the project was to validate a gas migration model using an in situ gas migration experiment.
 
Description of the experiment

Design:

MEGAS E4 gas injection experiment
MEGAS E5 gas injection experiment
 
Protocol/explanation :
In situ gas injection experiments have been carried out at two different locations in the HADES URF on behalf of the MEGAS project. The first experiment, referred as MEGAS E4 gas injection experiment, was carried out from a vertical multipiezometer installed under the bottom of the old access shaft. The second experiment, referred as MEGAS E5 gas injection experiment, took place in the Test Drift using a device composed of four multipiezometers. In both tests and for safety reasons we used Helium as injected gas, since its properties are the closest to Hydrogen.
 
MEGAS E4 gas injection experiment
Two gas injection experiments were carried out from the vertical multipiezometer: from screens no. 6 and 9. Therefore, the water contained in these screens was first emptied using a slight gas overpressure. Local pore water pressures in screens 6 and 9 were respectively equal to 1.34 and 1.75 MPa abs before the gas injection experiments. In screen no. 6, a pulse test using a gas overpressure of 1 MPa has first been applied for a period of 27 hours. The following day, the gas overpressure was increased up to 1.25 MPa for 26 hours. After five days, a second pulse test with a 1.25 MPa gas overpresure was carried out for two hours. The gas inflow and the pressure evolution in the different screens were monitored.
 
These gas injection tests were followed by a hydraulic test carried out in screen no. 11.
 
MEGAS E5 gas injection experiment

A hydraulic campaign has first been carried out nine months after the installation of the experimental device. Therefore, screen no. 20 was put for 44 days in contact with the atmosphere. The water outflow and the pressure drop in the surrounding screens were monitored. The hydraulic parameters were calculated with a validated computer code using an analytical solution. Let us note that the system had not reached a steady-state value yet.

A gas injection experiment was carried out from this same screen months after the hydraulic campaign took place. The local pore water pressure was equal to 1.67 MPa g just before the gas injection. Increments of 0.1 MPa were weekly applied to the gas pressure, with an original overpressure of 0.1 MPa, until a gas breakthrough was established.
Two hydraulic tests were subsequently carried out, in screens no. 20 and 17 on the central multipiezometer.

It should be noted that the MEGAS E5 experimental device has been further used for another gas injection test, followed by a radioactive tracer test in the frame of the PROGRESS project.
 
Instrumentation :
 
MEGAS E4 gas injection experiment
  • Bourdon manometers for the pore water pressure measurements in the peripheral screens·
  • A pressure transducer connected to a X-t recorder was used to monitor the gas injection pressure
  • A magnetic displacement transducer was used to monitor the gas flow-rate
  • Two different water injection set-ups were used for the hydraulic test:
    • a syringe pump directly connected to screen no. 11, allowing an automatic setting of the pressure, and indicating the volume variation in function of time as an output
    • see figure below. The water flow-rate was estimated by gravimetry.
MEGAS E5 gas injection experiment
  • Pressure transducers for the pore water pressure measurements in the screens
  • A paper recorder was used for the data acquisition
  • Two switchable magnetic displacement transducers were used to monitor the gas or the water flow-rate
  • The constant alimentation gas pressure was ensured by an electro-valve (see figure below)
 
 
Status/timing/planning :
The project is completed. The timing took place as indicated below:
 
MEGAS E4 gas injection experiment
  • 1986-12-18: installation of the vertical multipiezometer
  • 1992-12-09 till 1993-01-12: water expulsion from screen no. 6
  • 1993-01-20 till 1993-01-27: gas injection test through screen no. 6
  • 1993-02-08 till 1993-03-08: water expulsion from screen no. 9
  • 1993-03-08 till 1993-05-26: gas injection test through screen no. 9
  • 1994-06-01 till 1996-03-04: hydraulic test in screen no. 11
 
MEGAS E5 gas injection experiment
  • 1992-10-22: installation of the central multipiezometer
  • 1992-11-27 till 1992-12-01: installation of the peripheral multipiezometers
  • 1993-09-20 till 1993-11-03: hydraulic campaign
  • 1994-04-11 till 1994-07-06: gas injection test through screen no. 20
  • 1994-11-15 till 1995-04-04: hydraulic test in screen no. 20 part 1
  • 1995-05-08 till 1996-03-27: hydraulic test in screen no. 17
  • 1996-05-08 till 1996-06-19: hydraulic test in screen no. 20 part 2
  • 1996-11-19 till 1996-12-06: hydraulic test in screen no. 13 (PROGRESS project)
  • 1997-02-27 till 1998-02-28: gas injection test through screen no. 13 (PROGRESS project)
  • 1998-04-08: start of the tracer (HTO) migration test (PROGRESS project)
 
Associated works:
The MEGAS project included both experimental and modelling work packages concerning gas migration in a clay host rock. This project was followed by the PROGRESS project, which enlarged the scope to fractured low-permeability host rock and rock salt., and included scientific work dedicated to the prediction of gas generation rates. A gas status report was written on behalf of a joint collaboration between the European Commission and the NEA. Gas migrationn tests have also been carried out in the frame of the RESEAL experiment in different backfill materials.
 
Results of the experiment:
 
MEGAS E4 gas injection experiment
The gas breakthrough was quite lower than expected (excess gas pressure of 0.6 MPa instead of 1.25 MPa). The creation of one (or more) preferential pathway in the upwards direction has been established. This can be explained by the local geomechanical stress distribution around the multipiezometer. The hydraulic test showed a dependence between the water injection pressure and the measured hydraulic conductivity. This is a further evidence of the strong coupling between hydraulic and geomechanical parameters in Boom Clay.
 
MEGAS E5 gas injection experiment
A gas breakthrough to screen 19 occurred after 44 days at a gas pressure of 2.36 MPa, i.e. at an excess gas pressure of 0.69 MPa. A preferential pathway between these filters was established. During the first days after this gas breakthrough, the pore pressure in screens 11 to 14, located above the injection multipiezometer, increased by 0.03 to 0.04 MPa. A few days later, the pore pressure in screens 10 and 15 increased by 0.02 MPa. A similar observation could be made in the screens located on the right multipiezometer. On the left multipiezometer, only screens 2 and 8 indicated a pore pressure increase. The spatial distribution of these pressure changes showed neither a spherical nor a cylindrical symmetry around the injection filter. Since such a symmetry would have been expected in thecase of a purely hydraulic phenomenon, this is a supplementary indication that the creation of (a) preferential pathway(s) is coupled with a mechanical effect. The figure below shows the pore pressure evolution in the screens adjacent to the gas injection screen no. 20.
 
The results of the hydraulic tests carried out in screen 20 show a progressive re-saturation in the vicinity of the former gas injection screen.(see figure). Let us note that the last measurement point was taken more than a year after the preceding point.
 
The results of the hydraulic test carried out in the screen no. 17 (showed on the figure below) indicates a constant hydraulic conductivity for water injection pressures comprised between the original pore water pressure, and an excess water pressure (i.e. the difference between the water injection pressure and the original pore water pressure) of 1.47 MPa. For excess water pressures ranging from 1.47 to 1.88 MPa, the measured hydraulic conductivity progressively increases, showing a mechanical effect. Eventually, a hydrofracturing phenomenon has been established at an excess water pressure of 1.88 MPa (the pore pressures in screens no.17 and 18 became equal, and a very important pore pressure increase was observed in screen no. 10, located above screen no. 17 at a distance of 98 cm and slightly in the direction of the experimental gallery). This pressure of 1.88 MPa might correspond to the local minor effective stress component.
 
 
Conclusions:
In a radioactive waste repository located in a plastic clay layer, there might be, according to the chosen concept and to the waste type, a gas pressure build-up generated by gas produced mainly from the anaerobic corrosion of the metallic waste packages. It might eventually lead to a breakthrough following the path of least resistance in the clay. Gas would flow through preferential pathways which directions will be orthogonal to the lowest principal effective stress tensor component. These paths are highly unstable. A modelling exercise considering potential effects of such gas induced pathways on the long-term safety of the repository according to existing HLW and MLW categories and concepts has been reported by Volckaert and Mallants (2001).
 
Bibliography:
  • Horseman, S. T., J. F. Harrington, "Evidence for thresholds, pathways and intermittent flow in argillaceous rocks", in Proc. of the NEA/EC Workshop: Fluid Flow through Faults and Fractures in Argillaceous Formations, Berne, Switzerland, 10-12 June 1996, pp.85-103 (OECD ed.), 1998.
  • Ortiz, L., S. T. Horseman, "Gas Migration through Bentonitic Engineered Barrier Systems and through Non-indurated Clays", in Proc. of the NEA/EC Workshop: Gas Generation, Accumulation and Migration in Underground Repository Systems for Radioactive Waste: Safety-Relevant Issues, Reims, France, 26-28 June 2000, pp75-79 (OECD ed.), 2001.
  • Ortiz, L., M. Impey, S. Einchcomb, "Characterization of Gas Flow in Boom Clay, a Low Permeability Plastic Rock", in Proc. of the NEA/EC Workshop: Fluid Flow through Faults and Fractures in Argillaceous Formations, Berne, Switzerland, 10-12 June 1996, pp.243-256 (OECD ed.), 1998.
  • Ortiz, L., G. Volckaert, P. De Cannière, M. Put, M. A. Sen, S. T. Horseman, J. F. Harrington, M. D. Impey, S. Einchcomb, "MEGAS: Modelling and Experiments on Gas Migration in Repository Host Rocks", Final Report-Phase 2, European Commission Report EUR 17453 EN, 1997.
  • Ortiz, L., G. Volckaert, D. Mallants, "Gas generation and migration in Boom Clay, a potential host rock formation for nuclear waste storage" Eng. Geol. 64 (2-3), pp.287-296, 2002.
  • Ortiz, L., G. Volckaert, M. Put, "The MEGAS E5 Experiment: A Large 3-D In Situ Gas Injection Experiment for Model Validation", GEOVAL '94 Validation Through Model Testing, Paris, France, 11-14 October 1994, pp.151-162 (OECD ed.), 1995.
  • Rodwell, W. R., A. W. Harris, S. T. Horseman, P. Lalieux, W. Müller, L. Ortiz Amaya and K. Pruess, "Gas Migration and Two-Phase Flow through Engineered and Geological Barriers for a Deep Repository for Radioactive Waste", Joint EC/NEA Status Report EUR 19122 EN, 1999.
  • Rodwell, W. R. (ed.), "Research into Gas Generation and Migration in Radioactive Waste Repository Systems (PROGRESS Project)", Final Report, European Commission Report EUR 19133 EN, 2000.
  • Volckaert, G., B. Dereeper, M. Put, L. Ortiz, A. Gens, J. Vaunat, M. V. Villar, P. L. Martin, C. Imbert, T. Lassabatère, E. Mouche, F. Cany, "A large-scale in situ demonstration test for repository sealing in an argillaceous host rock (Reseal project-Phase 1)", Final Report, European Commission Report EUR 19612 EN, 2000.
  • Volckaert, G., D. Mallants, "The Treatment of Gas in the Performance Assessment for the Disposal of HLW and MLW in Boom Clay", in Proc. of the NEA/EC Workshop: Gas Generation, Accumulation and Migration in Underground Repository Systems for Radioactive Waste: Safety-Relevant Issues, Reims, France, 26-28 June 2000, pp125-128 (OECD ed.), 2001.
  • Volckaert, G., L. Ortiz,, P. De Cannière, M. Put, S. T. Horseman, J. F. Harrington, V. Fioravante, M. D. Impey, "MEGAS: Modelling and Experiments on Gas Migration in Repository Host Rocks", Final Report-Phase 1, European Commission Report EUR 16235 EN, 1995.