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MAQUETTE, a critical project for ANDRA to determinte the thershold of thermal fracking of rock for radioactive waste

Deep geological storage

ANDRA is the French Agency responsible for identifying, implementing and granting the safe management and disposal of all the French radioactive waste, in order to protect the present and future generations from the risks inherent in such substances. ANDRA oversees the Cigeo project (Industrial Centre for Geological Disposal) consisting of a deep geological facility aimed for disposing of the highly radioactive long-lived waste. Within this frame, the Underground Laboratory of Bure has been built as an exceptional tool to perform the research activities required to implement Cigeo.

Maquette is a critical experiment performed in Bure Laboratory aiming to determine the threshold of thermal fracking of the host rock where the waste will be disposed. It will get important scientific data that will increase the knowledge of Thermo-Hydro-Mechanics behavior and the modelling capacity of these phenomena.

The experiment is composed by 10 heated boreholes 20 m long and 172 mm of diameter: two boreholes in the center and eight in the periphery. A 10 m long heater has been installed at the bottom of each borehole. The instrumentation is composed by 450 high accuracy thermo-resistances installed in contact with the borehole inner casing surface and 40 thermocouples to monitor inner temperatures at the heaters. The system is completed by six monitoring boreholes, three of them for pressure observations and the other 3 for crack detection using geophones.

COPYRIGHT: ANDRA (Agence Nationale pour la gestion des déchet radioactifs)

CHALLENGE

Several elements were critical to ensure the success of the project:

  1. The boreholes must be as watertight as possible in order not to dissipate pressure increases induced by the heat (up to 180 bar)
  2. Accurate temperature control at the casing of the boreholes (< 0.1oC) is needed
  3. The temperature sensors must be in contact with the inner surface of the borehole casing
  4. The system needs to be reliable in order not to fail during the heating phase, which would compromise the odds to frack the rock
  5. The entire system should be retrievable because the experiment will be repeated in other location of the laboratory.

SOLUTION

ARQUIMEA was contracted to perform every related aspect to the heated boreholes and heating operation of the experiment, that is to say: design, validation, fabrication, installation, operation and maintenance of: 10 heaters, casing and cementation works, control system and temperature instrumentation. Among the aspects to highlight are the following:

  • Design of casing threads to assure water-tightness at the casing couplings. Standard pipes have been used modifying their threads and using Viton O-rings. Boreholes have 172 mm diameter, while the used casing is 139.8 mm of outer diameter and 120.1 mm of inner diameter.
  • Methodology to cement the annular between casing and rock with two types of resin. The 16 meters at the bottom end (heated zone) were filled with a high temperature resin, while for the first 4 meters a second epoxy resin was used. To allow the pouring of the resins, 4 packers filled with an expansive resin were installed in a reaming of 220 mm diameter performed in the first 4 meters of each borehole.
  • Design and construction of modular 10 m long heaters composed by 4 modules 2.5 m long, watertight and connectable between them. Each heater has a redundant mineral insulated resistor coiled in an aluminum tube and protected by an outer stainless-steel tube of 101.6 mm of outer diameter. The maximum power of each heater is 3300 W.
  • ARQUIMEA has taken advantage of its expertise in Shape Memory Alloys (SMA) to design and construct SMA resorts that help enter the heaters with the temperature sensors folded and deploy them against the casing inner surface once the heating is launched.
  • Design and construction of the control system based on angle-phase thyristors commanded by a PLC. Two control modes were implemented: by temperature and by power. The control system is completely redundant (as the heaters) to minimize any possible failure during the experiment.

 

RESULTS

Every subcomponent of the system has been tested and validated by ANDRA before final installation. During installation only one sensor out of 450 was damaged (far above the usual installations of this type).

Two tests have been performed so far. During the first one, performed between June and August 2019, a temperature control strategy was used. The system successfully responded within the specifications of the experiments although no cracking has been observed. A second test, power controlled, was launched on January 2020. This time, observations pointed out that the rock cracking was reached. These observations must be confirmed after the cooling of the rock foreseen for June 2020.

The whole system will be removed and installed again in other location of the laboratory to confirm results.