Backfilling

In Belgium, it is planned to dispose of long-lived and high-level waste in an underground facility. After disposal of the waste, the galleries will be back¬filled to provide stability to the galleries and to limit the amount of voids in the repository. To achieve those goals, the backfill material has to have a good flowability, a negligible bleeding, and a limited shrinkage. A limited grain size is also required to allow the injection of the backfill material. Despite the fact that the backfill supports the gallery lining, its strength must be low enough to enable the retrieval of the waste packages. The backfill material has to be chemically compatible with the Boom Clay and the waste packages. This means that it should not unduly perturb the clay or disposal packages. The thermal conductivity of the backfill material in the galleries containing high-level and thus heat-generating waste must be high enough to allow sufficient dissipation of the decay heat into the surrounding clay.

Based on these objectives, material requirements were specified and the development of a backfill mixture was carried out. Initially, the mix composition was optimized in the laboratory. Thereafter, the backfill process of a gallery section was simulated. The investigations illustrate that this mixture can be transported via pipelines through the shaft and drifts and would fill completely the backfill sections in the galleries. Measurements of the porosity, the pore solution composition, the thermal material properties, and the strength illustrate the compliances with the requirements and the feasibility of backfilling the disposal galleries.
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The Belgian National Agency for Radioactive Waste and enriched Fissile Material ONDRAF/NIRAS is studying the disposal of low and medium activity level, long-lived waste (category B) and high activity, heat-generating waste (category C) in an underground facility. The repository is built at a reference depth of approximately 230 m in the Boom Clay host rock. Two shafts are built for personnel and material transfer and to provide ventilation during the construction and operation of the repository. A third shaft will be constructed for the waste transport. The shafts are connected via horizontal access galleries. The disposal galleries are constructed perpendicular to the access galleries. They are blind or dead-end galleries with a diameter of approx. 3.0 m and a length of 1,000 m. Fig. 1 shows an overview of the repository layout.
The galleries in the clay will be lined with concrete wedge blocks. In the order to transport and to support the waste packages after disposal, the galleries are outfitted with a concrete floor. It is planned to backfill the galleries section by section with a cement-based material, because grout injection is assumed to offer better opportunities for achieving the industrial performance that is required to backfill such volumes in a relatively short period of time. The current planning assumes a volume of the sections of approximately 85 m³, which will be backfilled in three and a half hours. Seals will be placed at the front-end of the disposal galleries.

The main functions of the backfill mortar are (1) isolating the waste by forming an extra barrier to the waste, (2) providing the galleries with stability and thus avoiding a gallery collapse and (3) reducing the voids in the repository which is a regulatory requirement.

As the backfill needs to realize a high filling degree, it has to show good flowability, negligible bleeding, and limited shrinkage. The grain size is limited to allow the injection of the backfill material. Another important requirement for the backfill follows from the potential requirement for waste retrievability. This means that the strength of the backfill has to be sufficient low so that the backfill can be removed at a later stage. In addition, a high porous backfill might be envisaged as it can provide a storage volume for gas generated in the repository and consequently limit the gas pressure build-up. This is in particular important for the category B waste for which the gas generation is expected to be more significant than for the category C waste.
The backfill material has to be chemically compatible with the Boom Clay host formation and any other component of the disposal system like the gallery lining and the waste disposal packages. This means that it should not unduly perturb the clay or disposal packages. Finally, the thermal conductivity of the backfill material in the category C waste disposal galleries must be high enough to allow sufficient dissipation of the heat from the category C waste into the surrounding clay. Furthermore, it has to be thermally stable under the maximum temperature that will occur in the backfill material.

In Belgium, it is planned to dispose of long-lived and high-level waste in an underground facility. The galleries will be backfilled after the disposal of the waste. The backfill mainly has a mechanical function: providing stability to the galleries and filling the void space in the galleries. Despite the fact that the backfill supports mechanically the gallery lining, its strength must be low enough to enable the retrieval of waste packages. The objectives require that the material will fill up almost completely the void volumes. It furthermore needs to provide a chemical and thermal environment consistent with the design concept. Thus, it may not disturb the alkaline corrosion-protective environment and cause no formation of mobile radionuclide complexes. It must be ensured that no excessive expansion or shrinkage will occur which could damage the gallery walls. In addition, the backfill may guarantee the dissipation of the decay heat.

The priority task in the mineral industry is to increase the efficiency of the exploited and infrastructure, equipment, including the creation of transport systems that have high performance while reducing the cost of transporting mineral raw materials and products of its processing. One-third of the world's mining companies use their mining systems at their enterprises to create a space. Backfilling activities are carried out in order to improve the safety of mining operations, namely, to control mining pressure, reduce possible mineral losses during excavation, prevent the occurrence of underground fires, as well as sudden outbursts of coal and gas, protect against destruction of objects in residential areas. Particular attention should be paid to the use of hydro-folding as a measure to prevent technogenic-tectonic earthquakes during long-term and continuous development of deposits. Taking into account the peculiarities of the materials used as a base when preparing filling mixtures, the specificity of the process of moving them to the bookmark site, necessary for obtaining a backfill array of required characteristics, is becoming increasingly important to use fundamentally new transport technologies for filling mixtures.

Backfilling and sealing are integral parts of the multi barrier concept of a geological disposal facility (GDF). General tasks of the backfill are to stabilize openings, to minimize the void volume that can be filled with water or brines, and to ensure a favorable chemical milieu with regard to the overall disposal system. Depending on these functions and the conditions provided by the GDF design and the safety concepts, backfilling materials have to comply with a wide scope of requirements.

ONDRAF/NIRAS the competent Belgian organization for radioactive waste management proposes to build a GDF in a poorly indurated clay host rock. Low- and intermediate-level radioactive waste (B-waste) will be conditioned in concrete monoliths B and high-level, heat-generating waste (C-waste) in so-called Supercontainers (SC). The SCs consist of overpacks embedded in concrete and a steel envelope. The two types of waste packages will be disposed of in separate fields of the future GDF.

In the framework of a technical support project with ONDRAF/NIRAS, DBE TECHNOLOGY GmbH has developed reference backfilling materials. It is planned to backfill the remaining voids inside the disposal galleries stepwise after the emplacement of a specified number of waste packages. As the space underground is restricted and to generally minimize operational activities underground, the preferred concept is to mix the backfill above ground and to pump the mixture via a piping system into the backfill segments. Consequently the backfill material has to remain in a flowable condition for the time needed for the transport process. After filling the segments, the backfilling material shall harden without swelling or significant shrinkage to homogenous bodies. Additionally, according to the current ONDRAF/NIRAS retrievability concept, the strength of the backfilling bodies has to be low enough to allow a later excavation of waste packages should that be required.

Initially, these general tasks suggest the development of a universally usable material for the backfilling of all galleries. Nonetheless, the different characteristics of the B- and C-waste packages and differences in the designs of the emplacement fields require the specification of individual catalogues with many common, but also several different material requirements for the development of backfilling material for B- and C-waste disposal galleries.

The requirement of chemical compatibility makes it necessary to use a cement-based backfill and all potential mixtures must meet the criteria for hydraulic backfilling. However, particularly high demands have to be specified for the time of workability and flowability of the backfill of the C-waste field due to its large extension resulting in pipeline lengths of up to 4000 m.

Other deviating requirements for material properties of the two backfill types originate from the long-term behavior of the two waste types. For instance, the pore volume of the B-waste backfill must be large enough to allow gas flow and thereby minimize a possible gas pressure built-up, due to the degradation of organics. In contrast, a lower porosity is favored to achieve a higher thermal conductivity for a better dissipation and removal of the C-waste decay heat.  Moreover, for the C-waste backfill a minimum pH value was prescribed with the objective to guarantee a long-term passivation and corrosion resistance of the SC steel surface.

After defining the requirement catalogues, the next step was dedicated to the selection of suitable raw materials based on the knowledge of technological properties of available high-quality materials. For instance, Portland limestone cement was selected as the binder of the B-waste backfill and a Portland cement for the C-waste backfill to achieve the required high pH value in this waste field. Another example is the use of sand aggregate, which is allowed in the B-waste field, while only limestone powder and aggregate were considered for the development of the C-waste backfill to safely prevent alkali-silica reactions at elevated temperatures.

Usually, the time span of workability of low-porosity Portland cement mixtures is limited, whereas the backfilling of the voids in the C-waste field require an exceptional long potlife. This example demonstrates that different requirements often have contrary consequences for the material selection and vice versa. Consequently, one focus of the backfill development was to identify compositional ranges that guarantee the respective material property. Finally, a combination of the individual „conformity fields“ results in the optimal solution for the standard operating conditions as well as a compositional range that guarantees compliance with the required material specifications. These interrelations and the general strategy used to reach the optimal solution will be demonstrated for the two development lines of the B- and C-waste backfill. The principle strategy can be adapted to many underground repositories and conventional mines.

The priority task in the mineral industry is to increase the efficiency of the exploited and infrastructure, equipment, including the creation of transport systems that have high performance while reducing the cost of transporting mineral raw materials and products of its processing. One-third of the world’s mining companies use their mining systems at their enterprises to create a space. Backfilling activities are carried out in order to improve the safety of mining operations, namely, to control mining pressure, reduce possible mineral losses during excavation, prevent the occurrence of underground fires, as well as sudden outbursts of coal and gas, protect against destruction of objects in residential areas. Particular attention should be paid to the use of hydro-folding as a measure to prevent technogenic-tectonic earthquakes during long-term and continuous development of deposits. Taking into account the peculiarities of the materials used as a base when preparing filling mixtures, the specificity of the process of moving them to the bookmark site, necessary for obtaining a backfill array of required characteristics, is becoming increasingly important to use fundamentally new transport technologies for filling mixtures.

The Belgian radioactive waste management agency ONDRAF-NIRAS plans to dispose of long-lived and high-level radioactive waste in galleries of an underground repository constructed in clay. For stabilization purposes the disposal concept provides a concrete backfilling of the disposal galleries. Plans call for mixing the concrete in above ground facilities and pumping it through the shaft and drifts into the galleries. Upon hardening the backfill will guarantee a corrosion-retarding environment and reduce the mobilization of radionuclide complexes. To avoid an impairment of the gallery walls swelling of concrete must be limited. In addition, the backfill should contribute to the dissipation of the decay heat of the radioactive waste and – according to appropriate legal requirements – its strength must be low enough to enable the retrieval of the waste packages. DBE TECHNOLOGY GmbH was contracted to develop a suitable backfill material and to demonstrate the feasibility of the backfill process. Laboratory tests were carried out including investigations on the pumpability and the flow or spreading behavior of the mixture. Examinations of pore solutions were performed to evaluate the chemical environment with regard to corrosion limitation. Thermal parameters of the backfill material were determined, which will be used as input parameters for future numerical thermal calculations. Mock-up tests were performed to investigate the flow behavior of the grout in 2 m long Plexiglas tubes, which were designed to simulate a section of the disposal galleries. The pump pressure and the pressure in the pipeline were registered in order to develop the design of the backfill system.

To optimize or restore the barrier function of different host rocks or backfill materials, BGE TECHNOLOGY GmbH developed injection materials and grouting techniques to seal any cavities in rocks or backfill materials. In order to stabilize the rock and backfill bodies and to seal cracks, fractures, or pores, suspensions and particle-free injection materials are applied. In saline environments, the particle-containing grouts are based on magnesium oxide. In non-saline areas, mixtures with classic cements as well as reactive and nonreactive fillers are used. Beyond a certain size, particles cannot penetrate into small fractures or pores, which are potential pathways for fluids. As a result, the range of use of these grouts is limited, and water glass is used to seal finer flow paths. However, experience has shown that the penetration of particles can be improved if water glass is injected first and the grouts are injected immediately afterwards. This effect can be explained by a lubrication of solid surfaces and the effect that water glass dissolves salts before gelation and hardening can start. This finding suggests combining water glass, as carrier fluid, with reactive additives. The development work led to novel grout families that also have many other benefits. At elevated injection pressures, the water glass and non-inert solutions are squeezed out of the mixtures, which themselves act as an independent injection agent. This way, it is possible to fill or seal fractures and pore systems of very different sizes in one injection step for the very first time.