Thomas Lautsch, Beate Kallenbach-Herbert and Sebastian Voigt

Geology

The Asse salt structure is located in the Southwest of the Hercynian Mountains in Northern Germany. In geological times, compressive tectonic movements ruptured the overlying rock strata covering the salt layers of the Zechstein series deposited 250 million years ago. Consequently, the rheological salt ascended between the layers of the lower and upper Buntsandstein forming the steep Asse salt structure with 40 to 70 degree dip in the Northern and Southern flanks. The structure strikes over a length of about 4 kilometers West/ East and is only a few hundred meters wide across strike at the top of the structure.

The formation of the Asse structure was quite intense compared to neighboring structures like the much more gentle Elm salt structure. The ascent induced intense tectonic faults to the overburden and reduced the coherence of the rock masses by breaking the overburden into blocs (see Figure 1). In addition, the internal structure of the Asse salt dome indicates a turbulent and rapid genesis of the formation. Individual layers rose at different rates and the sequence of salt layers within the salt structure as well as the topography is now irregular. Characteristic is the vertical position of anhydrite in the center of the structure. Anhydrite is one of the typical geological markers of the Zechstein salt sequence. At the surface, you see two separate ridges with a gentle trough in-between. This unconformity is another indication of rupture and discontinuity caused by the rise of the structure.

Fig. 1: Rise of the Asse salt structure. From left to right: first rupture of the flat deposit by compressive tectonic stress, second lift of overburden strata and rise of salt, third actual geological setting with blocked overburden and complex and steep salt structure.

The Asse salt dome rose near to the surface. Therefore, the top of the structure is close to near- surface groundwater. The overburden at the Southern flank of the structure has layers of porous sandstone and fragmented limestone (see Figure 2). Consequently the salt structure is always close to potential aquifers (water bearing strata), even at depth. In case the integrity of the salt structure is compromised, near surface groundwater may enter mine-openings travelling along aquifers and faults within the overburden and internal cracks within the salt structure. On its path, the groundwater dissolves salt minerals and turns into brine.

Fig. 2: Geological Cross-section showing blocked layers of sandstone and limestone in the overburden of the Zechstein salt sequence (crosscut from Southwest to Northwest, from left to right), Source: BfS.

Mining

The specific mining vocabulary varies between mining methods and mining districts. Most mining methods extract valuable minerals by creating cavities or voids, named stopes or rooms. They leave pillars to protect the structural integrity of the underground workings. Sometimes horizontal pillars are named roof. In underground repositories for radioactive waste, often the term emplacement chamber describes the rooms designated for accepting the waste.

This paper uses the terms stope and pillar when describing the conventional part of the mine and the term emplacement chamber for the stopes, where the waste is stored.

Mining salt from the Asse structure started in 1906. In total, around five million m³ of rock salt and potash has been mined until the 1960s with the largest mining district close to the Southern flank of the structure (see Figure 3).This district contains 131 stopes, each up to 60 meters on strike, 40 meters across strike and 15 meters high. The stopes are configured in 12 rows and 9 levels. The bearing structure of the mining district close to the Southern flank consists of 12-meter wide vertical pillars between the ribs of the stopes and 6-meter thick horizontal pillars between floor and roof of the stopes. In total, the district stretches over 260 meters in height, from 490 to 750 meter below surface, and 650 meters along strike. A 20-meter wide barrier pillar in the center splits the mining district into an Eastern and a Western part, between the fourth and the fifth row, counted from the West. The thickness of the salt barrier between stopes and overburden rises from only 7 meter at the top to > 20 meter at the bottom of the mining district.

Fig. 3: Cross-section from South (left side) to North (right side), showing shafts, entries and stopes of the Asse mine within the salt structure.

Backfilling the mine started in the 1980s. Up to today a total of 4.4 million m³ of stopes and entries have been backfilled. Stopes have been backfilled pneumatically using 1.8 million m³ of rock salt material from mining lower levels and 2.2 million m³ from waste piles of neighboring mines. Later on 0.4 million m³ of remaining voids and entries have been concreted using Sorel concrete made of acid-base cement and rocksalt, a recipe geochemically stable even in potash dominated brines. Currently 1.2 million m³ are still active for compression, mostly as porous volume of the pneumatically backfilled rocksalt. It is planned to backfill another 0.2 million m³ of entries and voids until 2028.

Mining induced Ground Movement

Basic Principles

An empty stope cannot carry overburden. Therefore, neighboring structures have to carry additional load, as the total load caused by the weight of the overburden stays the same. The mining activity redistributes in-situ stresses and causes additional stress to the bearing structure of the mine, namely the vertical and horizontal pillars. The rock masses later have a tendency to homogenize the stress level again by moving into the stopes.

Movement of the rock masses is welcome, as it reduces stress peaks in the close vicinity of the stope and subsequently the risk of destroying the structural integrity of the surrounding rock masses. If well designed, the remaining high stress zone does not exceed the stability of the pillars. The contour of the stope remains integer, stresses are redistributed away from the stope and a new arch is formed further away. Mechanical roof support structures out of concrete and steel predominantly stabilize and reinforce the contour if needed.

While this interaction of excavation, stress redistribution and deformation works in every material, there are significant differences between rock types. Hard rock has little deformation; its strength allows withstanding stress peaks without breaking. The hard rock accumulates stress; its brittle behavior may lead to sudden failure by breaking, if stopes become too large. Salt like the Asse material has far less strength compared to hard rock. Instead, it has rheological behavior and creeps without breaking, when homogenizing the stresses by moving into the cavity. You may mine large stopes with little ground support, as the stress/strength management of salt is quite effective. If you compare hard rock and salt, much larger rock mass movement accompanies the formation of a new stable stress environment in salt. In a way, salt follows the principle “the smarter one gives way”.

For stability of any underground excavation, it is necessary that the rock mass movement is declining and a new stable balance of stress and strength develops before the cavity collapses.

Every stability is limited in time; at the end every void in the deep underground closes. This behavior of the rock masses is known as “horror vacui” of the underground environment. The mine design stabilizes the underground workings for the time of utilization, typically some decades. In the long run the inevitable closure of the voids is beside the gas formation of corrosive processes one of the key drivers of the mobilization of nuclides within fluids in the deep underground, subject to the safety case of any repository of nuclear waste.

Movement of the Southern Flank in Space and Time

The bearing structure of the Asse mine is closely monitored regarding the deformation of pillars and seismicity. The intention is to control the rock mass movement by backfilling in such a way, that the deformation is as uniform as possible to prevent new fluid path to develop. A key indicator is the horizontal compression of the pillars. Because the Southern flank steeply dips, and rock masses typically move perpendicular to the dip, the rock masses move largely horizontally into the mine. Analysis of the data shows, that 90% of the pillar compression has its origin in the movement of the Southern flank into the mine .Only a minor part of the compression comes from movement of the central and Northern parts of the mine into the stopes. Therefore, compression rates are a good indicator for the rock mass movement of the Southern flank.

Figure 4 shows the trend of pillar compression rates over time since the start of the mining activities. The blue lines represent different measuring points at different levels in the Southern mining district. During the decades of active mining the Southern flank moved at a constant speed of well below 10 cm/a into the stopes of the mine. After mining ceased, movement accelerated in the 1980s, 1990s and 2000s up to a top speed of 20 to 25 cm/a. Since the turn of the century, the pace of the rock mass movement slows down. Currently the rate is slightly below 10 cm/a.

Fig. 4: Pillar compression rates over time, mining period until the 1960s, waste emplacement in the 1970s, destabilization and backfilling of the Southern flank in the 1980s and 1990s, consolidation in the 21^st century.

There is a central barrier pillar between row #4 and row #5. It splits the subsidence trough of the Southern flank into a wider Eastern part with an accumulated maximum of 7-meter movement of the flank into the mine, measured horizontally. The more narrow Western part peaks at 5.5 meter, the central pillar is deformed by 3 meter. The inhomogeneous movement of the Southern flank into the mine causes steep gradients in the center of the structure, consequently strains and stresses cause cracks and ruptures (see Figure 5)

Fig. 5: Accumulated pillar compression in the Southern flank, measured at different points with extensometer and/or inclinometer [1].

The more horizontal overburden subsidence of the Southern flank into the stopes causes a vertical subsidence of the surface topography (see figure 6). The surface subsides by maximal 0.4 meter. The relatively steep flanks of the surface subsidence trough indicate the reduced cohesiveness of the overburden caused by the tectonic block formation [2] (see Figure 7).

Fig. 6: Rock mass movement of the Southern flank causes surface subsidence, indicated with blue signature [1].

Fig. 7: Surface subsidence trough, with cross-sections top East- West, bottom North – South [2].

Explanation

The stress redistribution during the mining period of the Southern district caused the bearing structure of the mine to creep. The structural integrity stayed intact. Deformation was proportional to excavation. The bearing structure was fully functional and protected the mine by accepting the additional stresses and redistributing them by movement.

After mining suspended, the rock mass movement continued in the 1970s. The bearing structure increasingly started to fail by breaking instead creeping and subsequently lost load capacity. After accumulating around 2 meters of compression, specifically the horizontal pillars between floor and roof of the stopes cracked and the movement of the Southern flank into the stopes accelerated. Obviously, the design of the bearing structure was not sufficient for the extended life time of many decades for the Asse mine. The extraction ratio of up to 60 % in the Southern mining district was too high for long term stability.

The rock masses of the overburden are tectonically fragmented at the Southern flank of the Asse structure. The coherence of the formally joint layers of rock is reduced. When the rock masses get in motion, blocks of rock can move along slicks and slides. Continued movement reduces the shear strength of the overburden further. The load on the bearing structure of the mine continues to rise. Movement accelerates the resulting compression of the pillars too. The rock masses of the overburden deform and finally crack not only more and more pillars, but also the salt barrier between the stopes and the overburden. After 5 meter of pillar compression the moving rock masses increasingly hit the backfill material, which was brought into the mine in the 1990s. The backfill material built up counter-pressure against the incoming Southern flank.

Rock mass movement decelerated from 2000 onwards until today and total pillar compression reached up to 7 meters in some places. It will take possibly another 2 meters movement over the next decades, until compaction of the backfill material is completed and rock mass movement finally ends.

Brine Influx

When deformation of the Sothern flank accelerated, the salt barrier between mine and overburden cracked. The stopes and the fluid paths of the overburden connected hydraulically, starting in the top of the Eastern trough, close to the barrier pillar in the center. There the salt barrier is thin and the deformation gradient is large.

Brine influx was first noticed in the 1980s and rose to 12.5 m³ per day up to now. The main water intake moved over the years to the Western side of the central barrier und downwards by 120 meter. The exact fluid path is unknown (see Figure 8). Changes in flow rate and intake location during the last 30 years happened sudden and stepwise from one stable level to the next. It seems that shifting blocks in the overburden opened new fluid paths along faults, slicks and slides and closed existing ones. This caused sudden changes of the brine influx, both in rate and location.

Fig. 8: Brine influx shown schematically with brine entering the salt structure at roughly -500 meter, then continuing its flow downwards to the water intake. Small picture right top shows schematically the damage to the horizontal pillars, small picture right bottom shows the main intake at -658 meter.

The specific weight of the brine diminished slightly over the last 30 years. This indicates a washing out of the fluid path, as the solution of salt by the groundwater when flowing into the mine obviously decreased.

The Southern flank will continue to move into the mine. Because of the variance in pillar dimension, the movement will not be uniform. The overburden, the protective salt barrier within the salt structure and the pillars are partially cracked. Therefore, it is possible, that in the next decades new fluid paths develop and already existing paths continue to wash out, the flow rate increases or the main intake location moves further down within the mine and approaches the radioactive waste at the -750 meter level. These scenarios may not develop at all, or trends will be linear. But it is also possible, that these developments unfold dynamically.

Retrieval of the radioactive Waste

Baseline

In recent years, many repository concepts for high active waste include the option for waste retrieval, both during operations and after closure. This is an engineered approach, with specific design considerations for waste canisters, mine layout and backfill technology. The vocabulary in use is retrieval and retrievability for the readiness to do so.

In contrast to this, the Asse project recovers waste canisters of unknown condition out of unstable geotechnical environment. This paper therefore uses the term recovery for the process within the emplacement chamber and retrieval for the entire process including the engineered process of transferring the overpacks from the emplacement chamber to the waste treatment and interim storage facility.

Between 1967 and 1978, about 47,000 m³ of low- and intermediate-level radioactive waste was emplaced in the Asse II mine on behalf of the German government. For this purpose, existing stopes on the 511-meter level (medium-level waste) and on the 725- and 750-meter levels were used as emplacement chambers. Due to increasing public criticism of the pIans for the closure of the mine under the mining law, the facility was later placed under nuclear law. Since the long-term safety required by nuclear law for the stored waste cannot be proven with the existing knowledge and uncertainties about the hydrogeological situation of the Asse, it was decided in 2009 to recover the waste. The legal basis was created in § 57 b of the Atomic Energy Act.

Recovery and retrieval is a complex undertaking that will require several decades to prepare and execute. It includes all underground and surface process steps that involve the handling of radioactive materials, starting with the activities for recovering the radioactive waste and ending with interim storage (see Figure 9). The basic procedure is described in the recovery and retrieval plan [1].

The particular challenge of waste-recovery is to reconcile basically contradictory requirements in one concept: On the one hand, the mine must be stabilized by backfilling and closure measures in such a way that occupational safety for underground workers is ensured, that beyond-design-basis brine influx (AüL) is avoided as far as possible, and that its potential effects are reduced as much as possible. On the other hand, accesses to the waste chambers must be maintained or established in order to recover the waste. The excavations and underground measures required for this purpose must not have an unacceptable impact on the stability and hydrogeological conditions.

Fig. 9: Retrieval scheme.

Figure 9 shows the main steps of the retrieval process, starting underground with the recovering of the waste from the emplacement chambers (ELK). Via the retrieval mine that is being built for this purpose and the associated new shaft 5 (see figure 10), the waste is transported to the surface in a special overpack. In the above ground facilities the waste will be characterized as well as treated and packaged (conditioned). Afterwards it is stored in an interim storage facility.

Figure 10 shows in red the emplacement chambers within the existing mine. The future retrieval mine with its shaft (shaft 5) is situated to the right of the existing mine. This new infrastructure is necessary to enable an efficient recovery operation of the waste under the conditions of nuclear safety requirements.

Fig. 10: Existing mine and retrieval mine as well as emplacement chambers at -750 m (in red) [1].

Design parameters

In addition to the existing geological and hydrogeological situation, special challenges for recovery arise from the condition of the waste containers, which is not precisely known. It must be assumed that a considerable part of the containers is no longer intact, i.e. at least leaking or destroyed to such an extent that handling, e.g. by means of a grab, is no longer possible. In addition, it can be assumed that the containers cannot be easily detached from the surrounding salt crust. While emplacing containers, they have been embedded in crushed salt. Furthermore, the documentation of the waste does not allow a clear determination of the radioactive inventory of each container.

The handling and transport of radioactive waste underground must meet the safety requirements for the operation of a nuclear facility. Potential incidents during handling must be controlled. Radiation protection of personnel must be ensured at all times, both during normal operation and in the event of incidents. Radioactive emissions – especially after opening of the emplacement chambers – must comply with the permissible limits for the personnel below and above ground and for the general public.

The technology to be used for recovery must be designed to take into account the geological and hydrogeological conditions as well as the safety-related and radiological requirements. In addition, it must be able to react to the challenges posed by the waste containers. This includes, for example, dealing with defective casks or casks that are firmly trapped by the salt crust, as well as with the uncontrolled behavior of casks in stacks and dumping cones.

The detailed planning of the recovery technology for the radioactive waste is carried out on the basis of the available concept studies, initially for the waste chambers on the -511 and -725 meter levels. By using small-volume excavation technologies, the structural integrity of the mine will be affected as little as possible. For the recovery of waste from the chambers on the -750-metre level a comparison of two technological approaches is currently being carried out.

Figure 11 shows the status of concept planning for recovery technology. Different recovery techniques are being developed, tested and used for the respective emplacement conditions. For the individual chambers on the 511 m and 725 m levels, the current plans envisages a grapple that is guided on the floor or via a rail system on the roof of the chambers. For the geotechnically unstable chamber rows on the -750 m level, sequential recovery with immediate backfilling for stabilization is preferred. Alternatively, the use of large-volume shield tunnelling machines is considered.

Fig. 11: Recovery technology adapted to specific conditions of emplacement chambers [1].

Precautionary and Emergency Measures

According to the provisions of the Atomic Energy and Mining Law, precautionary measures against possible operational incidents and accidents must be taken as a prerequisite for recovery. In this context, above all, the possibilities of a direct attack by an AüL on the waste chambers before and during retrieval must be prevented or reduced as far as possible. To achieve this, the BGE pursues a concept of emergency planning that provides for so-called precautionary and emergency measures (see Figure 12, [1]).

Fig. 12: Delimitation of precautionary and emergency measures of emergency planning with naming of exemplary measures.

The most important measure is the backfilling of the remaining void volumes and the construction of hydraulic barriers around the waste. However, even after the backfilling measures have been completed, a void volume of almost 1 million m³ will remain, mostly as pore space in the backfill.

It is also important to seal the stopes and cracked pillars below the current main intake location on the -638 meter level in order to block any internal flow path that may develop within the excavation field. For this purpose, an injection screen is to be installed underneath the collection point in the next few years.

Constructing a new retrieval mine outside the existing mine building also contributes to the precautionary measures. In this way further weakening of the central part of the geotechnically stressed mine infrastructure and a further loss of integrity is avoided.

Outlook

The hydrogeological conditions will be clarified by extensive exploration measures. In particular, the exact location of the border between salt structure and overburden as well as the possible flow paths at layer boundaries, in aquifers and along fault paths must be clarified. A central element of the safety assessment for the retrieval will be the proof that no additional hydrogeological risks will arise from the further interventions in the salt structure associated with the project. To this end, a large number of core drillings will be made from below and above ground. An elaborate three-dimensional reflection seismic survey was carried out in the winter of 2019/2020. Only with precise knowledge of the critical hydrogeological areas can the new mine building for recovery be constructed in such a way that it avoids possible hydrogeological risks, such as approaching the salt envelope or potential flow paths.

Bibliography

[1] Plan zur Rückholung der radioaktiven Abfälle aus der Schachtanlage Asse II–Rückholplan; BGE 2020

[2] Schachtanlage Asse II: Gebirgsbeobachtungsgespräch, BGE 2020, YouTube

[3] Dr.-Ing. Oleksandr Dyogtyev: Numerische Analyse des Tragverhaltens komplexer gebirgsmechanischer untertägiger Systeme mit filigranen Strukturen bei Anwesenheit von Imponderabilien; Dissertation, TU Clausthal, 2017

Authors:

Thomas Lautsch

Beate Kallenbach-Herbert

Sebastian Voigt

Bundesgesellschaft für Endlagerung (BGE) mbH

Peine, Germany

Beate.Kallenbach-Herbert@bge.de