atw - International Journal for Nuclear Power | 06.2021

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atw Vol. 66 (2021) | Issue 6 ı November DECOMMISSIONING AND WASTE MANAGEMENT 20 also be deformed, burst as well as shattered, and it is assumed that there is a matrix of (possibly solidified) crushed salt around the casks. [2] Partial-face excavation machine At the tunnel face (left in Figure 3) the salt rock of the pillars or the packages inside the emplacement chambers can be loosened and lifted with the help of the tools. A vacuum can be created between the tunnel face and the machine to prevent contamination from being carried into the interior of the machine via radioactive particles in the air. [2] The driving of the emplacement chambers and the pillars creates a cavity that has to be backfilled behind the machine. For this purpose, sorel concrete is placed both as shotcrete and as extruded concrete (see Figure 5). In this way, the rock can be stabilised and at the same time a structure can be created on which the partial-face excavation machine can support itself for further travel. In this so-called ‚cavity filling‘ (see | Fig. 4. Rear view of the partial-face excavation machine with cavity filling [2]. Figure 4), two tunnel tubes are cut out for the period of retrieval. After suitable repacking for transport, the materials can be removed through the larger tunnel tube (disposal tunnel) (see also Figure 3), the smaller tube serves as access for personnel and for the transport of working materials ( passenger and utility tunnel). After successful retrieval and salvage of the partial-face excavation machines, the two tubes can be backfilled. Both the cavity filling and the backfilling of the tunnel tubes (transportation tunnels) are made of sorel concrete. [2] Sorel concrete has already been used for the production of flow barriers and various backfillings in the Asse II mine. The main components of sorel concrete are magnesium oxide as a binder and crushed salt as aggregate, which are mixed with magnesium chloride solution. [4] Once the tunnel face has been completed, the machine can support itself against the cavity filling by means of thrust cylinders and move forward. When advancing, the annular gap that becomes free behind | Fig. 5. Internals and machine equipment of the partial-face excavation machine, e.g. thrust cylinders and ( inner) tailskin seal [2]. the tailskin can be filled with grout immediately. At the end of the shield shell, an (inner) tailskin seal is provided, which seals the interior of the machine against the annular gap. [2] In Figures 3, 4 and 5, the thrust cylinders in the rear part of the machine (tailskin) are shown in red, the tailskin seal in blue. In the course of the in-depth investigations, an outer tailskin seal was added to the partial-face excavation machine designed in the study. This seals the steering gap against the annular gap, and prevents material from the annular gap from getting around the shield or up to the tunnel face (see Figure 6). This ensures that the annular gap is filled conclusively under pressure. This is particularly important in the area of the pillars between the emplacement chambers, as this minimises contamination carry-over between the chambers and the barrier effect of the pillars can be maintained to the maximum. Differentiated consideration of the tailskin seal Due to the technical and legal framework conditions existing in a repository, it is necessary to mirror all components and systems of a shield machine against the resulting special requirements, to evaluate their applicability and functioning, and to make adjustments if necessary. For the investigations of the two tailskin seals, a comprehensive catalogue was compiled in each case, in which various loads play an essential role. The mechanical loads on both seals are exceptionally high due to the square cross-sectional shape of the shield machine. This is intensified by damage in the roofs, which can lead to loosening on the shield skin of the machine, and thus also on tailskin seals. In addition, chemical stresses occur, mainly from the liquid sorel concrete and saturated sodium chloride solution. The influence of ionising radiation on the tailskin seals is considered negligible due to the low activity inventory. Based on this catalogue, conventionally used tailskin seals were evaluated with regard to their suitability for shield tunnelling in the Asse II mine. Different aspects have to be given priority in the requirements for the respective tailskin seal. The inner tailskin seal must primarily fulfil a barrier effect towards the trailing annular gap, usually by building up a high pressure difference between the areas to be separated. The reason for Decommissioning and Waste Management Investigations of the Tailskin Seal During the Retrieval Concept ‘Shield Tunnelling with Partial-face Excavation’ in the Asse II Mine ı Birte Froebus
atw Vol. 66 (2021) | Issue 6 ı November | Fig. 6. Schematic of an inner and outer tailskin seal in partial-face excavation machine with description of the steering gap and annular gap [own figure]. this is the classification of radiation protection areas according to §52 of the Radiation Protection Ordinance (German: Strahlenschutzverordnung = StrlSchV), which defines the annular gap as a ‚exclusion area‘, while the interior of the machine represents a ‚controlled area‘ (see Figure 6). For the outer tailskin seal the focus is on high wear resistance to obstacles outside the machine. As a result of the investigations, an inner tailskin seal was first designed which combines various conventional design elements. The basis for this is a wire brush seal with external spring plates and three sealing chambers, which is modified with a lubricant compression line and enlarged sealing chambers. With the help of the pressurised tail seal compound, this can meet the high demands on the barrier effect. In addition, the spring plates at the rear of the annular gap, in combination with the lubricant used, make it easier to start up the machine, as brushes in contact with the liquid backfilling (grout inside the annular gap) would stick together and be damaged over time. The lubricant itself also protects against the aggressive chemical environment in the adjacent annular gap. The enlarged sealing chambers increase the protection of the sealing system with a kind of spring effect – both in case of post-fractures from the rock mass and in case of occurring convergences. Subsequently, the outer tailskin seal was designed from three rows of spring plates, which are also modified with lubricant injection lines. The arrangement of three seal rows ensures sufficient redundancy, which is why the lubricant supply was also designed redundantly, so that the outermost seal row always has an associated supply line. In order to prevent the spring plates from being torn off by obstacles – such as barrel bundles from the emplacement chambers or rock fragments from the surrounding salt rock – their bearing was embedded in the shield casing and thus protected. The number of seals in a row also ensures a certain redundancy here. Conclusion Within the scope of the investigations, comprehensive findings were obtained for the problems occurring during shield tunnelling in the Asse II mine. So far, not all components have been investigated in a differentiated manner. In further investigations, for example, aspects such as the interactions between the shield machine and the surrounding salt rock or the synergies and disadvantages of the parallel tunnel routes could be determined. It has been shown that the various situations of the partialface excavation machine with changing framings – such as salt rock, already hardened sorel concrete from preceding machines or contents of the emplacement chambers – represent a special challenge. Especially when feeding from cavity filling, the stability of the lateral walls along the travel distances plays an important role. Against this background, for example, the sequence of the partialface excavation machines would have to be adjusted so that the first machine starts in the direction of travel on the left in order to be able to support itself against the salt rock in the south – especially in the area of the emplacement chambers (see Figure 2). The grouting (of the annular gap) in the area of the emplacement chambers could also be considered in more in-depth investigations. A further aspect would be the compatibility of the tunnelling concept with the emergency planning for a possible flooding of the Asse mine. At present, the reduced barrier effect of the drilled-through pillars between the emplacement chambers as well as the volume of the five shield machines and their underground infrastructure represent significant disadvantages of shield tunnelling with partial-face excavation. The retrieval of radioactive waste from the Asse II mine is scheduled to begin in 2033, in this sense: Glück auf! References [1] Plan zur Rückholung der radioaktiven Abfälle aus der Schachtanlage Asse II – Rückholplan, Verfasser: Bundesgesellschaft für Endlagerung mbH Peine/Remlingen, Stand: 19.02.2020 [2] Machbarkeitsstudie für die Methode „Schildvortrieb mit Teilflächenabbau“ – Studie zur Eignungsfähigkeit und zum Entwicklungsbedarf von Gerätschaften/ Werkzeugen für den Einsatz in der Schachtanlage Asse II, Herrenknecht AG im Auftrag des Karlsruher Instituts für Technologie (KIT), Technologie und Management des Rückbaus kerntechnischer Anlagen (TMRK), Schwanau, Karlsruhe; Stand: 13.05.2015 [3] Maidl, B., Herrenknecht, M., Maidl, U., Wehrmeyer, G. 2011. Maschineller Tunnelbau im Schildvortrieb. Ernst & Sohn [4] Schachtanlage Asse II: Nachweis der Langzeitbeständigkeit für den Sorelbaustoff der Rezeptur A1; ERCOSPLAN, TU BAF ( IFAC), IFG; Stand: 10.08.2018 Author Birte Froebus Research Assistant at Karlsruhe Institute of Technology, Karlsruhe, Germany birte.froebus@kit.edu Birte Froebus studied civil engineering at the Karlsruhe Institute of Technology (KIT), specialising in geotechnical engineering. In her master thesis, she took the opportunity to combine mechanised tunnel construction with the retrieval of radioactive waste, for which she was awarded the WiN-Germany Prize 2020/21. She then started working as a research assistant at the Institute for Technology and Management in Construction (TMB) in the ’ Deconstruction and Decommissioning of Conventional and Nuclear Buildings‘ department. DECOMMISSIONING AND WASTE MANAGEMENT 21 Decommissioning and Waste Management Investigations of the Tailskin Seal During the Retrieval Concept ‘Shield Tunnelling with Partial-face Excavation’ in the Asse II Mine ı Birte Froebus
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atw - International Journal for Nuclear Power | 06.2021

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