WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

Concepts and Tests for the Remote-Controlled Dismantling of the Biological Shield and Formwork of the KNK Reactor – 13425 Sylvia Neff*, Anja Graf*, Holger Petrick*, Stefan Rothschmitt *, Stefan Klute**, and Dieter Stanke*** *WAK Rückbau- und Entsorgungs- GmbH, P.O.Box 12 63, 76339 EggensteinLeopoldshafen, Germany; **Siempelkamp Nukleartechnik GmbH, Am Taubenfeld 25/1, 69123 Heidelberg, Germany; ***Siempelkamp NIS Ingenieurgesellschaft mbH, Industriestraße 13, 63755 Alzenau, Germany ABSTRACT The compact sodium-cooled nuclear reactor facility Karlsruhe (KNK), a prototype Fast Breeder, is currently in an advanced stage of dismantling. Complete dismantling is based on 10 partial licensing steps. In the frame of the 9th decommissioning permit, which is currently ongoing, the dismantling of the biological shield is foreseen. The biological shield consists of heavy reinforced concrete with built-in steel fitments, such as formwork of the reactor tank, pipe sleeves, ventilation channels, and measuring devices. Due to the activation of the inner part of the biological shield, dismantling has to be done remote-controlled. During a comprehensive basic design phase a practical dismantling strategy was developed. Necessary equipment and tools were defined. Preliminary tests revealed that hot wire plasma cutting is the most favorable cutting technology due to the geometrical boundary conditions, the varying distance between cutter and material, and the heavy concrete behind the steel formwork. The cutting devices will be operated remotely via a carrier system with an industrial manipulator. The carrier system has expandable claws to adjust to the varying diameter of the reactor shaft during dismantling progress. For design approval of this prototype development, interaction between manipulator and hot wire plasma cutting was tested in a real configuration. For the demolition of the concrete structure, an excavator with appropriate tools, such as a hydraulic hammer, was selected. Other mechanical cutting devices, such as a grinder or rope saw, were eliminated because of concrete containing steel spheres added to increase the shielding factor of the heavy concrete. Dismantling of the biological shield will be done in a ring-wise manner due to static reasons. During the demolition process, the excavator is positioned on its tripod in three concrete recesses made prior to the dismantling of the separate concrete rings. The excavator and the manipulator carrier system will be operated alternately. Main boundary condition for all the newly designed equipment is the decommissioning housing 1

WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

of limited space within the reactor building containment. To allow for a continuous removal of the concrete rubble, an additional opening on the lowest level of the reactor shaft will be made. All equipment and the interaction of the tools have to be tested before use in the controlled area. Therefore a full-scale model of the biological shield will be provided in a mock-up. The tests will be performed in early 2014. The dismantling of the biological shield is scheduled for 2015. INTRODUCTION The compact sodium-cooled nuclear reactor facility (KNK) was a prototype nuclear power plant of 21 MW electric power on the premises of the Karlsruhe Institute of Technology (KIT), the former Forschungszentrum Karlsruhe (FZK), in Eggenstein-Leopoldshafen [1]. The plant was operated first from 1971 to 1974 as KNK-I and then from 1977 with a ”fast core “ as fast-breeder power plant KNK-II. The plant was shut down in 1991, dismantling started in 1993. Decommissioning to the “green field” is planned to be completed by 2020. According to the decommissioning concept, the plant is to be dismantled completely in ten steps. For each of these steps, a decommissioning permit is applied for. The first eight decommissioning steps have already been completed. In particular, the fuel elements and sodium were disposed of, no longer required systems were shut down, the cooling towers and the machine hall were demolished. Secondary and primary sodium cooling circuits were disassembled completely. Presently, the ninth decommissioning step is being accomplished. Within the 9th decommissioning permit, the reactor tank and all internals have already been dismantled completely. Disassembly of the thermal shield was completed in May 2011. Now, removal and dismantling of the primary shield are being prepared [2]. After that, the formwork and the activated part of the biological shield will be dismantled as the last activity under step 9 (s. Fig. 1).

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WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

Thermal shield (refractory lining 8 Mg)

Primary shield (cast iron 90 Mg) Activated part of the biological shield and reactor formwork (heavy concrete 330 Mg, steel 45 Mg)

Fig. 1: Reactor shaft of KNK with thermal shield, primary shield, and activated part of the biological shield Dismantling of the formwork and the activated part of the biological shield is presently being planned. Testing of the necessary remotely controlled equipment is being prepared. DESCRIPTION OF THE BIOLOGOCAL SHIELD The biological shield around the reactor shaft and around the already dismantled fuel element storage pool consists of a heavy concrete component of 12.6 m height. The diameter of the reactor shaft varies from 2.10 m to 3.83 m. The formwork is made of sheet steel of 10 to 12 mm thickness. For mounting in the heavy concrete, it is equipped with 200 mm wide anchor plates at distances of 500 mm to 800 mm. The biological shield is made of barite concrete with a density of 4.2 kg/dm³. For shielding purposes, steel spheres (Ø 4 to 16 mm) were added. The heavy concrete of the biological shield was activated by neutron radiation from the reactor core. Maximum Co-60 activity of the concrete is 8*105 Bq/g, it decreases from the inside to the outside. Based on the results of radiological studies, the amount of activated heavy concrete ( 0.5 Bq/g) was determined. Verification was made by sampling. The release contour expected is shown in Fig. 2. 330 Mg of concrete are expected to be activated. 3

WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

Fig. 2: Biological shield of KNK with release contour and dimensions Apart from the formwork, the release contour contains steel internals that have to be considered by dismantling. Among these internals are several pipelines of 345 to 1005 mm in diameter, the mounting of the reactor lid, five ionization measurement chambers in the center of the shaft, and the support of the primary shield of 75 mm in thickness. The steel quantity totals about 45 Mg. DISMANTLING OF THE BIOLOGICAL SHIELD It is envisaged to dismantle the activated part of the biological shield and to subject the inactive structures and the containment to release measurement and subsequent conventional demolition. Due to the radiological boundary conditions, dismantling of the activated part of the biological shield will be performed remotely. As the concrete is reinforced with steel spheres, removal by cutting techniques (milling, drilling, sawing) is impossible. Under the impact of external forces, the spheres contained in the concrete will detach easily and rotate between the tool and the concrete structure. As a result, they will interfere with the cutting process and make it impossible. In this connection, a number of preliminary tests were conducted. For concrete 4

WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

disassembly, use of a demolition tool (ABG) with a tripod based on a series excavator is envisaged (see Fig. 3). Within the framework of the 8th decommissioning permit, favorable experience was gained from the use of an excavator for the demolition of a 1000 mm thick sphere-reinforced heavy concrete ceiling.

Fig. 3: Demolition tool (ABG) for dismantling heavy concrete Due to the concrete linings of the steel components, it is impossible to use mechanical techniques for dismantling the formwork and the internals within the activated biological shield. In the course of preliminary tests, a new plasma cutting technique with a non-transferred arc and a consumable electrode (anode) – the so-called hot wire technique – was found suitable for dismantling (see Fig. 4). Compared to conventional plasma cutting, this technique is less sensitive to the material to be cut and the cutting geometry.

Fig. 4: Principle of plasma cutting (left) and hot wire cutting (right) The cutting tools and the manipulator required for guiding and handling are adapted to a manipulator carrier system (TMS) that interlocks in the reactor shaft (Fig. 5). 5

WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

Fig. 5: Manipulator carrier system The demolition tool and the manipulator carrier system have to be adapted to the cutting tasks as well as to the geometry of the reactor shaft and of the decommissioning housing. The decommissioning housing is located directly above the reactor shaft and provides for a shielding, separation of ventilation, and storage place of the remote handling devices. The decommissioning housing was built at the beginning of the 9th decommissioning permit and is adapted to every dismantling step. Hoisting gear of demolition excavator

Cell crane with adapter plate

Reactor shaft cover

Fig. 6: Decommissioning housing above the KNK reactor shaft For the remote-controlled activities in the reactor shaft, the manipulator carrier system will be attached to the cell crane via the adapter plate that also contains most of the media supply connections for the manipulator carrier system. Then, the manipulator carrier system will be lowered into the shaft through the open reactor shaft cover. In the shaft it will interlock for the 6

WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

dismantling of the steel components. When the manipulator carrier system will no longer be required or when the demolition excavator is to be lowered into the reactor shaft, interlocking of the manipulator carrier system will be loosened and the latter will be pulled up into the decommissioning housing with the cell crane and put down onto a stand with a maneuverable carriage. For repair, the carriage with the manipulator carrier system can be moved into a shielded area of the decommissioning housing. The demolition tool will be attached to a crane system coupling (KSK) of the hoisting gear and lowered into the reactor shaft for dismantling. The demolition tool will be put down at the location of dismantling and it will then remove the concrete in the clear spaces of the tripod. Then, the demolition tool will be rotated by about 60 degrees in order to remove the remaining concrete. In this way, the manipulator carrier system and the demolition tool will be operated alternately in a ring-wise manner from the top to the bottom.

Fig. 7: Dismantling of the biological shield with the demolition tool For rubble removal, a funnel with a grid plate (150 x 150 mm) will be installed in the lower part of the reactor shaft. The dismantled steel components will be collected on the grid and transported upwards into the decommissioning housing using a magnetic gripper. There, these components will be packed into T-170 l compaction drums and removed from the housing via a double-lid lock. The concrete rubble and parts of the reinforcement will fall through the grid plate and then be transported from the reactor shaft via an opening to the primary cleaning cell. There, packaging into steel containers will take place.

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WM2013 Conference, February 24 – 28, 2013, Phoenix, Arizona USA

Fig. 8: Funnel, grid plate, and transport system in the lower part of the reactor shaft CONCLUSIONS Due to the limited space and activation during operation, dismantling of the biological shield of KNK represents a big challenge and is being planned thoroughly by KNK. The demolition tool, the manipulator carrier system, inclusive of the manipulator and the extensive equipment for hot wire cutting, as well as the waste disposal facilities have to be procured for mock-up testing and optimization, if necessary, prior to use in KNK. KNK plans to start testing in early 2014. REFERENCES 1. J. DUX, B. EISENMANN, J. FLEISCH, A. GRAF-FRANK, J. MINGES, W. PFEIFER, E. PRECHTL, M. URBAN “Decommissioning and Dismantling of Prototype Reactors and Fuel Cycle Facilities at the German Karlsruhe Site – Progress and Challenges” Proc. WM2010 Conference, Phoenix AZ, Paper No. 10088 (2010) 2. S. ROTHSCHMITT, A: GRAF, S. BAUER, S. KLUTE, E. KOSELOWSKI, K. HENDRICH “Testing and Commissioning of a Multifunctional Tool for the Dismantling of the Activated Internals of the KNK Reactor Shaft” Proc. WM2013 Conference, Phoenix AZ, Paper No. 13524 (2013)

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