________________ A joint project between Herrenknecht AG and KIT, funded by the Federal Ministry of Education and Research is developing a method for removing reinforced concrete.
The specific removal of reinforced concrete is a central issue, especially when decommissioning nuclear installations. The objective is to selectively remove the contaminated material and forward the remaining material, which makes up the majority of the total plant, into the normal recycling loop.
Removing the reinforced concrete using our experience in hard rock cutting technology
Removing radioactively contaminated reinforced concrete structures (i.e. reinforced concrete surfaces, cracks in the building structure and internal installations) without involving personnel is a major challenge in decommissioning nuclear plants. In this situation all containment areas - the area that protects the environment against radioactive contamination in the event of a fault - must be decontaminated. The primary objective is to selectively remove the contaminated material and dispose of it, in order to be able to conventionally recycle the remaining material, which makes up the major part of the total mass in the plant. It must be possible to clean all surfaces of the building structure, so they can be released from regulatory control and so that contaminated building materials can be clearly separated from the non-contaminated.
In a collaboration between the Institute for Vehicle Technology and Mobile Machines (Mobima) and the Institute for Technology and Management in Construction (TMB) at the Karlsruhe Institute of Technology and Herrenknecht AG, a corresponding method is being developed. Within the scope of the joint project "Innovative demolition of bulky reinforced concrete structures”, funded by the Federal Ministry for Education and Research (BMBF), a system is being developed for the spatially limited, remotely handled and defined removal of highly-reinforced concrete structures.
At the present time, several methods to be applied to all interior walls of nuclear plants are available for surface decontamination, which is only a few millimeters thick. As a rule, only concrete is removed, however the demolition and selective deep removal of reinforced concrete structures in the vicinity of installations or breakouts still pose a problem. So far, no automated or automatable procedure is available for the in-depth removal of the concrete together with the reinforcement. Likewise, there is no procedure for dealing with cracks at the present time, and the reinforced concrete can only be reamed out to a very limited extent, for example at depths of 30cm.
The main problem is that brittle and abrasive concrete and the tough reinforcing steel cannot be cut at the same time. There are some procedures that can deal with both materials separately. The simultaneous, efficient handling of steel and concrete, especially under the difficult conditions in nuclear plants, is, however, not possible at this time.
By comprehensively analyzing removal procedures and their interaction, a procedure has been found which enables an automated process flow under the stringent requirements in nuclear facilities. Space constraints, however, strongly limit the construction size and the mass of the overall system. Moreover, using auxiliary and cooling media like water as well as large, high-performance carrier equipment is also not possible because of the risk of spreading contamination. This and a variety of other requirements were used as a basis for determining appropriate cutting techniques.
A combination of undercutting and milling technology is the solution
By combining an activated undercutting technique (similar to a vibrating chisel) for the efficient removal of the concrete using only a milling technique (known in mechanical steel machining) to remove the reinforced concrete structures, an automatable process combination was found. Both technologies can be incorporated into a tool carrier and allow for the defined removal of the reinforced concrete with relatively low cutting forces. Initial tests were carried out in a first trial demonstration on the TMB testing grounds.
The continuous further development of the system and cutting technology by incorporating the latest findings from test series should lead to a further improvement of the system in terms of efficiency and process reliability for practical use in a nuclear installation. Thus, we have come closer to the goal of enabling the respective companies for the first time to remove highly reinforced, heavily radioactively contaminated reinforced concrete in an automated and remote handling form while reducing the use of personnel in activated areas during decommissioning work.
The comprehensive experience of Herrenknecht AG in cutting stones and especially in hard rock cutting techniques provided the prerequisites for this project. At the same time, it has made it possible to use and expand the existing expertise into other technical areas (away from tunnel construction).