The Greifswald nuclear power plant is located in Lubmin in West Pomerania. The town is near Greifswald and Wolgast on the Baltic Sea. Construction work on the first of eight planned nuclear power blocks of Soviet manufacture was begun there in 1967. By 1979, four reactors had gone on line. They supplied a significant proportion (about eleven percent) of East Germany’s electrical power. At the time of German reunification, the reactor of Block 5 was in trial operation, Block 6 had already been equipped with all of its components, and Blocks 7 and 8 were still in the construction phase.
Upon reunification, the Federal Republic of Germany’s Ministry of the Environment commissioned a safety analysis of the Greifswald reactors. Serious deficiencies were found in Blocks 1 through 4. Bringing them up to Western safety standards would have been very difficult. Block 5 was more modern, and would have been easier to re-equip. However, due to the high costs involved and the uncertainty of receiving official approval, no investor could be found. It was therefore decided in 1990 to shut down all of the reactors and turn the Greifswald nuclear power plant back into green fields.
Removing contamination
Once the systems and equipment in nuclear power plants have been dismantled, the interior spaces must be decontaminated. Only when the operator can prove on the basis of measurements that radiation is below the permissible level can the buildings in question be released from the requirements of the German Atomic Energy Law. In older plants in particular, deeper layers of floors, walls and ceilings are often contaminated, for example due to crack formation or the failure of protective coatings, and the layers of materials that have to be removed differ from spot to spot.
Previously this had been done using various methods, all of which were labor-intensive and generated noise and dust. In addition, uneven surfaces were often produced which led to difficulties when making measurements. Moreover, no one person may work more than six hours a day in a nuclear power plant. Productivity is further limited by the protective clothing which is necessary, and by the heavy tools, which weigh several kilograms. What is more, providing relief personnel is expensive, since only skilled nuclear plant workers may be used.
Precision and high performance
About 2.3 million square meters of concrete surfaces have to be decontaminated on the floors, walls, and ceilings of the Greifswald reactor blocks. How to solve this task automatically is currently being demonstrated by SINA Industrieservice GmbH & Co. KG of Pforzheim, Germany. A subsidiary of the Swedish Studsvik group, SINA is the foremost German service company in the fields of radiation protection, decontamination, restoration and disposal. Drawing on its many years of experience with safety issues, SINA has developed a patented processing unit, the “Bolero”. This unit is built around a KR 125 six-axis jointed-arm robot from KUKA Roboter GmbH of Augsburg, Germany.
In selecting KUKA Roboter as a partner, SINA relied consistently on the quality and expertise of a market leader. Of greatest interest to the service company was the continuous availability of the KR 125, because in this case a malfunction could result in considerable costs. This is especially true for areas with critical deadlines, since subsequent operations would also be seriously affected.
The solution that was found features precise removal of the contaminated material, thus minimizing the amount of radioactive debris which must be disposed of at high cost. In addition, large surface areas can be processed quickly. For example, the robot can strip off up to ten square meters of concrete per hour. Its working speed can be increased even further if only layers of paint are to be removed. Executing these tasks manually would be slower and thus significantly less cost-effective.
A broad spectrum of uses
Bolero is suitable for concrete, masonry, clay brick and sand-lime brick, as well as for gas concrete, plaster, anti-contamination paints and other wall coatings. The six-axis robot is equipped with a tool changing system, and has the flexibility to strip walls in almost any geometry.
The processing unit demonstrated at the Greifswald nuclear power plant comprises the KUKA robot and its PC-based controller; a processing tool driven by compressed air; a compressor, which supplies the tool with approximately 5000 liters of compressed air per minute (at 8 bar); and an industrial vacuum cleaner, which conveys all of the removed material and dust particles through a cyclone separator and into an R-200 drum, thereby eliminating the need for enclosures and supplementary ventilation equipment. The processing tool is equipped with an array of 32 carbide rams. There is also a rubber strip attached to the tool, creating a continuous seal between the tool and the wall.
In addition to the advantages already mentioned, Bolero significantly reduces personnel requirements and improves working conditions, for example regarding noise and respiratory protection; in some cases respiratory protection may no longer be needed at all.
The processing unit was subjected to a test phase lasting one year, which was completed in mid-2002. This phase was primarily used to optimize the extraction system and the type and form of the tool. SINA then tested the targeted removal of surfaces in reactor block 7, which was not contaminated.
Automatic removal and measurement
Before any surface processing is begun, Bolero detects the geometry of the wall to be decontaminated and generates a coordinate system with the aid of reference points. The KUKA robot, which has a collision safeguard and can be equipped with an anti-contamination protective suit, generates the coordinate system by using a teaching device. With this it makes measurements at four points within its maximum work envelope. The data are determined in about five minutes and stored in the robot controller.
The KR 125 then sets down the teaching device and picks up the processing tool, which weighs 120 kg. It uses this tool to remove the contaminated surface layer to a defined depth, with accuracy to the nearest millimeter, based on the data that were determined previously. The depth varies between one and ten millimeters, depending on the specific operation and the material.
Finally the robot picks up a measuring probe and verifies whether any contamination which remains is below the statutory limit values. If the limits are exceeded, the robot automatically exchanges its tool again and reworks the relevant areas. This is repeated until the surface in question is completely cleansed and may legally be demolished. All of the data are saved by the computer, such as the position of the measured surface, the time required for measurement and the result. These data can be cited in connection with the release procedure to prove that the contamination is under the limit values.
To enable the probe to make measurements with the required accuracy, the KR 125 must leave behind a smooth surface every time. While conventional detectors can measure an area of 133 square centimeters in each cycle, the robot can cover 2000 square centimeters in the same amount of time. The time required for measurement also depends on the types of nuclides present.
From a single location, the KUKA robot can process a surface four meters high and five meters wide. To reach higher wall surfaces or ceilings, the KR 125 is lifted using a crane or electric chain hoist and placed on a movable, stress-analyzed scaffold. The scaffold is designed so that the robot can be positioned at different levels up to 20 meters high. In addition, Bolero can be broken down quickly into component parts, making it easier to get to locations which are otherwise hard to access. Moreover, the setup time after these moves is quite short.
Important for the local economy
The dismantling of the Greifswald nuclear power plant is the largest project of its kind in the world, and is thus an important factor in stabilizing the economy in West Pomerania and the surrounding regions. To date the German Federal Ministry of Finance, as the sole owner of the Greifswald nuclear power plant, has allocated over 1.5 billion euros for decommissioning and shutdown of the facilities. Land which is not needed for the remaining dismantling work is being sold to investors as sites for industrial firms. Lubmin is also to remain in the electricity-generating business. Plans exist to build two gas / steam turbine power plants – a type which is being used more and more in many countries for cost-effective and environmentally friendly power generation.
Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld, Germany