Development and testing of a laser-based decontamination system

Abstract

Decontamination of radioactive concrete surfaces may be necessary during operation or decommissioning of nuclear power plants. Usually only the upper layers of the concrete structure are contaminated and are removed using labor-intensive mechanical milling processes. Production of a large amount of dust, which can lead to secondary contamination, is inherent to these processes. Improvements in high-energy laser technology have now made it possible for laser radiation to be used in decontamination technologies for the removal of concrete layers. A decontamination unit comprising a diode laser with a beam power of 10kW in continuous wave (CW) mode in combination with an autonomous manipulator was developed for use in nuclear plants. The laser beam melts the concrete surface to a depth of approximately 5mm. Compressed air jets then detach the molten layer from the concrete surface and convey it to a suction system, with which it is transported to a collection container. Most of the radionuclides are trapped in the solidifying melt particles, which form an extremely stable effluent well suited to long-term storage. A relatively small amount of dust is generated in the process. Because there is no backlash during energy transfer, the laser device carrier can be designed to be lightweight and flexible. A specially developed manipulator that can move freely along walls and ceilings by means of suction plates is used for the carrier unit. This results in short setup times for preparing for use of the device and minimal personnel exposure to the radiation. Experiments were conducted on a concrete wall to demonstrate the functionality of the overall system in realistic conditions. An optimal ablation rate of 2.16m²/h at an ablation depth of 1–5mm was achieved. Today’s commercially available diode lasers with powers higher than 50kW enable ablation rates of >10m²/h to be achieved and hence make these laser-based systems competitive alternatives to mechanical systems.