The Release of Radionuclides in the Laser Decontamination Process

The decommissioning of nuclear installations requires the decontamination of radioactively contaminated concrete surfaces in order to minimize the amount of radioactive waste to be disposed of as well as the exposure time of the staff during this works. The rapid progress in the development of laser technology has yielded high-performance diode lasers whose radiation can be guided over a long distance by means of glass-fibre optical units. This opens up the possibility of implementing unconventional laser-based decontamination processes. The aim of the method presented here is to combine melting and contactless ablation of a radioactively contaminated concrete surface by means of a laser beam with waste product conditioning. It is intended to design the process in such a way that a maximum of the radioactivity present at the surface is incorporated in the glass melt (= conditioning of waste products). The glassy granulate obtained is very well suited for direct final storage due to its physical and chemical properties. The portion of radioactive isotopes that are released in the process, but not incorporated during the ablation process is selectively deposited in a cooled electro-filter. To prove the effectiveness of the method, research was focused on decontamination experiments conducted on concrete samples contaminated with ¹³⁷Cs, ⁶⁰Co and ⁸⁵Sr. Furthermore, the chemical composition of the concrete samples was varied (quartzitic, quartzitic-calcitic) to take account of the different release conditions in real concrete structures. The experiments showed that ⁸⁵Sr and ⁶⁰Co are highly soluble in the glass melt. Their release rate is very low as they have a relatively high boiling point. ¹³⁷Cs also exhibits a great affinity to the glass melt, but is more easily released again in the high temperature range due to its low boiling point of approx. 700 °C. The released portion of ¹³⁷Cs is then deposited in the upstream electro-filter. The overall assessment is that the intended decontamination process with simultaneous conditioning of waste products is basically feasible using today’s laser technology. The special advantage can be seen in the great versatility and easy control of the laser unit that is equipped with a fibre-optical system. Furthermore, laser ablation can be set up as a low-dust process, which minimizes problematic secondary contamination.