Marion Herrmann

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 137 Cs, 60 Co and 85 Sr. 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 85 Sr and 60 Co are highly soluble in the glass melt. Their release rate is very low as they have a relatively high boiling point. 137 Cs 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 137 Cs 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.Copyright

NON-OXIDE CERAMICS - CHANCES FOR APPLICATION IN NUCLEAR HYDROGEN PRODUCTION

Research and development are increasingly focusing on the provision and utilization of heat in the high-temperature range above 900 °C, in particular under the aspect of resource-saving energy technologies. On the one hand, the exploitation of the high-temperature range helps to improve the efficiency of energy conversion processes; on the other hand, the provision of high-temperature heat makes it possible to utilize innovative thermochemical processes, which in turn represent environmentally compatible processes. An example to be quoted here is the thermally induced production of hydrogen by the iodine-sulfur process. The high temperatures alone place extremely high requirements on the materials to be used so that metallic materials soon reach their limits of application. If additionally chemically aggressive process media are used, as in the iodine-sulfur process, basically only ceramic materials can be considered as construction materials. In this application, notably silicon carbide (SiC) is favored owing to its excellent high-temperature properties. The possible technical fields of application of such high-performance ceramics can be broadly extended provided that suitable, highly efficient joining methods are available for these ceramics. In addition to its use as a constructional ceramic, SiC can principally also be used as a functional ceramic. For this purpose, the basic ceramic is modified with different additives, providing it with electrical properties that permit its application as a full ceramic heat conductor or sensor. In this case, it also holds true that a suitable joining method for making electrically conductive joints will extend the fields of application considerably. Laser-based joining technologies are being developed for both applications at the Dresden University of Technology. The research work presented here notably focuses on laser joining of electrically conductive SiC ceramics. In addition to a CO2 laser, a diode laser has been used. Basically, electrical connection has been made in two ways. In the first variants, graphite pins are inserted into the joining zone as electrically conductive bridges. In an alternative concept, the oxidic glass filler itself is made electrically conductive with additives. Like that a full ceramic heating conductor joined by means of laser radiation has been tested. The temperature resistance and functionality of the laser-joined heating conductor could be fully demonstrated.Copyright

Laser Joining of Ceramics: A Contribution to High Temperature Range Application of Ceramic Components

Non-oxide ceramics, such as silicon carbide (SiC) and silicon nitride (Si3 N4 ), have excellent properties that make the materials interesting for application also in the nuclear sector. Due to their exceptional resistance to high temperatures, aggressive and abrasive media as well as nuclear radiation, the materials seem to be particularly suitable for developments in such fields as high-temperature reactors ((V)HTR) and peripheral systems (e.g. for hydrogen production). To simplify and thus to enable the technical application of these high-tech ceramics, the Dresden University of Technology has developed a laser beam joining process. This opens up many possibilities, e.g., to encase HTR fuel elements (as well as spheres and composites) in SiC, to encapsulate highly radioactive waste in SiC or to build a highly efficient heat transformer using high-temperature energy from VHT reactors. The progress made in laser beam technology in the last few years is a major element that has contributed to the developments achieved to date. Research has been focused mainly on the following three areas: (1) optimization of the laser parameters in combination with the advancement of oxide brazing fillers, (2) transfer of the basic technology to other high-tech ceramics like oxide ceramics, and (3) application of the laser process to develop electrically conductive joints. The possibility to laser join also Al2 O3 and ZrO2 ceramics has created the opportunity to produce full ceramic sensors for (V)HTR specific applications at low cost. This requires adaptation of laser technology to the special properties of oxide ceramics. These are markedly less resistant to thermally induced stress than non-oxide ceramics, placing high requirements on laser process control. Another peculiarity is the property of oxide ceramics to be partly transparent to the laser wavelengths emitted by diode lasers (808 nm and 940 nm), with the result that the ceramic material is not heated primarily at the surface but inside its volume. This produces joint seams inside ceramic components even without any excessive thermal stress. The R&D work has made it possible to produce novel sensors for the high-temperature range that are also highly resistant to aggressive media. It is considered a further advantage that this joining technology has no special requirements regarding the process atmosphere such as vacuum or inert gas, which ensures that the process lends itself well to automation.Copyright