Wolfgang Lippmann

Investigation of the use of ceramic materials in innovative light water reactor – fuel rod concepts

Abstract The RWTH Aachen and the TU Dresden have undertaken a joint research effort, the goal of which is the development of innovative fuel rods which would constitute a high-melting reactor core. An additional sintered silicon carbide (SSiC)-encasement of the UO 2 pellets within the zircaloy cladding was planned. Various designs for the construction of the absorber rods were developed in order to achieve a failure temperature in excess of 1200°C. At the RWTH Aachen, a series of depleted UO 2 pellets were enclosed in gastight SSiC capsules through reaction sintering. The capsules were checked for leaks, and their thermomechanical behavior was analyzed after thermal strain; the capsules were heated to 1800°C (maximally 2070°C) in oxidizing water vapor as well as in air. Further series of experiments were conducted in order to examine the chemical behavior of the SSiC pellets in the presence of various reactor component materials at high temperatures. SSiC was heated to 1800°C while in contact with the following substances: zircaloy, steel, corium material, UO 2 , Ag–In–Cd-alloy, HfO 2 , Dy 2 O 3 , Gd 2 O 3 , Sm 2 O 3 , BN, and B 4 C. With the exception of steel and corium material containing steel, the substances proved relatively inert in reactions with SSiC, such that their use in combination with SSiC can be judged to be favorable.

Biotechnological hydrogen production by photosynthesis

Microbiological photosynthesis is a promising tool for producing hydrogen in an ecologically friendly and economically efficient way. Certain microorganisms (e.g. algae and bacteria) can produce hydrogen using hydrogenase and/or nitrogenase enzymes. However, their natural capacity to produce hydrogen is relatively low. Thus, there is a need to optimize their core photosynthetic processes as well as their cultivation, for more efficient hydrogen production. This review aims to provide a holistic overview of the recent technological and research developments relating to photobiological hydrogen production and downstream processing. First we cover photobiological hydrogen synthesis within cells and the enzymes that catalyze the hydrogen production. This is followed by strategies for enhancing bacterial hydrogen production by genetic engineering, technological development, and innovation in bioreactor design. The remaining sections focus on hydrogen as a product, that is, quantification via (in‐process) gas analysis, recent developments in gas separation technology. Finally, a discussion of the sociological (market) barriers to future hydrogen usage is provided as well as an overview of methods for life cycle assessment that can be used to calculate the environmental consequences of hydrogen production.

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

Ceramic High-Temperature Heat-Pipes

Ceramic heat pipes and heat pipe based heat exchangers are tailored for automatically heat removal and heat distribution in thermally, chemically and abrasive high stressed systems. The manufacture of silicon carbide heat pipes was carried out. These were filled with sodium or zinc and sealed by laser brazing using metallic and glassy solder materials. High-temperature performance tests revealed a stable operating regime for both ceramic heat pipes with sodium and zinc as working fluid, respectively. Specifically the heat transferred by a zinc filled heat pipe of 22 mm in diameter and 750 mm in length accounted for 600 W at a temperature difference of 400 K. Notably the internal heat transfer capacity of the working fluid was even higher however, the total heat transfer was limited by the external active heat transfer area of the heat pipe. In order to evaluate the long-term stability of the heat pipes, particularly with respect to the joining seam, manufactured heat pipes are currently being tested in long-term annealing experiments at a temperature of 1000 °C under a variety of corrosive atmospheres.Copyright

Development of a Manipulator-Supported Laser Decontamination System

Efficient decontamination technologies are needed for both decommissioning and safe operation of nuclear power plants. Up to now room walls as well as other structures contaminated with radionuclides have mainly been decontaminated using mechanical removal processes. The aim of such processes is to remove the wall surfaces to a contaminated depth of several millimeters. This generates a large amount of dust, which can lead to secondary contamination, and is associated with high personnel and/or technical expenditures. Advances in high-power lasers allow the use of laser radiation to remove contaminated layers. These layers are melted to depths of up to 5 mm by a laser beam. Most of the radionuclides are dissolved in the melt. The molten layer is detached from the wall by pulsed compressed air jets and solidifies to form small particles in which the radionuclides are fixed. The particles are then suctioned up by means of a negative pressure. The advantages of the innovative process are based on minimal dust formation and force-free coupling of the laser unit and the wall, allowing a very lightweight and flexible overall design of the system. Because the laser processes can easily be automated and controlled by remote control, the personnel exposure time within contamination areas can be minimized in an efficient manner.The present report describes the development and testing of a laser decontamination system for the removal of radioactively contaminated concrete wall layers. A diode laser with a power of 10 kW is used as the radiation source. The laser energy is delivered via a fiber optic cable up to 100 m in length to the laser tool, which is situated on a specially designed autonomous manipulator. The manipulator moves over the wall surface to be processed by means of pneumatic suction plates. For velocities of 400 mm/min and a removal depth of 2 to 3 mm, a removal rate of 1.2 m2 per hour could be achieved. The laser focus area on the processed concrete surface is 10 mm × 45 mm. Using today’s commercially available lasers with powers of > 50 kW, removal rates higher than 6 m2 per hour are possible.Development efforts revolving around the presented technology are focused on two main areas which can be understood as spinoffs of the present research: increasing the removal rate using more powerful lasers and adapting the technology for use in decontamination of chemically contaminated surfaces, e.g., for pollutant-free stripping of PCB-containing wall coatings.Copyright

Laser Decontamination of Paint Coated Concrete in Nuclear Plants

Laser technology offers an efficient decontamination of surfaces contaminated by Polychlorinated Biphenyls (PCB). PCB is a component of protective coatings on concrete walls in NPP’s up to the 1980s-years. State of the art is a manual and mechanic ablation, which then poses danger of a secondary contamination and needs a second treatment in a hazardous waste incinerator. A 10 kW diode laser in continuous wave mode and wavelength of 915–1030 nm rises up the surface temperature for ablation of coatings and thermal decomposition in one process step. Meanwhile, decontamination rates of 6.345 m2/h have been operated using a laser spot of 45 × 10 mm2. An experimental facility with a three-barrier-system to contain toxic material has been designed and constructed. First experimental investigations use epoxy wall coatings for an optimization of the process and to calculate a simulation of the ablation process. Further experimental investigations for ablation of PCB-coatings can be operated safely. A second development is an in-situ measurement system to detect the thermal decomposition of PCB by a laser induced fluorescence (LIF) system by the project partner TU Bergakademie Freiberg (TUBAF), Germany.Copyright