Wolfgang Hering

Overview of the HCPB Research Activities in EUROfusion

In the framework of the EUROfusion’s Power Plant Physics and Technology, the working package breeding blanket (BB) aims at investigating four different BB concepts for an EU demonstration fusion reactor (DEMO). One of these concepts is the helium-cooled pebble bed (HCPB) BB, which is based on the use of pebble beds of lithiated ternary compounds and Be or beryllides as tritium breeder and multiplier materials, respectively, EUROFER97 as structural steel and He as coolant. This paper aims at giving an overview of the EU HCPB BB Research and Development (R&D) being developed at KIT, in collaboration with Wigner-RCP, BUTE-INT, and CIEMAT. The paper gives an outline of the HCPB BB design evolution, state-of-the-art basic functionalities, requirements and performances, and the associated R&D activities in the areas of design, functional materials, manufacturing, and testing. In addition, attention is given also to the activities dedicated to the development of heat transfer augmentation techniques for the first wall and the corresponding testing. Due to their nature as design drivers, a brief overview in the R&D of key HCPB interfacing areas is given as well, namely, the tritium extraction and recovery system, the primary heat transfer and power conversion systems, and safety topics, as well as some specific activities regarding the integration of in-vessel systems through the BB. As concluding remarks, an outline of the standing challenges and future R&D plans is summarized.

Validation of TRACE in the Field of Liquid Metal Heat Transfer

The validation of system codes like TRACE is an ongoing task especially in areas with limited or almost no application like liquid metal flow. Therefore, extensive validation efforts are necessary to increase the confidence in the code predictions. TRACE has been successfully validated and applied to lead-alloy cooled systems. The results gained with lead-alloy coolants could be extrapolated to other liquid metals with the necessary care. Nevertheless, dedicated investigations with the different liquid metals are mandatory to confirm the extrapolations. In the present case, the validation work focuses on liquid metal heat transfer in pipes and rod bundles under forced convection. To take advantage of a greater data base, several liquid metals have been implemented into the code. In addition, new coolants allow supporting analysis of liquid metals loops which are in the design or construction stage. Concerning the validation, several experiments have been found, conducted by other investigators, which are modeled with the modified TRACE version. The results indicate that the chosen heat transfer models for pipe and bundle flow are applicable. In case of deviations, physical sound reasons can be provided to explain them.Copyright

Investigation of Local Heat Transfer Enhancement in Generic Liquid Metal–Cooled Fuel Assemblies with Empirical Models [in press]

Abstract The heat transfer in liquid metal–cooled rod bundles is modeled with a knowledge-based best-estimate system code. Thereby, the focus is on the heat transfer enhancement due to flow perturbations. These perturbations are caused by local geometrical variations, such as sudden expansions and contractions, in the flow channel. The accurate calculation of the heat transfer is important for the safety demonstration of, e.g., subassemblies. Safety-related parameters, such as fluid and wall temperature, have to satisfy certain limits during normal and off-normal operation as well as during accidents. Up to now, fully developed flow is assumed for heat transfer in liquid metal–cooled rod bundles. The effects of local heat transfer enhancements were ignored in best-estimate system codes. The currently used empirical heat transfer models are functions of the Péclet number only. Several experimental and numerical investigations show that flow perturbations induce higher heat transfer due to increased turbulences, accelerated flows, and secondary motions. In this paper, the effects of the entrance region and the presence of spacer grids on the heat transfer are investigated. Empirical models for that are selected and applied. These empirical models are functions of the Péclet number, the geometrical perturbation, and the distance from the perturbation in the flowing direction. The calculated heat transfer coefficients at the bundle entrance and in the vicinity of spacer grids are twice as high compared to bare rod bundles under a fully developed flow condition without any flow perturbation. Because of the higher heat transfer, lower wall temperatures are to be expected. This provides additional safety margins during normal and off-normal operation as well as during accidents. Furthermore, the considerable increase of heat transfer shows that existing perturbations have to be considered to obtain accurate and reliable results.

Development of high temperature liquid metal test facilities for qualification of materials and investigations of thermoelectrical modules

Three classes of experimental liquid metal facilities have been completed during the LIMTECH project aiming the qualification of materials, investigation of thermoelectrical modules, investigation of sodium transitional regimes and fundamental thermo-dynamical flows in concentrating solar power (CSP) relevant geometries. ATEFA facility is dedicated to basic science investigation focussed on the alkali metal thermal-to-electric converter (AMTEC) technology. Three SOLTEC facilities are aimed to be used in different laboratories for long term material investigation sodium environment up to a 1000 K temperature and for long term tests of AMTEC modules. The medium scale integral facility KASOLA is planned as the backbone for CSP development and demonstration.

Design and construction of the ATEFA facility for experimental investigations of AMTEC test modules

The Alkali Metal Thermal-to-Electric Converter (AMTEC) is an electrochemical cell that requires a high temperature heat source to generate electricity. At KIT the AMTEC technology is being investigated focusing on the use of concentrating solar energy as heat source. First a review on AMTEC technology is given. Further, the design and realization phases of the AMTEC Test Facility (ATEFA) and AMTEC test cell are presented, including the data acquisition and control system and two key technology developments: a ceramic to metal joint for high temperatures (800 – 1000 °C) and the magnetron sputtering of cathode layers on the ceramic electrolyte. The sheet resistance of several electrode samples has been analyzed using the 4-point probe technique and the microstructure of the cathode layer has been examined using the scanning electron microscopy (SEM).