Heiko Barnert

The Re-Evaluation of the AVR Melt-Wire Experiment Using Modern Methods With Specific Focus on Bounding the Bypass Flow Effects

The AVR (Arbeitsgemeinschaft Versuchsreaktor) is a pebble bed type helium cooled graphite moderated high temperature reactor that operated in Germany for 21 years and was closed down in December 1988 [1]. The AVR melt-wire experiments [2], where graphite spheres with melt-wires of different melting temperatures were introduced into the core, indicate that measured pebble temperatures significantly exceeded temperatures calculated with the models used at the time [3]. These discrepancies are often attributed to the special design features of the AVR, in particular the control rod noses protruding into the core, and to inherent features of the pebble bed reactor. In order to reduce the uncertainty in design and safety calculations the PBMR Company is re-evaluating the AVR melt-wire experiments with updated models and tools. 3-D neutronics thermal-hydraulics analyses are performed utilizing a coupled VSOP99-STAR-CD calculation. In the coupled system VSOP99 [4] provides power profiles on a geometrical mesh to STAR-CD [5] while STAR-CD provides the fuel, moderator and solid structure temperatures to VSOP99. The different fuel histories and flow variations can be modelled with VSOP99 (although this is not yet included in the model) while the computational fluid dynamics (CFD) code, STAR-CD, adds higher-order thermal and gas flow modelling capabilities. This coupling therefore ensures that the correct thermal feedback to the neutronics is included. Of the many possible explanations for the higher-than-expected melt-wire temperatures, flow bypassing the pebble core was identified as potentially the largest contributor and was thus selected as the first topic to study. This paper reports the bounding effects of bypass flows on the gas temperatures in the top of the reactor. It also presents preliminary comparisons between measured temperatures above the core ceiling structure and calculated temperatures. Results to date confirm the importance of correctly modelling the bypass flows. Plans on future model improvements and other effects to be studied with the coupled VSOP99-STAR-CD tool are also included.Copyright

Design evaluation of a small high-temperature reactor for process heat applications

Abstract The high-temperature reactor is also suitable for process heat application, in particular in smaller size units. On the basis of the results of R & D and demonstration work on coal refinement some improvements for small high-temperature reactor for process heat applications are discussed. These are: increase of the gas outlet temperature, the gas inlet temperature, the decrease of the overall system pressure, and non-integration of the primary circuit components. Some of these design evaluations are based on the results of the pre-project for the extension of the AVR-reactor in Julich to be used as a process heat plant.

Advanced pebble bed high temperature reactor with central graphite column for future applications

Abstract Design evaluations of the advanced pebble bed high temperature reactor, AHTR, with central graphite column are given. This reactor, as a nuclear heat source, is suitable for coal refinement as well as for electricity generation with closed gas turbine primary helium circuit. With this design of the central graphite column, it is possible to limit the core temperatures under the required value of about 1600°C in case of accident conditions, even with higher thermal power and higher core inlet and outlet temperatures. The designs of core internals are described. The after heat removal system is integrated in the prestressed concrete reactor pressure vessel, which is based on the principals of natural convection. Research work is being carried out, whereby the spherical fuel elements are coated with a layer of silicon carbide, to improve the corrosion resistance as well as the effectiveness of the fission products barrier.

New perspectives and pre-feasibility results of the process heat HTR work

Some pre-feasibility results from the design evaluations of the advanced high temperature reactor AHTR for process heat applications are given, e.g. coating of the spherical fuel elements with silicon carbide as a fission product barrier, design of the central graphite column and the graphite top reflector without any metallic structure, after heat removal system based on natural convection for the primary and secondary circuit, venturi jets to reduce the depressurization rate of the helium in case of damage to the primary ducts etc.. On the basis of these results it is possible in the future, that AHTR can be designed in such a way, that any type of damage of the plant or due to any operator error, poses no hazard to the public. n nWith these design perspectives and attendant simplification of the plant design, it is possible to reduce the investment costs by 10 to 15%, so that the economic competitiveness of AHTR will improve in the future. n nIt has been further shown, that coal gasification with a high temperature reactor reduces CO2-emissions by a factor of about two by the methanol production and also by the electricity generation in comparison to the conventional methods.

After heat removal by a closed secondary cooler circuit on the example of the advanced high temperature reactor AHTR 500 with central graphite column

Abstract The application of natural convection in connection with an after heat removal concept in general supports the claim for an inherent safety concept for advanced high temperature reactors (HTR). The effectivity of such an after heat removal (AHR) concept will be explored exemplarily for the process-heat reactor AHTR 500 with central graphite column by a thermohydraulic simulation of a secondary cooler circuit which is thermally connected with the primary circuit. This coupling is undertaken by an AHR-cooler located in the upper part of the graphite column. The heat removal from the secondary circuit is taking place outside of the reactor by a secondary heat exchanger under the assumption that the latter is cooled by a water capacity flow on an ambient temperature level. The developed calculation model determines iteratively the dynamic and thermal positions of equilibrium in the primary and secondary circuit which in the after heat removal mode of operation are exclusively run by natural convection. Different types of design for the central column heat exchanger (coaxial tube, U-tube and helically coiled tube heat exchanger) have been compared. For the secondary heat exchanger a parallel tube design has been supposed. The choice of the secondary flow medium as well as the most important limiting quantities influencing the transmission of heat via the secondary circuit during the after heat removal mode of operation are subject of a parameter study.