(359b) Experimental and Computational Investigations of Plenum-to-Plenum Heat Transfer and Gas Dynamics Under Natural Circulation in a Prismatic Very High Temperature Reactor

The prismatic Very High Temperature Reactor (VHTR) is one of the promising candidates for the Next Generation Nuclear Plant (NGNP). In a typical prismatic block type VHTR design (350 MWth Modular High Temperature Gas-Cooled Reactor (MHTGR)), graphite hexagonal blocks (which are either fuel or reflector blocks) are stacked on top of each other to form columns and the hexagonal arrangements of those columns form the core of the prismatic block type design4,5. Each fuel block has circular holes for fuel and coolant that are aligned axially with those of the other blocks over the entire length of the column. The fuel holes contain the fuel compacts made of the TRISO particles within a graphite matrix, while the coolant holes are aligned axially to form coolant channels. The central and side graphite blocks in the prismatic core are replaceable reflectors while those at the outer periphery are permanent side graphite reflectors placed between the side replaceable reflectors and the core wall. During normal operation, helium enters the reactor from bottom, flows to the upper plenum of the core through the inlet riser holes in the permanent side reflectors, flows through the coolant channels from top to bottom and cools the core, and finally exits through the lower plenum. Natural circulation is one of the passive safety features of prismatic VHTRs to remove core decay heat during off-normal shutdown and accidental scenarios7 (e.g., Loss of Forced Cooling Accident (LOFA)). It transfers the decay heat from the inner (hotter) core to the outer (colder) periphery which will then be dissipated by conduction and radiation heat transfer to the Reactor Cavity Cooling System (RCCS). Natural circulation plays important role during accident scenarios and is characterized as one of complicated phenomena from CFD code validation point of view. Increasingly more attention is been given to CFD codes and to models verification and validation in order to provide reliable and practical tools to design prismatic VHTR and to analyze its passive safety cooling system. Previous findings suggest that natural circulation in simplified geometries cannot be directly extended to VHTR applications and demands experimental benchmark data from a scaled integral and separate effects test facility mimicking VHTR operation during natural circulation.

This study will provide the much needed knowledge to advance the understanding of natural circulation in prismatic blocks and benchmark data to develop simulation methods which can integrate accidental events, and can be used in validation of models used in safety analysis and commercial CFD codes. Advanced and sophisticated techniques such as; fast response flash mounted heat transfer probe, hot wire anemometry, and gaseous tracer technique will be implemented and utilized in a novel way and for the first time in this proposed work that will serve for both separate-effects measurements. The objectives of this study are; to develop a scaled separate-effects experimental setup that can be operated under different intensity of natural circulation, to advance the knowledge and understanding of the natural circulation phenomena in the VHTR core, and to provide experimental benchmark data which is much needed and is missing in the literature for verification and validation of Computational Fluid Dynamics (CFD) codes.

The scaling methodology has been reviewed and already a scaled-down experimental setup has been developed. The setup preserves a scaling down ratio ¼ axially and ¼ radially with reference to Oregon State University – High Temperature Test Facility (OSU-HTTF). Preliminary experimental and computational studies are being carried out for the proper selection of installation locations of the measurement techniques. The preliminary results have shown that a natural circulation is achieved in the setup and there is a good matching between the experimental and computational results.

The gas dynamics and heat transfer measurements could be obtained by using the novel advanced techniques mentioned earlier. The new integrated fast response flash mounted heat transfer probes can measure both the local heat flux and the surface temperature simultaneously. Hot wire anemometry technique can be used to determine local velocity in various location of interest in the core. In addition the gaseous tracer (GT) technique is available for accurately measure the residence time distribution (RTD) in a complex flow structure by injecting impulse or step change gas tracer and monitoring its concentration at various locations in the geometry.