SUMMARYCore bypass flow has been one of key issues in the very high temperature reactor (VHTR) design for securing core thermal margins and achieving target temperatures at the core exit. The bypass flow in a prismatic VHTR core occurs through the control element holes and the radial and axial gaps between the graphite blocks for manufacturing and refueling tolerances. These gaps vary with the core life cycles because of the irradiation swelling/shrinkage characteristic of the graphite blocks such as fuel and reflector blocks, which are main components of a core's structure. Thus, the core bypass flow occurs in a complicated multidimensional way. The accurate prediction of this bypass flow and counter-measures to minimize it are thus of major importance in assuring core thermal margins and securing higher core efficiency. Even with this importance, there has not been much effort in quantifying and accurately modeling the effect of the core bypass flow.The main objectives of this project were to generate experimental data for validating the software to be used to calculate the bypass flow in a prismatic VHTR core, validate thermofluid analysis tools and their model improvements, and identify and assess measures for reducing the bypass flow. To achieve these objectives, tasks were defined to (1) design and construct experiments to generate validation data for software analysis tools, (2) determine the experimental conditions and define the measurement requirements and techniques, (3) generate and analyze the experimental data, (4) validate and improve the thermofluid analysis tools, and (5) identify measures to control the bypass flow and assess its performance in the experiment.
Design and construction of experimental facilities:The experimental designs, which consist of the air test led by the Korean Atomic Energy Institute (KAERI) at Seoul National University (SNU) and the matched-index-of-refraction (MIR) and air tests led by Idaho National Laboratory (INL) at Texas A&M University (TAMU), were completed in FY 2008. The air tests use air as a working fluid to obtain data for the validation of core thermofluid analysis codes; the MIR experiments are designed to produce data for the validation of computational fluid dynamics (CFD) codes. Based on the experimental designs, the construction of the multiblock air test facility and the MIR test facility were completed in FY 2009. A considerable portion of the experimental design efforts were directed at precisely controlling the bypass gap sizes and also the laser diagnostics designed to produce the validation data.
Determination of experimental conditions and development of measurement techniques:A methodology for estimating gap size distribution in the prismatic cores was developed by KAERI in FY 2008 and applied to a reference prismatic core. The results were used in selecting the gap size and distribution in the test matrix. The influence of core restraint mechanism and replacement of side reflector on bypass gap distribution and hot spot fuel temperature was inv...