Pebble-bed core design option for VHTRs - Core configuration flexibility and potential applications

Gas-cooled nuclear reactors have been receiving specific attention for Generation IV possibilities due to desired characteristics such as relatively low cost, short construction period, and inherent safety. Attractive inherent characteristics include an inert, single phase helium coolant, refractory coated fuel with high temperature capability and low fission product release, and graphite moderator with high temperature stability and long response times. The passively safe design has a relatively low power density, annular core, large negative temperature coefficient, and passive decay heat removal system. The objective of the U.S. DOE NERI Project is to assess the possibility, advantages and limitations of VHTRs with fuel loadings containing minor actinides. This paper presents the analysis of pebble-bed core configurations. Whole-core 3D models for pebble-bed design with multi-heterogeneity treatments in SCALE 5.0 are developed to compare computational results with experiments. Obtained results are in agreement with the available HTR-10 data. Actinide fueled VHTR configurations reveal promising performance. With an optimized pebble-bed model, the spectrum shifting abilities become more apparent. Effects of altered moderator to fuel ratio, Dancoff factor, and core and reflector configurations are investigated. This effort is anticipated to contribute to a facilitated development of new fuel cycles in support of future operation of Generation IV nuclear energy systems. (authors)