This report details a fully coupled neutronics thermal hydraulics reference plant model for a gas-cooled high-temperature pebble-bed reactor. The multiphysics model is developed on the Nuclear Regulatory Commission's Comprehensive Reactor Analysis Bundle available on the Idaho National Laboratory high-performance computer, which natively and seamlessly couples Griffin, Pronghorn, and the BISON Multiphysics Object-Oriented Simulation Environment-based applications. Griffin provides the reactor physics capabilities, including depletion to the equilibrium core, k-eigenvalue, adjoint, and transient solutions. Pronghorn solves the porous medium equations for the fluid regions and conduction in the solid regions. BISON solves thermal conduction problems for the average and tristructural isotropic pebbles in the pebble-bed core thus providing the fuel and moderator spatial fields for each pebble burnup group. The neutronics feedback relies primarily on fuel and moderator temperatures. This report presents our results for the coupled steady state equilibrium core and a protected loss of flow event. Although this model is prototypical regarding capabilities in the Comprehensive Reactor Analysis Bundle, its results are consistent with published work by the Institute of Nuclear and New Energy Technology in China and other research entities. iii • Javier Ortensi-conceptual development, cross sections, neutronics model development, multiphysics analysis • Sebastian Schunert-conceptual development, code development (Nuclear Energy Advanced Modeling and Simulation), thermal fluids model development This work would not have been possible without the contributions from the Idaho National Laboratory Griffin development team: