Development and Applications of Jump-In Equilibrium Modeling Methods for Pebble-Bed High-Temperature Gas-Cooled Reactors

Pebble-bed high-temperature gas-cooled reactors (PB-HTGRs) are one of the proposed Generation IV reactor designs that have garnered significant interest in recent years from both private industry and government. PB-HTGRs offer many advantages, including online refueling capabilities and higher operating temperatures. However, high-fidelity computational modeling of pebble-bed reactor (PBR) designs, from reactor start-up to operation at equilibrium, is more challenging compared to conventionally fueled reactors due to the continuous movement of fuel pebbles through the reactor during operation. Jump-in equilibrium modeling methods were developed to reduce the computational burden associated with generating equilibrium models of PBRs. This work will first seek to identify a set of simplifying assumptions that can be used when performing jump-in equilibrium modeling. The focus of these simplifying assumptions will be on the appropriate fidelity of the inclusion of temperature, core representation during transport and depletion calculations, core discretization, burnup, and flow speed variation. As part of identifying these simplifying assumptions, the Axial Zone Equilibrium Modeling (A-ZEM) method and the Axial-Radial Equilibrium Modeling (AR-ZEM) method were developed as iterations upon a previously established jump-in equilibrium modeling method developed at Oak Ridge National Laboratory. After identifying the appropriate simplifying assumptions for different applications of jump-in modeling, this work will explore the available applications of these methods for PBR modeling and the advantages offered by jump-in equilibrium modeling compared to other higher-fidelity methods. The applications investigated include the use of composite moderators, transient accident analysis, transuranic fuel use, and scenarios involving pebble misuse.