Semi-two-dimensional transient modeling of decay heat removal to an external reactor cavity cooling system for a gFHR with effects of heat generation pattern

In prior work, we introduced a prototypical natural circulation water-based reactor cavity cooling system (RCCS) design for a pebble-bed generic Fluoride-salt-cooled High-temperature Reactor (gFHR) based on a onedimensional (1-D) model with evaluation of decay heat removal at limiting states. The 1-D steady state model is useful at the preliminary design stage, but it is subject to many limitations that can affect its predictive capabilities when system performance and temperature distribution are concerned. In the present work, we extend the previous model to a two-dimensional (2-D) model to examine the effects of reactor axial power distribution, initial in-core axial temperature distribution, axially varying reflector thickness, and axially varying temperature-dependent thermophysical properties. The model is considered semi-2D as only the radial heat transfer from the core to the RCCS is explicitly considered. The present model integrates the decay heat generation, storage, and removal in time to predict the transient performance of the RCCS and estimate the effective temperature distribution of different components in the system as a function of time. A key finding of this work is that the 1-D steady model produces relative RCCS performance results consistent with the more complex 2-D transient models with varying power and initial temperature distribution despite substantial differences in the values predicted for the performance and temperature distribution in the system. This makes the 1-D modeling suitable for RCCS design optimization only when operating far from temperature limits.