The SCDAP/RELAP5/ATHENA code was extended to enable the code to perform transient analyses of two leading candidates for the next generation of nuclear power plants, namely the Pebble Bed and Block-Type (prismatic) High Temperature Gas Reactors (PB-HTGRs and BT-HTGRs). Models in five areas of reactor behavior were implemented into the code. First, models were implemented to calculate the transfer of decay heat by conduction and radiation from the center of the heterogeneous cores of these reactors to their outer surfaces. Second, models were implemented to calculate the transfer of decay heat from the outer surfaces of these reactor cores to ultimate heat sinks such as the atmosphere. Third, models were implemented to calculate the flow losses and convective heat transfer in the core of a PB-HTGR. Fourth, models were implemented to calculate the oxidation of graphite due to the ingress of air. Fifth, a model was implemented to calculate the ingress of air into the reactor core by molecular diffusion from the point of break in the coolant system. The extended code was applied to the analysis of conduction cooldown accident in a BT-HTGR. The analysis indicated that the new models in SCDAP/RELAP5/ATHENA are properly interfaced with the previously developed and assessed fluid behavior models in the code. Preliminary results indicate that post-shutdown decay heat can be adequately removed from BT-HTGRs by natural circulation of air from the atmosphere.

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