Modeling and Transient Analysis for the Modular Pebble-bed Advanced High Temperature

The Pebble Bed Advanced High Temperature Reactor (PB-AHTR) is a liquid-salt cooled reactor that uses conventional TRISO fuel. Due to the high thermal inertia of the liquid salt coolant, the PB-AHTR allows for a much higher power-density core than traditional gas-cooled high temperature reactors. High power density has a favorable economic impact. In 2007, the baseline design 2400MWth PB-AHTR was modified to incorporate a modular core with smaller pebbles (between 3 to 4 cm in diameter) flowing inside a large number of separate channels, inside a set of graphite reflector blocks. This configuration has a number of potential advantages over the large, homogenous core that was studied previously. The addition of graphite reflectors allows for a further increase in heavy metal loading, therefore reducing the number of pebbles required and lowering fuel costs and spent fuel volume. A series of PB-AHTR transients were analyzed using the thermal-hydraulics code RELAP5-3D with the intent of determining an acceptable core power-density range while still meeting criteria for peak fuel and structural material temperatures. The range under consideration (between 15 to 30 MW/m 3 ) is much higher than the power density of a typical gas-cooled reactor core (4.8 to 6.5 MW/m 3 ). The initiating events considered were Loss of Forced Cooling (LOFC) transients as well as an Anticipated Transient Without Scram (ATWS) that consists of a LOFC transient without scram. Results presented in this paper show that the PB-AHTR response to the LOFC is very promising. Peak temperatures during an ATWS are also within the ASME code temperature range for high temperature alloys such as Alloy 800H. Results from parametric studies determining the optimal design for the decay heat removal system and subsequent configurations for the core and pebbles are presented.