The modular Pebble Bed Advanced High Temperature Reactor (PB-AHTR), with a nominal power output of 900 MWth, is the most recent US Berkeley design for a liquid fluoride salt cooled reactor. Due to the high volumetric heat capacity of the primary coolant, the PB-AHTR operates with a high power density core with a similar average coolant temperature as in modular helium reactors. The reactivity control system for the PB-AHTR uses a novel buoyantly-driven shutdown rod system that can be actively or passively activated during reactor transients. In addition to a traditional active insertion mechanism, the new shutdown rod system is designed to also operate passively, fulfilling the role of a reserve shutdown system. The physical response of the shutdown rod was simulated both computationally and experimentally, using scaling arguments where applicable, with an emphasis on key phenomena identified by a preliminary PIRT study. This paper discusses preliminary results from this effort.

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