Conceptual design, experiments, and analysis for the core of an FHR-16 test reactor

Advanced high temperature reactors (AHTRs) use liquid fluoride salt as a coolant, along with high-temperature, coated particle fuel in pebble form. They also use a modular Direct Reactor Auxiliary Cooling System (DRACS) for decay heat removal, that allows this class of reactors to be scaled with only small design changes from small to large sizes. A key step in the development of the technology involves the construction of a test reactor, to provide an integrated test of the reactor safety systems and fuel. This report discusses the design of a radially-zoned, annular pebble core for a fluoride cooled, high temperature test reactor (FHR-16), designed to have a nominal power output of 16 MWth. Furthermore, it discusses specific issues involving design and radial zoning of a three-fuel-zone annular reactor core. A scaled pebble recirculation experiment for a 30 deg. sector of the FHR-16 core was constructed and tested using high-density polyethylene spheres to simulate fuel pebbles. Pebble zones within the core consist, from the outside toward the inside, of a radial layer of inert graphite blanket pebbles, then thorium seed pebbles, and finally low-enriched uranium (LEU) driver fuel pebbles, layered around a solid center graphite reflector. Experimental and numerical simulation results discussed herein illustrate the feasibility of keeping distinct radial pebble regions within the core throughout the entire height of the test section. In addition, a two-dimensional numerical simulation of granular flow and mixing is presented to benchmark our hypothesis that separate distinct regions are feasible with this core geometry. Neutronic simulations were performed to confirm that criticality and substantial fuel burn up can be achieved using low-enriched (<20%) uranium fuel with thorium blankets. Lastly, a sensitivity study of the effects of fuel element mixing is presented to demonstrate how the neutronic behavior of an FHR-16 reactor changes due to mixing of pebbles between radial zones. (authors)