SUMMARY

The Advanced High Temperature Reactor (AHTR) being considered in this study is a Fluoride-Salt-Cooled High-Temperature Reactor (FHR) developed by ORNL. This reactor design is based on a hexagonal prismatic fuel element 5.5 meters tall, it is moderated by graphite and the fuel form is a dispersion of TRISO particles into a graphite matrix.
The AHTR reference design is based on a traditional batch refueling approach, which requires to shut down the reactor and replace/reshuffle a certain amount of fuel assemblies in the core at a specific frequency. Several options have been evaluated in the design process, in order to maximize the lifetime of a single batch and optimize the use of fuel. However, the relatively short cycle and poor fuel utilization are intrinsic features of this family of reactors, due to the low heavy metal loading in the core and insufficient moderation. It must be noted that the heavy metal loading and the moderation are competing aspects: an increase of the heavy metal loaded into the core would reduce the amount of volume available for moderation, and vice versa. Since the fuel is expected to be more expensive than the fuel of light water reactors, this issue might challenge the economic viability of the AHTR.
The overarching objective of this work is the development of a novel approach to refueling that will eliminate or at least ameliorate the problem presented in the previous paragraph. The approach that is intended to be analyzed in this work is the continuous on-power refueling, or on-line refueling. The refueling procedure is performed at full power or at almost full power (the reactor is not shut down) and a single assembly is removed per each refueling operation.
This study will perform neutronic and thermal-hydraulic analysis of the on-line refueling, followed by an evaluation of the mechanical design of the process and the quantification of the economic advantages deriving from the implementation of this procedure.