(510a) A Multi-Phase Continuum Approach to Modeling the Performance of a Fluidized Bed Nuclear Reactor

The Generation IV International Forum has identified six designs to be considered for the future nuclear power systems to be built and operated in 2030 and forward. Of the six designs, two are gas cooled, two are liquid metal cooled and one is a molten salt reactor. Research in the area of fluidized bed systems is of importance to several of the potential Generation IV reactor designs. The conceptual design of the fluidized bed nuclear reactor proposed by Kloosterman, Golovko, van Dam & van der Hagen (2001) is a simple design consisting of a vertical graphite-walled tube with graphite bottom and top reflectors partially filled with TRISO-coated fuel particles made of UO₂ fuel kernel surrounded by three layers of pyrolytic carbon and silicium carbide modeled as one moderator shell. Helium coolant flows from bottom to top through the reactor to cool the fuel particles and when its flowrate reaches a critical velocity, the particle bed expands. As a result, more neutrons are allowed to escape to the graphite walls where they are moderated and the reactivity of the core increases. However, if the bed expands beyond a critical core height, neutrons start to leak away and reactivity decreases. The expanded core height is strongly dependent on the particle size and density distributions in the core. Moreover, instabilities in the form of bubbles and particle clusters, which can form due to size and/or density distributions, can significantly affect reactivity and core neutronics requiring further investigations.

Fluidized beds are currently used extensively in the petrochemical and pharmaceutical industries and there is extensive chemical engineering modeling work on the steady state and time-dependent hydrodynamics of gas- and liquid-fluidized beds with one particle species (see Jackson, 2000). However, models incorporating particle size and density distributions have been examined to a much lesser extent. This work considers a multiphase approach which makes use of volume-averaged mass and momentum balances for the fluid and particle phases independently and generalizes the monocomponent equations proposed by Anderson & Jackson (1967). The model consists of a multi-solid mixture of fluidized particles consisting of N particle species that mimics the particle size and/or density distribution of the TRISO fuel particles. The linear stability of the fluidized reactor core will be examined using hydrodynamic stability theory and a multi-fluid continuum approach to determine if and when the fluidized reactor core becomes hydrodynamically unstable. A reactor model, consisting of the proposed multi-phase continuum model along with a thermal hydraulics model and a neutron kinetics model, can be combined to investigate the dynamic stability of the reactor and the relationship between the expanded reactor core height, core neutronics, reactivity and coolant mass flowrate.