Conceptual design of a fluidized bed nuclear reactor

A new type of nuclear reactor is presented that consists of a graphite-wall ed tube partly filled with TRISO-coated fuel particles. Helium is, used as a coolant that flows from bottom to top through the tube, thereby fluidizin g the particle bed. Only when the coolant flow is large enough does the rea ctor become critical because of the surrounding graphite that moderates and reflects the neutrons. The fuel particle designed for this reactor is stro ngly undermoderated and has a temperature coefficient of reactivity that is sufficiently negative. The outer diameter is 1 mm with a fuel kernel diame ter of 0. 26mm. The fuel enrichment (16.7%) and the core inventory (120 kg of uranium) inherently limit the maximum power to 16 MW(thermal). A lumped- temperature point-kinetics model has been made that describes the fluidizat ion of the particle bed, coupled to the thermal hydraulics and the neutroni cs of the core. The model has been linearized around the stationary solutio n, and the transfer function from coolant mass flow rate perturbations to r eactor power fluctuations has been calculated. From a root-locus analysis, the reactor operation is shown to be stable with respect to small variation s of the coolant mass flow rate around the stationary operation points. Tra nsient analyses with the nonlinear reactor model show that for the three tr ansients considered (a step in the coolant mass flow rate, a decrease of th e coolant inlet temperature, and a loss of heat sink), the fuel temperature remains well below 1600 degreesC. Recommendations are made for further res earch.