For the further development of nuclear fission reactors, the feasibility of a new concept is evaluated. It concerns a fluidized bed reactor in which carbon particles with a uranium core are fluidized and cooled by a high velocity pressurized helium flow. Nuclear reaction takes place if the bed is in fluidized conditions at a void fraction above 80% and it stops as soon as the bed is unfluidized, i.e. when no coolant flow is available. The feasibility has been evaluated by design calculations of the nuclear reactor physics and heat transfer, and by an experimental study of an appropriate large particle high velocity fluidized bed. During the experimental study, pressure measurements and gamma-ray transmission measurements have been performed along the height of a Geldart D powder fluidized bed in a 38 cm diameter column. From these measurements, axial void fraction profiles have been calculated, the regimes have been analysed and the frequencies of the major bed fluctuations have been determined.