Cocurrent downward flow of a settled bed of particles in a liquid medium through a straight tube with a restricting orifice at the lower end was studied in connection with a “paste” type mobile fuel system developed by Atomic Power Development Associates, Inc. for a fast breeder reactor. It was found that the excess liquid flow rate around the particles, and other related physical quantities, can be satisfactorily evaluated through already existing moving bed correlations. However, in order to determine the absolute, rather than excess or relative, flow rates of both liquid and particles, a new understanding and correlation are needed by which the flow through the restricting orifice of the system can be coupled with the flow in the straight tube section above the orifice. New correlations, using two dimensionless quantities, i.e., and effluent paste density expressed as the ratio of particle flow rate to total flow rate, and an “in-orifice” modified Reynolds' number, were developed for both wetted orifice (i.e., paste discharging through an orifice into a liquid phase) and nonwetted orifices (paste discharging through an orifice into a gaseous phase). Square edge orifices were employed as well as tapered edge orifices. The ranges of the principal variables covered experimentally are as follows: particle size: 60 to 325 mesh (1.7 mils to 9.8 mils); particle density: 2.6 gm/cc to 18.9 gm/cc; liquid viscosity: 0.004 to 0.2 cm2/sec; particle flow rate: 5 to 40 cc/min; orifice diameter: 0.075 in. to 0.199 in.