In this study, two pool blowdown experiments were conducted on simulated PWR fuel rods filled with spherical lead pellets as a surrogate fuel having similar density to UO2. These experiments were performed at conditions similar to those expected during the second heatup phase of a loss-of-coolant accident in a conventional light water reactor. The rods were pressurized with a small volume of nitrogen gas at 4.0 MPa to rapidly expel the surrogate fuel particles through a pre-fabricated rupture in the rod. Subsequent dispersion of the particles was captured with a high-speed camera at an acquisition rate of 800 frames per second in order to properly record the transient at a resolved time scale. Initial images revealed contrasting mechanical behavior in the case of a single rod when compared to that in a representative 5x5 section of a fuel bundle. Pressure history on each experiment showed that there is not a significant difference in the depressurization rates of rods with or without the surrogate fuel. Selected particles were tracked in each experiment and overlaid to visualize the differences in the effective range. For the case of the bundle, most of the particles are deposited in the adjacent subchannels, while for the single rod, the mean free path is much longer. Calculated particle displacement and velocity trajectories are compared against theoretical models based on a force balance of a single particle, and show good agreement, with possible errors arising in the transient nature of the drag coefficient, and uncertainties in particle mass.
