Dispersion of Surrogate LWR Fuel Experiments Under LOCA Conditions

In this study a series of experiments were performed subjecting surrogate nuclear fuel rods to high-pressure transients to induce fuel dispersion representative of the expected conditions of a fuel rod during a hypothetical loss-of-coolant accident. Experiments were conducted on like-for-like pressurized water reactor geometries in both a single-rod and rod-bundle configuration. In the rod-bundle configuration, a matched index of refraction techniques was employed to provide optical access to the bundle internals and to view the surrogate fuel dispersion event. Both configurations used small lead pellets as a surrogate fuel and were observed with a high-speed camera to capture the transient on a resolved timescale. For the single-rod experiments, the test rod was subjected to pressure transients at 4.0, 8.0, and 12.0 MPa multiple times, and for the rod-bundle experiments, the rod was subjected to 8.0 MPa transients in order to compare mechanical behavior against the single-rod test at 8.0 MPa. For both configurations, the results showed highly variable behavior in both the quantity of fuel dispersed and the mean displacement relative to the burst rod origin, likely due to statistical variations in the internal fuel stack orientation. Measurements of the rod plenum internal pressure showed no discernible difference in depressurization rates at a given pressure, indicating the likelihood that the mass flow rate is limited by the valve orifice in the current experimental configuration. The bundle tests also showed that a 5 × 5 array appears to be too small to capture the full spatial distribution of dispersed fuel, thus future tests will employ a larger bundle size and particle collection technique.