Flow Dynamics in a Fuel Bundle With a Particle Bed

Abstract The pressure reduction in the RCS and temperature excursion of the cladding during a postulated loss-of-coolant accident (LOCA) can lead to cladding rupture. Fuel fragmentation can occur at any burnup, but the fuel fragment sizes decrease with increasing burnup, thereby increasing the potential for relocation and dispersal via the rupture opening. Research is ongoing to better understand fuel fragmentation, relocation, and dispersal (FFRD). However, the research to date is focused largely on understanding the mechanisms generating the fragmentation and quantifying the resulting fragment sizes and amounts within the fuel rod. There is a need to understand the consequences of the dispersal from the fuel rods. Some of the dispersed particles can settle onto a spacer grid below the rupture location forming a particle bed. Framatome is researching the behavior of fluid flow through and around a particle bed that has the characteristics of fuel particles that have been dispersed from rupture locations within a fuel bundle. Understanding the behaviors and changes in the flow patterns caused by the presence of the particle bed are of importance as they relate to the cooling behavior of the fuel pins during a LOCA transient. The presence of the particle bed can either divert steam and droplet flow away from the location near the particle bed region or may slow the flow of steam and droplets through the particle bed. A reduction of flow in the local region of the particle bed may adversely impact the cladding temperature during the transient. Tests for the benchmarking of flow patterns and cooling dynamics for these kinds of conditions are limited. Consequently, Framatome has performed full scale fuel bundle flow testing to provide needed benchmarking data. The testing includes bundle geometrical aspects under scaled LOCA conditions which include clad swelling, rupture locations, spacer grids, mixing grids, the presence of a particle bed, and entrance and exit effects. Multiple sizes of particles including mixtures of sizes are taken into account as well as multiple bed depths and different bed formations in the testing. Additional testing has been performed to establish baseline characteristics of fuel particle beds in general (fluidization behavior, packing fractions, etc.). The data collected has provided results leading to in-depth understanding of the flow patterns and pressure drops in and around the particle bed with a high level of detail. This data can then be used to develop and benchmark tools capable of simulating the flow patterns within a nuclear fuel bundle with a particle bed. An increased understanding of how steam and droplets interact with a particle bed in the geometrical conditions found in a nuclear fuel bundle is a requirement for developing new safety methods.