X-ray and Gamma-ray Tomographic Imaging of Fuel Relocation Inside Sodium Fast Reactor Test Assemblies During Severe Accidents

Abstract The present work reports on x-ray and gamma-ray high-spatial resolution computed tomography measurements of the Pin Bundle Metallic Fuel Relocation (PBR) assemblies tested in the Metallic Uranium Safety Experiment (MUSE) facility at Argonne National Lab (ANL). The aim of the study was to characterize fuel relocation structures that develop during severe core accidents pertaining to SFR assemblies. In particular, two test assemblies (PBR-1 and PBR-2) were analyzed; the investigation includes but is not limited to advanced core disruption recreated in the PBR-1 assembly, and cladding breach recreated in the PBR-2 assembly. The x-ray tomography measurements were able to resolve small quantities of relocated fuel; with increased presence of relocated fuel, the x-ray measurements spatially mapped the material but could not resolve the inner regions of these because of the increased photon attenuation. The gamma-tomography measurements showed improved results, resolving the relocated structures in great detail. It is shown that the upper plenum of the PBR-1 assembly where the molten uranium was initially inserted presented high structural damage, reflected by the partial and complete disintegration of the central rods. Relocated fuel filled the subchannels, adhering to surviving cladding walls and the assembly casing. In the lower portion of the measured section, the tomogram degrades due to photon starvation effects hinting at the increased amount of relocated fuel potentially plugging the assembly; flow blockage in this section was difficult to determine due to the tomogram's degradation from photon starvation. Small fragments were observed further down the assembly, dislodged from the initial insertion of the molten material. This section was used as an unperturbed assembly reference, with a calculated blockage of less than 1% from the present fragments. The PBR-2 assembly was characterized by columnar relocation structures propagating through the subchannels. Three of the relocation structures were captured in the measured section, with evidence of cross migration on to adjacent subchannels. The measured section in this assembly captures the leading edge of two molten slug structures. The calculated flow blockage was estimated to be 16% in the planes where the three relocated structures are present, but this quickly decreases to approximately 5% past the leading edge of two of the molten slug structures.