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High-resolution two-phase flow data in the rod bundle are important in the development and validation of high-fidelity models for computational fluid dynamics and subchannel codes, in particular, those pertaining to light water reactor cooling systems. The Michigan Adiabatic Rod Bundle Flow Experiment (MARBLE) has been constructed as a modular assembly of an 8 × 8 lattice rod bundle to simulate scaled pressurized water reactor and boiling water reactor subchannel assemblies. To establish a high-spatial resolution database of the void fraction in the reactor fuel assembly geometries, tomographic measurements were performed with the High-Resolution Gamma-ray Tomography System, which was designed and built in house; the detector system has a spatial resolution of less than 1.0 mm using 240 LYSO (Lu1.8Y0.2SiO5) scintillators with a fan-beam array. In the present study, the local void fraction was measured with the MARBLE facility under various air-water flow conditions (jg = 0.04 to 0.85 m/s and jl = 0.12 to 0.77 m/s) covering from bubbly to cap-turbulent flows. The local void fraction was also successfully measured under nonuniform and asymmetric air bubble distribution conditions with an investigation of the effect of spacer grids and mixing vanes on void drift across subchannels. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This work describes development and testing of a high resolution fan-beam gamma tomography system. This includes an array of 240 LYSO scintillators read out by silicon photomultipliers and a set of modular electronic readout boards for each eight detectors. Each readout channel consists of an amplifier and two counters with individually software-controllable energy thresholds, such that the difference in counts between the two thresholds corresponds to an energy region of interest. Control and readout of the modules is done remotely via Wi-Fi. While the system works with any gamma source, tomographic imaging tests were made using a nominally 15 Ci Ir-192 source. Reconstructed images demonstrate a spatial resolution of about 1 mm. The readout electronics module represents a flexible and scalable basis which can be used in computed tomography systems for a wide range of sources, geometries, and applications. [ABSTRACT FROM AUTHOR]