Feasibility of fluorescent X-ray computed tomography to verify the homogeneity of Mo/Gd contained in nuclear fuels: A Monte Carlo simulation

We studied the feasibility of a fluorescent X-ray computed tomography (FXCT) system to monitor the uniformity of nuclear fuels; this system could be used for research reactors or power reactors. In general, nuclear fuels contain non-fissile metals for various purposes. To avoid the use of high-enriched uranium (HEU) fuels, U–Mo alloy fuels have been studied to lower the enrichment factor and to enhance the uranium density of fuels to be used in research reactors. (U, Gd)O₂ fuel is another uranium-metal alloy in which gadolinium (III) oxide (Gd₂O₃) powder is added to uranium dioxide (UO₂) to adjust the reactivity of the fuel and enhance the performance of UO₂. To guarantee the combustion stability of fuels in reactors, it is important to verify the homogeneity of alloy metals contained in the fuels. The FXCT system, which analyzes materials based on characteristic X-rays emitted from atoms, could be used to verify the homogeneity of the materials contained in nuclear fuels. External radiation sources are not necessary to induce characteristic X-rays from the fuel, as the gamma rays emitted from ²³⁵U stimulate the fuel atoms. To verify the feasibility of the system, we measured the energy spectrum of a UO₂ pellet, in which small metal pieces of Gd and Mo were attached to the surface. Based on simulation studies, 2D and 3D radiographic images of UO₂ fuels with Gd and Mo were reconstructed to verify the effectiveness of the FXCT system.