2017 | AFC ACCOMPLISHMENTS 31 Accomplishments: As an example of the thorough characterization described above, investigations on UN/U-Si composite fuels are explained below.At a pulsed neutrons source in combination with a pixilated time-of-flight imaging detector (provided by AntonTremsin, UC Berkeley), energy-resolved neutron imaging is possible by recording neutron transmission data for each pixel of the imaging detector.This allows to measure the areal density of isotopes for each pixel by fitting the data over a several 10 eV energy range, covering several absorption resonances, with the known neutron cross-sections. Data analysis is accomplished by utilizing the SAMMY code, developed at ORNL, and fitting data for each of ~100,000 pixels covered by a typical fuel pellet (Adrian Losko, LANL). Using tomographic reconstruction codes, the areal densities are converted to atoms per volume of a voxel in the CT, which can be converted to gravimetric density with the molar mass of each isotope. From the CT reconstruction of isotopes present in the samples, slices for isotope densities or other analysis can be conducted. Figure 1 gives an example of 3D isotope densities in UN/U3Si5 and U3Si5 pellets with different enrichment levels encapsulated in a stainless steel container (provided by Andy Nelson, LANL).The average enrichment levels measured by neutron resonance transmission analysis (NRTA) are within 0.1% of the nominal enrichment levels.While characterization with thermal neutrons, as shown on the left, does not allow to distinguish the highest (8.8%) and lowest (0.2%) enrichment level pellets, the reconstruction of the isotope density of 235U clearly allows that distinction. Figure 2 gives examples of the neutron transmission fits.A peak in the difference curve of the area where the 8.8% enriched material was interrogated originates from a 400 ppm contamination with tungsten, which has a resonance at 18.6 eV. Neutron diffraction complements the characterization by energy-resolved neutron imaging (SvenVogel, LANL). Figure 3 shows neutron radiographs of another set of UN/U3Si5 pellets (a) with the areal density for 238U derived from the data in (b).A somewhat higher scatter in the areal densities was observed for the data points corresponding to the left-most pellet. Neutron diffraction analysis of diffraction data from 2mm slices of the rodlet is conducted by fitting a model describing crystallographic and microstructural parameters to the measured diffraction intensities (c).While the UN and U3Si5 weight fractions are constant for all pellets (d, also showing the weight fraction of the steel container), the unit cell volumes of the hexagonal U3Si5 phase (e) are distinctly lower for the data points