Eddie López-Honorato

Tristructural-isotropic (TRISO) fuel and other coated particle nuclear fuels have been selected as candidate fuels for a wide range of microreactor concepts for terrestrial use and space exploration. The fabrication of these fuels largely relies on spout fluidized bed chemical vapor deposition, a process in which a batch of fuel kernels is subjected to flows of fluidizing and pyrolyzing gases, which are precursors to the desired coating composition. Kernel sphericity, particle size distribution (PSD), and surface roughness are critical parameters that can affect fluidization, coating quality, and ultimately fuel performance. Understanding the impact of PSD of the initial fuel kernel could allow the use of kernels with a wider fuel specification. If possible, reducing requirements on sphericity and particle size uniformity without compromising coating performance could reduce process cost, time, and waste generation. Therefore, a better understanding of their impact on the fluidization and coating characteristics is needed. This work used computational fluid dynamics coupled with discrete element modeling to assess the impact of polydispersity of the initial kernel population on fluidization for two mean particle sizes: 250 and 400 um. The initial particle size is normally distributed with standard deviations varying from 12 um (matching experimental conditions) to 63 um. The results show that preferential fluidization effects are increasingly present with wider PSDs and are more prevalent for the smaller mean particle size of 250 um. Increasing the PSD leads to an increase in coating thickness variations for the smallest particles and smaller coating thickness variations for the largest particles. The results have important implications to fabrication efforts, where consistency of coating thickness is of importance.

Silicon carbide (SiC) passive thermometry has emerged as a promising post-irradiation examination (PIE) technique for estimating irradiation temperature near the end of irradiation. While dilatometry techniques have been traditionally used to analyze prismatic samples after irradiation, Raman spectroscopy has recently been shown to provide comparable results by analyzing Raman-active phonon modes. In this work, Raman spectroscopy has been applied to the SiC layer of cross-sectioned irradiation tristructural isotropic (TRISO) particles from the AGR-5/6/7 experiment to evaluate the feasibility of particle-scale passive thermometry. Two-dimensional Raman mapping was used to measure the position of the SiC longitudinal optical (LO) phonon, which was then converted to an apparent irradiation temperature using a previously established empirical correlation. Particles from AGR-5/6/7 Compacts 2-2-1 and 5-1-3 were selected as their calculated time-averaged, volume-averaged (TAVA) temperatures (828°C and 706°C, respectively) fall within the range of the sensitivity of the experimental approach. The use of SiC thermometry was anticipated to confirm or highlight potential deviations from calculated end-of-life TAVA temperature across compacts. Four particles from Compact 2-2-1 and two particles from Compact 5-1-3 were selected based on 110mAg inventory (either some measurable activity or below the minimum detection limit) which is commonly used as an indicator of in-pile temperature variation of particles within the same compact. Across every particle selected it was determined that the average LO peak position was located around 968 cm-1 to 969 cm-1. Using the previously determined empirical correlation, this corresponds to an irradiation temperature around 950°C to 966°C, which does not align with the reported TAVA temperature values. This discrepancy in apparent irradiation temperatures likely reflects a combination of uncertainties in the calculated particle temperatures and differences in irradiation history between the present specimens and those used to establish the empirical Raman calibration such as neutron flux (damage rate) and SiC microstructure (as fabricated and irradiated).

The diametral compression test is an important way to evaluate fracture strength of the SiC coating in tri-isotropic (TRISO) nuclear fuel particles. The fracture strength of SiC coatings deposited under different conditions is measured by a modified crush test. The characteristic fracture strength (partial fracture strength) is reported using Weibull distribution functions with different estimators. In addition, the 90% confidence bound of the characteristic strengths are obtained from the 5% and 95% distribution. The scale parameter (the whole fracture strength) of SiC coatings is calculated, given that the fracture starts at the IPyC/SiC interface, of which together with the microstructure has important effect on fracture strength.