Numerical analysis of the effect of polydispersity on spout fluidized bed chemical vapor deposition for coated nuclear particle fuels

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Abstract
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.

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