Method for measurement of TRISO kernel and layer volumes by X-ray computed tomography☆
Introduction
Tristructural-isotropic (TRISO) particle fuels are the focus of significant experimental work due to widespread interest in TRISO-based reactor designs. The dimensions of the kernel and the coating layers are key parameters measured in TRISO characterization. Mean layer thicknesses are commonly measured in as-fabricated particles. Optical microscopy is used to measure particles polished to their mid-plane, while mean kernel diameters are commonly measured using optical shadow imaging [1]. More detailed data regarding kernel volumes and coating layers are commonly acquired using serial sectioning coupled with optical microscopy [2]. While this method is sufficient for basic characterization, it has two main shortcomings. First, the resolution of serial sectioning is limited by the step-size between imaging steps. This limitation may be mitigated by reducing the step-size, but there is a practical limit to the number of steps, particularly when working with irradiated particles in hot cell environments. Second, the process of serial sectioning may damage fragile irradiation microstructures, particularly in the buffer region, which commonly features internal fissures and tenuous connections to the adjoining inner pyrolytic carbon (IPyC) layer.
The alternative method for nondestructive characterization of TRISO particles in common use is x-ray computed tomography (XCT). In XCT, two-dimensional (2D) projections of a sample—or radiographs—are generated at varying sample rotations relative to the x-ray source and detector. Brightness and contrast within these radiographs are determined by the relative x-ray transmission through the sample. The radiographs are reconstructed into a single 3D image of the sample based on their specific angles. Historically, XCT of TRISO particles has been used mostly for qualitative analysis of as-fabricated and irradiated particles, with a focus on identification of defects and anomalies for further materialographic analysis [3,4].
An image analysis method has been developed to process tomographic datasets for TRISO particles and generate quantitative information regarding their geometry and structure. Compared to serial sectioning, this method provides higher resolution results while preserving the as-irradiated microstructures. These advantages are particularly useful in quantifying irradiation effects such as kernel swelling or buffer shrinkage. Combined with individual particle irradiation capabilities such as those offered by the Oak Ridge National Laboratory (ORNL) MiniFuel irradiation capsule [5], this image analysis method enables high-resolution measurement of kernel and layer dimensions on the same particle pre- and post-irradiation to quantify changes in a specific time, temperature, and flux environment.
Section snippets
Loading of particles
The key requirements for loading TRISO particles for XCT imaging are (1) the particle must remain stationary during imaging, and (2) sufficient radiation shielding must be provided for handling irradiated particles. When imaging unirradiated particles, two loading methods were used. In the first method, a single particle was placed in a Lucite holder with a notched conical cavity, as shown in Fig. 1. In the second loading method, a Kapton tube of an appropriate diameter was loaded with a series
Layer segmentation
The kernel and TRISO layers were segmented using software written in MATLAB. The segmentation process was designed to take advantage of the nested sphere geometry of TRISO particles to produce the best possible results.
Calculation of geometric parameters
Once segmentation of the kernel and TRISO layers was complete, geometric parameters characterizing each were calculated. These parameters were calculated at sets of approximately equidistant spherical angles from the centroid of the kernel. Kernel radius at each of 5000 spherical angles was taken to equal the position of the kernel/buffer boundary at that angle. Layer thicknesses were calculated at each of 5000 spherical angles by taking the difference between their outer and inner boundaries
Example results
Results illustrating the capabilities of this method in several example particles are given in the following subsections. While these examples show the type of data which can be gathered by this analysis, a full-scale implementation over extensive batches of particles has not yet been performed.
Conclusions
Characterization of individual TRISO particles by XCT has been expanded from primarily qualitative analysis of particle features to detailed quantitative analysis of particle geometry. Digital segmentation of the kernel, the coating layers, and additional particle features in tomographic datasets enables measurement of thicknesses, volumes, and curvatures to fully characterize each feature. This method may be applied to both as-fabricated and irradiated particles, and can support experiments
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
CRediT authorship contribution statement
Grant W. Helmreich: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Software, Supervision, Validation, Visualization, Writing - original draft. Dylan Richardson: Data curation, Investigation, Methodology, Writing - review & editing. Singanallur Venkatakrishnan: Investigation, Methodology, Writing - review & editing. Amir Ziabari: Investigation, Methodology, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was sponsored by the U.S. Department of Energy, Office of Nuclear Energy, through the Advanced Reactor Concepts ARC-Xe program and through the Idaho National Laboratory Advanced Reactor Technologies Technology Development Office as part of the Advanced Gas Reactor Fuel Development and Qualification Program.
References (12)
- et al.
Automated optical microscopy of coated particle fuel
J. Nucl. Mater.
(2008) Initial examination of fuel compacts and TRISO particles from the US AGR-2 irradiation test
Nucl. Eng. Des.
(2018)- et al.
First elevated-temperature performance testing of coated particle fuel compacts from the AGR-1 irradiation experiment
Nucl. Eng. Adn Des.
(2014) - et al.
Comparison of silver, cesium, and strontium release predictions using PARFUME with results from the AGR-1 irradiation experiment
J. Nucl. Mater.
(2015) - et al.
Evaluation of design parameters for TRISO-coated fuel particles to establish manufacturing critical limits using PARFUME
J. Nucl. Mater.
(2016) - et al.
Microscopic analysis of irradiated AGR-1 coated particle fuel compacts
Nucl. Eng. Des.
(2014)
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