Ming Fang, Ph.D. Candidate
Dr. Angela Di Fulvio, Director of Research
January 10, 2024 | 9:00am - 11:00am CST
This final examination will be held in 101A Talbot Laboratory.
Multi-Mode Imaging for TRISO-Fueled Pebble Identification
ABSTRACT: Tristructural-isotropic (TRISO) fuel is currently one of the most mature fuel types for pebble bed reactor (PBR) designs, such as pebble-bed high-temperature gas-cooled reactors (HTGCR) and fluoride-salt-cooled high-temperature reactors (FTR). Fuel units in PBR are made in the form of a billiard-size ball, which contains thousands of randomly-distributed TRISO fuel particles of 500-1000 µm diameter surrounded by graphite or silicon carbide (SiC). In PBRs, TRISO-fueled pebbles can be re-introduced into the reactor core several times to thoroughly burn the fuel. Reliable techniques to individually tag and identify fuel pebbles are therefore needed to maintain accountability of special nuclear material and gain a detailed understanding of pebble history, burnup, and structural integrity throughout the pebble’s lifespan. The identification techniques should be non-destructive, fast, and robust to meet the PBR’s operational constraints.
In this dissertation, two non-destructive assay (NDA) methods for TRISO-fueled pebble characterization, namely neutron multiplicity counting and X-ray computed tomography (CT), have been developed, simulated, and experimentally demonstrated. Based on these two NDA methods, unique identification of a pebble was achieved for a PBR core consisting of 100,000 pebbles in 150 s.
The identification relies on retrieving unique features associated with each fuel pebble, namely the 235U mass and the three-dimensional (3D) spatial distribution of TRISO fuel particles. Within this framework, a new type of neutron multiplicity counter (NMC) was designed to assess the 235U mass in TRISO fuel pebbles. The NMC is based on pie-shaped boron-coated straw (BCS) detectors and features superior gamma-ray insensitivity, thermal neutron detection efficiency, and assay accuracy, when compared to multiplicity counters based on 3He or organic scintillators. The NMC achieved a die-away time of 16.7 µs, approximately 61% lower than the 3He-based and 35% lower than the round BCS-based counter. A short die-away time is desirable to minimize the impact of random coincidence due to uncorrelated events and improve assay accuracy. The experimentally validated NMC model showed that this approach allows the estimation of the 235U mass in 100 seconds with a relative uncertainty below 2.5% (1-SD). The 3D spatial distribution of TRISO particles in a mock-up TRISO fuel compact was reconstructed with a North Star X5000 industrial CT system and custom image reconstruction algorithms. A point cloud registration-based identification algorithm was developed to retrieve the pebble identity in the presence of noises and arbitrary rotations. The CT scan, image reconstruction, particle segmentation, and pebble identification process took approximately 50 s for one pebble. An identification accuracy of 100% was achieved for a PBR core consisting of 100,000 pebbles.