Adrianna E. Lupercio
The urgent call to decarbonize our energy infrastructure, while simultaneously meeting growing energy demands, highlights the need for reliable and clean energy sources. Nuclear energy provides reliable, high-capacity baseload electricity while emitting zero greenhouse gases during operation. To adequately meet the energy needs of society and maintain economic viability, it is crucial to enhance the efficiency of nuclear power plant (NPP) operations. Upgrades to NPP operations require near-term Advanced technology Fuel (ATF), such as doped UO 2 , to increase the flexibility of plant operation without impacting safety margins. Small additions of metal oxide dopants are reported to increase grain size, thereby limiting fission gas release (FGR) and increasing pellet compliance to mitigate pellet-chemical interactions (PCI) and pellet-cladding mechanical interactions (PCMI). Prior to implementing doped UO 2 fuels into the reactor fleet, it is important to understand dopant effects on fracture behavior as it impacts fuel performance, such as thermal conductivity, and its tolerance to accident conditions. The availability of fracture data for irradiated and unirradiated UO 2 is limited, while only one study (N = 7, where N is the number of test samples) is available for unirradiated doped UO 2 . Hence, a knowledge-gap exists in the literature for fracture behavior of doped UO 2 fuel forms. The existing knowledge-gap in fuel fracture analysis partly stems from the inherent challenges in machining radiological materials into samples suitable for the traditional bend bar tests. Consequently, acquiring sufficient data to understand the stochastic fracture behavior of ceramic materials is difficult. The ball-on-ring (BOR) biaxial flexure test method utilizes simple right cylindrical geometries representative of commercial nuclear fuel, requires minimal surface preparation, and is tolerant of edge defects; these advantages reduce the time and cost of sample production. As a full understanding of the statistical fracture behavior for ATF concepts has not been established, this work aims to develop and establish the BOR test method to obtain statistical fracture data of undoped and doped UO 2 , providing insight into fracture behavior. Chapter two of this work presents a study performed to validate the BOR biaxial flexure technique using technical ceramics with well-known mechanical properties complemented with finite element analysis (FEA). Chapter three details a test case of CeO 2 and Ti-doped CeO 2 to obtain statistical fracture data. The CeO 2 material was selected as a surrogate for UO 2 to refine sample processing, characterization techniques, and the BOR test method for undoped and doped UO 2 . The research study performed on CeO 2 and Ti-doped CeO 2 was motivated by its use as an electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The Ti-CeO 2 samples were doped with 0.1 weight percent (wt%) TiO 2 and resulted in an increased characteristic strength (≈ 20%) and Weibull modulus compared to CeO 2 , making them a more robust option for IT-SOFCs. In the context of this collective study, it was intended to provide proof of concept for the BOR test method for the testing of UO 2 . Chapter four details the work to produce a benchmark dataset to establish the fracture behavior of undoped UO 2 using the BOR method. This work provides a robust dataset for a comparative analysis of the fracture behavior of doped UO 2 and future fracture studies of ATF concepts. Hertzian contact damage was observed for undoped UO 2 test batch 1 due to the small diameter loading ball (≈ 3 mm), which was no longer observed in test batch 2 with a larger loading ball (≈ 19 mm). The contact damage did not appear to influence fracture behavior as both datasets resulted in a characteristic strength and Weibull modulus that agrees with previously published transverse rupture strength (TRS) values for undoped UO 2 . In chapter five, the statistical fracture of doped UO 2 was acquired for UO 2 doped with TiO 2 or Cr 2 O 3 to investigate the effects of dopants on the fracture behavior of UO 2 . The interplay among grain size, dopant-induced defect structures, and pore size and distribution were explored. Both TiO 2 and Cr 2 O 3 doped UO 2 sample sets resulted in lattice contraction and a characteristic strength and Weibull modulus that were lower than expected based on density, pore size, and distribution. The increased grain size of TiO 2 doped UO 2 samples was expected to reduce the fracture strength, yet residual tensile stresses attributed to dopant incorporation in the UO 2 lattice had a greater impact on fracture behavior. Doped UO 2 samples resulted in a reduced fracture strength and with a larger scatter in TRS values. Collectively, the body of this work establishes a BOR test method for the rapid fabrication and mechanical testing of UO 2 and ATF concepts and presented a comparative analysis of fracture behavior for undoped and doped UO 2 fuels. The statistical fracture data presented in this study provide baseline data for enhanced fuel performance code predictions of fuel fracture behavior impacting phenomena during reactor operation. This work provides foundational analysis of fracture behavior to advance research on ATF concepts and assist in acceleration of fuel qualification for the current and future nuclear reactor fleets.