Fracture Behavior of Advanced Technology Fuels for Light Water Reactors

Author

Adrianna Elizabeth Lupercio, Boise State UniversityFollow

Publication Date

8-2024

Date of Final Oral Examination (Defense)

4-25-2024

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science and Engineering

Department Filter

Materials Science and Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Brian J. Jaques, Ph.D.

Supervisory Committee Member

Andrew T. Nelson, Ph.D.

Supervisory Committee Member

Corey Efaw, Ph.D.

Supervisory Committee Member

Mahmood Mamivand, Ph.D.

Abstract

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 UO₂, 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 UO₂ 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 UO₂ is limited, while only one study (N = 7, where N is the number of test samples) is available for unirradiated doped UO₂. Hence, a knowledge-gap exists in the literature for fracture behavior of doped UO₂ 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 UO₂, 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 CeO₂ and Ti-doped CeO₂ to obtain statistical fracture data. The CeO₂ material was selected as a surrogate for UO₂ to refine sample processing, characterization techniques, and the BOR test method for undoped and doped UO₂. The research study performed on CeO₂ and Ti-doped CeO₂ was motivated by its use as an electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The Ti-CeO₂ samples were doped with 0.1 weight percent (wt%) TiO₂ and resulted in an increased characteristic strength (≈ 20%) and Weibull modulus compared to CeO₂, 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 UO₂.

Chapter four details the work to produce a benchmark dataset to establish the fracture behavior of undoped UO₂ using the BOR method. This work provides a robust dataset for a comparative analysis of the fracture behavior of doped UO₂ and future fracture studies of ATF concepts. Hertzian contact damage was observed for undoped UO₂ 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 UO₂.

In chapter five, the statistical fracture of doped UO₂ was acquired for UO₂ doped with TiO₂ or Cr₂O₃ to investigate the effects of dopants on the fracture behavior of UO₂. The interplay among grain size, dopant-induced defect structures, and pore size and distribution were explored. Both TiO₂ and Cr₂O₃ doped UO₂ 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 TiO₂ doped UO₂ samples was expected to reduce the fracture strength, yet residual tensile stresses attributed to dopant incorporation in the UO₂ lattice had a greater impact on fracture behavior. Doped UO₂ 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 UO₂ and ATF concepts and presented a comparative analysis of fracture behavior for undoped and doped UO₂ 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.

Comments

https://orcid.org/0000-0002-8974-3008

DOI

https://doi.org/10.18122/td.2241.boisestate

Recommended Citation

Lupercio, Adrianna Elizabeth, "Fracture Behavior of Advanced Technology Fuels for Light Water Reactors" (2024). Boise State University Theses and Dissertations. 2241.
https://doi.org/10.18122/td.2241.boisestate