Fracture Behavior of Advanced Technology Fuels for Light Water
Reactors
Authors: Lupercio Adrianna Elizabeth 
Published: February 2025

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.