Toward Accelerated Qualification of ATF Claddings: An In-Situ Burst Testing Framework and Insights into Zr Alloy LOCA Behavior

This dissertation introduces a robust burst testing framework with in-situ measurement capabilities, for the purpose of accelerating the characterization and qualification of cladding materials under accident conditions, specifically design-basis loss-of-coolant accidents (LOCAs) in nuclear reactors. The framework combines digital image correlation and infrared thermography to generate high-fidelity thermomechanical data, providing insight into deformation mechanisms such as creep and plasticity during LOCA conditions. The framework was initially developed to evaluate accident-tolerant fuel (ATF) claddings, including Cr-coated Zr and FeCrAl alloys. However, testing of historic Zr claddings was conducted to demonstrate the framework’s versatility as well as address existing modeling gaps and expound on the deformation mechanisms of Zr during LOCAs.

In the first four chapters of this dissertation, historical use of burst testing is detailed, recent ex-situ testing of FeCrAl and Cr-coated alloys are presented, and the benefits of an in-situ framework for ATF systems are high-lighted. Subsequent chapters focus on Zr as a demonstration of the in-situ framework, where current modeling gaps are identified, the prevailing reliance on steady-state creep in LOCA models is challenged, and possible anisotropy effects are explored.

The steady-state creep assumption for Zr LOCA behavior was investigated with in-situ thermomechanical data, and a novel transition in deformation behavior during transiently heated burst testing was found. The onset of this behavior was compared to plasticity models which indicated the majority of ballooning deformation is likely related to plastic instability. Accelerated creep testing methods were also demonstrated on Zr alloys where creep parameters were recovered and compared to literature models.

Preliminary application of the validated framework to Cr coated Zr systems is presented to demonstrate the experimental resolution gained from implementing in-situ techniques. A change in deformation behavior related to ballooning was also identified during initial testing of Cr coated Zr. The characteristics of this ballooning process mirrored those of the uncoated reference, but were shifted by roughly 80 °C. This temperature shift corresponded with the observed difference in burst temperature between the coated and uncoated samples from ex-situ testing. This correlation suggested the coating delays ballooning rather than alter the deformation process.