Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Travis W. Knight

Abstract

A variety of normal operation and accident scenarios can generate thermal stresses large enough to cause cracking in light-water reactor (LWR) fuel pellets. Cracking of fuel pellets can lead to reduced heat removal, larger centerline temperatures, and localized stress in cladding, all of which impact fuel performance. It is important to understand the temperature profile on the pellet before and after cracking to improve cracking models in fuel performance codes such as BISON. However, in reactor observation and measurement of cracking is very challenging owing to the harsh environment and design of fuel rods.

Recently, a state-of-the-art experimental pellet cracking test stand was developed for separate effects testing of normal operations and accident temperature conditions, using thermal imaging to capture the pellet surface temperature for evaluation of thermal stresses and optical imaging to capture the evolution of cracking in real time. Induction heating was done using copper coils and molybdenum susceptors, which heated the pellets to a threshold temperature that is sufficiently high for the fuel material to conduct current. Thereafter, direct resistance heating was achieved by passing current through the specimen using a DC power supply to introduce volumetric heating to simulate LWR operating conditions. The pellets were held against nickel electrodes and mounted on a boron nitride test-stand. All the tests were carried out in a stainless-steel vacuum chamber. Simultaneous real-time dual imaging of the surrogate pellet surface was implemented using an optical and infrared camera system that was mounted along axial and perpendicular directions to the pellet surface, respectively. A beam-splitter was used to split the incoming radiation from the sample into two halves. While one of the beams was transmitted from the splitter through a bandpass filter to obtain optical images, the other beam was reflected from the splitter to the thermal camera to capture full-field temperature gradients of the as-fabricated pellet surface during cracking. A LabVIEW data acquisition system was set up for collecting useful data during experiments.

Cracking experiments were performed using surrogate fuel material including ceria (CeO2) which was useful for developing and demonstrating the experimental approaches but is also valuable in its own right for cracking model development and validation. A combination of induction and resistance heating was used to create an average temperature gradient of 300°C/cm before cracking and 249°C/cm after cracking was observed.

After validating the test stand and establishing the experimental conditions using surrogate ceria pellets, separate effects tests were conducted to study cracking in out-of-pile uranium dioxide (UO2) pellets which is useful for establishing benchmark test conditions and to collect data valuable for development and validation of cracking models using fuel performance codes such as BISON. A combination of induction and resistance heating was used to create an average temperature gradient of 236°C/cm and 193°C/cm before and after cracking respectively. The experimental results obtained here for single UO2 pellet can be used for validating the fracture models in BISON. Characterization of the pellets were done before as well as after cracking for understanding cracking behavior and physical properties of the UO2 pellets at ambient temperature.

The cracking patterns are somewhat different than those expected in a typical reactor because of the differences in operating thermal conditions and pellet microstructure. However, if the actual experimental conditions are to be reproduced in computational models, these out-of-pile tests on UO2 pellets provide relevant data for modeling purposes. The findings from this work will help improve confidence in fracture models used for fuel pellets under similar in-reactor conditions.

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