Separate-Effects Tests for Studying Temperature-Gradient-Driven Cracking in UO₂ Pellets

Abstract

A variety of normal operation and accident scenarios can generate thermal stresses large enough to cause cracking in ceramic fuel pellets. Cracking in fuel pellets can lead to reduced heat removal, higher centerline temperatures, and localized stress in the cladding—all which impact fuel performance. It is important to experimentally characterize the thermal and mechanical behaviors in the pellet both before and after cracking, which would help to improve cracking models in fuel performance codes such as BISON. However, in-reactor observation and measurement of cracking are very challenging due to the harsh environment and design of the fuel rods involved. Recently, an experimental pellet-cracking test stand was developed for separate-effects testing of pellet cracking under normal operations and accident temperature conditions using thermal imaging to capture the pellet surface temperatures in order to evaluate the thermal stresses and optical imaging to capture the evolution of cracking in real time. Experiments were performed using depleted uranium dioxide (UO₂) pellets, which are useful for collecting valuable data for development and validation of cracking models. A combination of induction and resistance heating was used to create thermal gradients similar to those seen in a reactor environment. Characterization of the pellets was conducted both before and after cracking. The cracking patterns are moderately different from those expected in a typical reactor because of the variations in the thermal conditions and pellet microstructures. However, when the actual conditions of these experiments are reproduced in computational models with sufficient precision, such out-of-pile testing on UO₂ pellets provides relevant data for modeling purposes.

Keywords:

• Uranium dioxide
• resistive heating
• cracking temperature
• dual imaging
• grain boundaries

Acknowledgments

This research is a part of integrated research project (IRP) IRP FC-1: Nuclear Energy University Program (NEUP) Project 16-10905 funded by the U.S. Department of Energy Office of Nuclear Energy NEUP under the DE-NE-0008531 research project. Jonathan McCreary and Jason Reynolds helped in conducting the experiments. The submitted manuscript has been co-authored by a contractor of the U.S. Government under contract DE-AC07-05ID14517.