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 is very challenging due to the harsh environment and design of 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 than those expected in a typical reactor, due to 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.

Temperature gradients in ceramic light water reactor (LWR) uranium dioxide (UO2) nuclear fuel pellets generate thermal stresses that cause fractures in the fuel, which begins early in the life of fresh fuel. The combination of heating due to fission and forced convective cooling on the exterior of LWR fuel rods generates a temperature profile that is difficult to replicate outside the reactor environment. In this study, a state-of-the-art experimental setup using electrical heating to study certain aspects of temperature driven fracture was built, and surrogate fuel materials such as ceria (CeO2) were used to validate the system. Cracking experiments were conducted on these surrogates by inducing reactivity-initiated-accident like temperature gradients in the pellets via induction and direct resistance heating. Induction heating was done using copper coils and molybdenum susceptors, which heated the surrogates 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 replicate 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. Some initial tests were conducted with a 2-color pyrometer that was later substituted with a forward-looking infrared thermal camera to capture the temperature profiles. A LabVIEW data acquisition system was set up for collecting useful data during experiments.

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This report describes the current state of research related to critical aspects of erosion and selected aspects of degradation of secondary components in nuclear power plants (NPPs). The report also proposes a framework for online health monitoring of aging and degradation of secondary components. The framework consists of an integrated multi-sensor modality system, which can be used to monitor different piping configurations under different degradation conditions. The report analyses the currently known degradation mechanisms and available predictive models. Based on this analysis, the structural health monitoring framework is proposed. The Light Water Reactor Sustainability Program began to evaluate technologies that could be used to perform online monitoring of piping and other secondary system structural components in commercial NPPs. These online monitoring systems have the potential to identify when a more-detailed inspection is needed using real-time measurements, rather than at a predetermined inspection interval. This transition to condition-based, risk-informed automated maintenance will contribute to a significant reduction of operations and maintenance costs that account for the majority of nuclear power generation costs. There is unanimous agreement between industry experts and academic researchers that identifying and prioritizing inspection locations in secondary piping systems (for example, in raw water piping or diesel piping) would eliminate many excessive in-service inspections. The proposed structural health monitoring framework takes aim at answering this challenge by combining long-range guided wave technologies with other monitoring techniques, which can significantly increase the inspection length and pinpoint the locations that degraded the most. More widely, the report suggests research efforts aimed at developing, validating, and deploying online corrosion monitoring techniques for complex geometries, which are pervasive in NPPs.

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