Stability of Nuclear Thermocouples

The mission of the Department of Energy, Office of Nuclear Energy (DOE-NE) is to advance nuclear power as a resource capable of meeting the nation’s energy, environmental and national security needs by resolving technical, cost, safety, proliferation resistance, and security barriers through research and development. In the pursuit of safer and more economic energy production from existing nuclear reactors and future generation IV reactors, new cladding, fuel, and structural materials are being developed. However, data on the performance of these materials in accident scenarios and relevant generation IV reactor conditions is limited. Currently, thermocouples are the most commonly used temperature monitors within nuclear reactors. The primary motivation of this dissertation was to investigate the stability of High Temperature Irradiation Resistant thermocouples (HTIR-TCs) in addition to commonly used thermocouples for temperature sensing in both high temperature and irradiative environments. The stability of thermocouples is dependent upon the resistance of the generated electromotive force (EMF) signal to drift during operation. As detailed below, this was demonstrated by understanding the materials science of the traditional build process for HTIR-TCs, followed by a novel stabilization method to reduce time and efficiency in the TC build process. Initially, the work focused on understanding the stabilization phenomena exhibited by the HTIR-TCs, and is detailed in Chapter Two. During the preliminary heat treatment, a secondary Nb3P phase formed within the Nb-P, along with an interaction region at the Al2O3/niobium interface. The formation of secondary phases within the niobium leg of the thermocouple causes an increase in the Seebeck coefficient and subsequently drift in the EMF signal. Stabilization of the HTIR-TC EMF signal was found to be dependent upon both the equilibrium of a diffusion interaction region at the Nb-P/Al2O3 interface and the formation of Nb3P precipitates. To enhance the transformation kinetics of the above interactions alternative heat treatment methods were studied which led to the development of a novel Joule heating method. Joule heating was investigated to reduce the time necessary to induce the transformations necessary for stabilizing the HTIR-TCs, and is described in Chapter Three. During short-term testing, Joule heating between 1.25 and 2.25 A for 30 minutes produced stability similar to that of HTIR-TCs heat treated between 1450 and 1650 °C for 6 hours. The stability observed in the HTIR-TCs is attributed to the formation of an interaction region between the thermoelements and the alumina insulation in addition to the precipitation of a Nb3P phase within the Nb-P thermoelement. As a result, Joule heating is able to reduce the length of time necessary to stabilize the generated EMF signal from 6 hours to 30 minutes—a 1200% improvement in efficiency. Models for the influence of irradiation on the stability of thermocouple EMF signals were developed using both empirical data and predictive modeling, and is described in Chapter Four. For thermocouples where transmutation dictates the rate of drift during operation, such as Type B, S, and C, the model predicts that the degree of drift in the EMF signal is dependent upon the concentration of transmutation products. For Type K, N, and HTIR-TCs, where the thermal neutron cross-sections of the thermoelements alloying elements are on the order of one to three barn, the drift in the EMF signal is not dependent on transmutation, rather it is dependent upon the operational temperature and the fast fluence. Finally, within Appendix A and B, a discussion is presented on the stability of alternative high temperature ceramic insulators with the niobium and molybdenum thermoelements of the HTIR-TCs. The motivation for this series of experiments was to define alternative ceramic insulators for use within the HTIR-TCs in place of the currently used alumina in the hopes of increasing the maximum service temperature above 1700 °C. This work initially focused on compiling both empirical data and thermodynamic data in the form of Ellingham diagrams for oxides, carbides, nitrides, and silicides with niobium and molybdenum. It was observed that each of down selected ceramics, AlN, SiC, H-BN, ZrO2 reacted with niobium at temperatures as low as 1700 °C. Independent of the work on alternative ceramic insulators, a separate study on the influence of oxygen concentration within the furnace atmosphere during the stabilization heat treatment on the mechanical properties of the HTIR-TCs niobium sheathing is also included in the Appendix. This work showed that the niobium sheathing of the HTIR-TCs was embrittled during heat treatment for 6 hours between 1450 and 1600 °C when exposed to partial pressures of oxygen as low as 8 ppm. Collectively, this body of work investigates the stability of nuclear thermocouples. A common theme throughout each chapter is the study of the relationship between the influence of processing and environmental conditions on the stability of the generated EMF signal of thermocouples through chemical analysis, crystallographic analysis, transformation kinetics, and mechanical behavior. This dissertation emphasizes the influence of heat treatment on the stability of niobium and molybdenum-based thermocouples, HTIR-TCs, and then branches out to develop an empirical and mathematical model to predict the drift behavior of thermocouples within nuclear reactors. Through increasing our understanding of the stability of HTIR-TCs and thermocouples in general, it may enable further innovation of reactor design to accommodate higher temperature operation and increased thermal efficiency.