| dc.description.abstract | In the commercial nuclear power industry, there is a vested interest in improving the properties of the reactor components. Improving nuclear reactor materials to resist the severe structural changes and degradation not only improves reactor safety and reliability, but also economical returns from decreased outages and up-keep cost. As the first barrier of fission product containment, the performance of fuel cladding in a nuclear reactor is a pivotal design parameter in ensuring reactor safety and reliability. An attractive solution to improve fuel cladding performance by enhancing the surface cladding properties while not affecting the bulk matrix material properties can be achieved through surface engineering techniques. Although surface modification techniques have been shown to improve cladding oxidation and corrosion resistance, debonding of the modified surface from the bulk matrix makes such approaches unsuitable for reactor applications.
In this dissertation, effects of advanced cathodic cage plasma nitriding on reactor cladding materials T91, HT-9, 316L, and Zircaloy-4 was investigated. The microstructual, corrosion and mechanical surface properties were evaluated as well as ion irradiation resistance and nitrogen diffusion kinetics of the nitrogen modified layer. Nitriding improved the mechanical surface properties of all investigated materials with steels and Zircaloy-4 hardness increasing to 14-16 GPa and 20-24 GPa respectively. The corrosion resistance in 3.5 wt% NaCl varied for all nitrided materials dependent on nitriding temperature and time. The corrosion rate of nitride 316L had the least improvement where the reference sample measurement was 1.9 µm/yr and nitrided sample at 454 °C for 45 minutes was 1.2 µm/yr. At higher temperatures and longer nitriding durations, the corrosion resistance was reduced. Nitriding of T91 and HT-9 significantly enhanced corrosion resistance for nitriding temperatures below 570 °C. The corrosion rate of nitrided Zircaloy-4 decreased by an order of magnitude for all samples above 920 °C.
The effective nitrogen diffusion coefficient, D0 and activation energy, Q of 316L is 23.01±6.55 cm2/sec and -1.65±0.02 eV/atom, respectively. The calculated D0 and Q for nitrogen in T91 are 9.91E-4 ± 1.97E-4 cm^2/sec and -0.82 ±0.02 eV/atom respectively. The calculated D0 and Q for nitrogen in HT9 are 5.16 ± 3.52 cm^2/sec and -1.46 ± 0.04 eV/atom respectively. At temperatures above 863 °C, the calculated D0 and Q were 706 ± 693 cm^2/sec and -3.00 ± 0.09 eV/atom respectively. The calculated D0 and Q for temperatures below 863 °C were 3.35E-2 ± 0.6E-2 cm^2/sec and -2.02 ± 0.02 eV/atom respectively. | |
