Corrosion and Hydriding of N Reactor Pressure Tubes

Along the fueled length of recently examined tubes, the waterside oxide layer thickness increases uniformly from /approximately/3 ..mu..m (the pre-installation value) at the inlet end to a maximum of over 50 ..mu..m near the peak neutron flux location. The outer surface oxidation levels are about an order of magnitude less. The oxide thicknesses have steadily increased with operating time, which was verified by a tube removal and examination program that spanned the 25-yr reactor lifetime. Oxidation rates were calculated from the waterside oxide layer thickness data. The in-flux oxidation rates are clearly enhanced (at a given temperature) relative to ex-reactor Zircaloy oxidation rates and to out-of-flux rates on actual pressure tubes. The in-flux oxidation rate varies exponentially with absolute temperature, similar to ex-reactor rates. In addition, the in-flux oxidation rate at a given temperature increases significantly as the oxide layer thickness passes from 20 ..mu..m. This last trend is in qualitative agreement with recent observations from high-burnup light water reactor (LWR) fuel rods, and with a ''Thick-Film Hypothesis'' derived from test reactor data on N Reactor tube materials in low- and high-oxygenated coolants. 11 refs., 11 figs., 3 tabs.