The Hanford Site N Reactor core includes approximately 1000 Zircaloy-2 pressure tubes that pass horizontally through the graphite moderator and contain the fuel elements and pressurized water coolant. The overall tube length is 16.1 m, and the central fueled length is 10.6 m. Over the fueled length, the coolant temperature increases from 204 to 282°C (399 to 540°F). Moist helium is maintained in the gas gap between tube and graphite moderator.
Along the fueled length of recently examined tubes, the waterside oxide layer thickness increases uniformly from ∼3 μm (the preinstallation value) at the inlet end to a maximum of over 50 μm near the peak neutron flux location. The outer surface oxidation levels are approximately 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-year 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 <15 μm to >20 μm. 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.
The hydrogen content in the tube wall along the fueled region follows the axial distribution of the oxide thickness. The hydrogen accumulation rate generally follows the oxidation rate. In particular, the hydrogen accumulation rate increases as the waterside oxidation passes from the thin-film (<15-μm) regime to the thick-film (μ20-μm) regime. The hydrogen content has a strong azimuthal distribution with the maximum occurring at the top of the tube, which is the lower temperature region. The hydrogen also has a strong radial distribution with the maximum occurring at the tube inner surface, which is the oxidizing (hydrogen source) surface and which is at the lower temperature.
In this paper, oxidation and hydriding trends are characterized, discussed, and analyzed in the light of current models and comparable LWR and ex-reactor data. Emphasis is given to results and insights generally applicable to in-reactor Zircaloy oxidation and hydriding.