Waterside corrosion, hydrogen pickup, and hydrogen redistribution in Zircaloy-2 pressure tubes during long exposure in N Reactor

The history and distribution of corrosion and hydriding in N Reactor Zircaloy pressure tubes have been monitored in a long-term surveillance program that has included sampling for local hydrogen concentration and metallographic determination of corresponding inner surface oxide layer thicknesses. General observations are: corrosion and hydriding are qualitatively well-correlated, and both are enhanced by radiation; the rates of corrosion and hydriding increase once an oxide layer of 12 to 15 ..mu..m is attained; and the ratio of hydrogen concentration to oxide thickness is highly scattered with maximum values corresponding to apparent pick-up rates greater than 100%. Azimuthal hydrogen content variations have been found that could account for the last observation above. The azimuthal and radial hydrogen distributions on a particular tube have been examined and analyzed. The following conclusions have been made: an azimuthal gradient exists in the wall-averaged hydrogen concentration, with maximum values at the top of the tube and minimum values at the bottom; radial (through-wall) surveys at the tube top and bottom reveal that it is the inner surface hydrogen which is relocated azimuthally; and combined thermal-hydraulic, thermal conduction and hydrogen diffusion calculations indicate that an inner surface azimuthal temperature gradient can exist within a tube, primarilymore » due to slight fuel element eccentricity, which is of proper direction and magnitude to relocate the inner-wall hydrogen to the degree observed. When the redistribution trends are accounted for, predictions of further increase in peak tube hydrogen concentrations due to further reactor operation can be made with more confidence.« less