Study of Zirconium Alloy Corrosion Parameters in the Advanced Test Reactor

Abstract

The influence of nuclear radiation on zirconium alloy corrosion was studied in the North (1D) loop of the Advanced Test Reactor (ATR). The loop was new and therefore was free of fissionable contamination. The principal corrosion parameters were: alloy composition (Zircaloy-2, Zircaloy-4, Zr-2 5Nb, and crystal bar zirconium); flux level (unirradiated, 6 × 1012 to 1.7 × 1014 neutrons (n)/cm2 s > 1 MeV); metallurgical condition (Zircaloy-4 and Zr-2.5Nb in two conditions); oxygen concentration (0.6 ppm 02 and < 0 05 ppm O2); and surface pretreatment (as-etched, thin prefilms, and thick prefilms). Maximum exposures were 61.6d (oxygenated) and 102.ld (deoxygenated) at 280°C, 2000 psig, in pH 10 LiOH. In the oxygenated test, corrosion rates were substantially accelerated at high-flux locations. At the low-flux location, corrosion rates were unaffected or slightly accelerated on Zircaloy-2 and -4, substantially accelerated on zirconium, and slightly reduced on Zr-2.5Nb. Thick prefilms tended to suppress accelerated corrosion in the oxygenated system In the deoxygenated test, corrosion rates were substantially accelerated for zirconium, but were only slightly above unirradiated values for the other materials. The results from the ATR loop paralleled previous results from the ETR G-7 loop, indicating that dissolved oxygen, rather than fissionable contamination, was the principal cause of accelerated corrosion in both systems. The Zr-2.5Nb alloy (quenched, cold-worked, aged) had the lowest weight gains and hydrogen absorption in both oxygenated and deoxygenated tests, but showed some pustule formation in the oxygenated test. Zircaloy-2 coupons which had oxidized rapidly to 120 to 160 mg/dm2 (2 0 to 2.6 mg/dm2d) in the oxygenated system were transferred to the deoxygenated system. After transfer the corrosion rates decreased to 0.5 to 0.6 mg/dm2d. Inspection of coupons under polarized light revealed evidence of oxide porosity on coupons undergoing accelerated corrosion.