Nuclear Technology / Volume 164 / Number 3 / December 2008 / Pages 465-473
Technical Paper / Materials for Nuclear Systems
After 30 yr of operation, the Experimental Breeder Reactor II (EBR-II) Superheater 710 at Argonne National Laboratory-West (now Idaho National Laboratory) was decommissioned. As part of its postservice examination, four duplex tube sections were removed and Charpy impact testing was performed to characterize the crack-arresting ability of nickel-bonded tube interfaces. A scanning electron microscopy (SEM) examination was also performed to characterize and identify changes in bond material microstructure. From room temperature to 400°C, all samples demonstrated ductility and crack-stopping ability similar to that exhibited by beginning-of-life samples. However, at a low temperature (-50°C), samples removed from the lower region of the superheater (near the sodium inlet) failed while those from the upper region (near the sodium outlet) did not. SEM analysis revealed that all the tube-tube interfaces showed evidence of iron diffusion into the nickel braze, which resulted in the formation of a multiphase diffusion structure. Yet, significant void formation was only observed in the bond layer of the tubes removed from the lower region. This may be due to a change in the crystal microstructure of one of the phases within the bond layer that occurs in the 350 to 450°C temperature range, which results in a lower density and the formation of porosity. Apparently, only the samples from the higher-temperature region were exposed to this transition temperature, and the resulting large voids that developed acted as stress concentrators that led to low-temperature embrittlement and failure of the Charpy impact specimens.