Volume 1, Issue 7 (July 2004)
Effect of Fabrication Variables on Irradiation Response of Crack Growth Resistance of Zr-2.5Nb
Specimens of pressure tubes made from Zr-2.5Nb by various fabrication routes were irradiated in research reactors to evaluate the response of their fracture toughness (represented by crack growth resistance, dJ/da) at 250°C to irradiation by fast neutrons (E > 1 MeV). The reference route consists of melting Kroll sponge and some recycled material, primary hot-working, extrusion at 815°C, and cold-working 27 %. Tubes made from ingots that were melted four times (quadruple melting) are initially tougher than tubes made from ingots that have been melted twice (double melting), and after irradiation the former retain close to 50 % of their original crack growth resistance while the latter retain less than 30 % of theirs. The high toughness and its retention in the quadruple melted material are attributed to low concentrations of chlorine. Tubes made from electrolytic powder contain fluorine rather than chlorine. Fluorine is less damaging than chlorine. Crack growth resistance and its retention with irradiation of tubes made from electrolytic powder are intermediate between those of tubes made from quadruple and double melted sponge. Lowering the amount of cold-work is beneficial, and the toughness of tubes made with less than 27 % cold-work correlates well with strength, both before and after irradiation. Lowering the extrusion temperature produces strong, tough tubes, but these tubes lose much of their crack growth resistance with irradiation. This loss in toughness is not associated with material chemistry but with a high sensitivity to strength.