DELAYED HYDRIDE CRACKING BEHAVIOR FOR ZIRCALOY-2 TUBING

The delayed hydride cracking (DHC) behavior for ZIRCALOY-2 tubing was characterized at temperatures ranging from 93-degrees-C to 288-degrees-C. Testing was performed on the three types of pressure tubes that were used in the construction of the Hanford N Reactor, Richland, WA. A two-stage cracking response was observed for all tube types, with a very high K sensitivity in the threshold (stage I) regime followed by a region that was insensitive or moderately sensitive to the applied K level (stage II). The stage II crack velocity was a thermally activated process with an apparent activation energy of 68 kJ/mol. This energy level was consistent with the theoretical activation energy associated with hydrogen diffusion into the triaxial stress field ahead of a crack. Above a critical temperature of 180-degrees-C, cracking occurred only when specimens were subjected to an overtemperature cycle. Below this critical temperature, DHC occurred regardless of whether specimens were heated or cooled to the test temperature. Differences in crack velocities and threshold responses for the various tube types were attributed primarily to variations in crystallographic texture. Postirradiation testing revealed that irradiation to a fluence of 7.7 x 10(21) n/cm2 caused a 50-fold increase in crack growth rates. Metallographic and fractographic examinations were performed to understand operative DHC mechanisms.