Volume 5, Issue 2 (February 2008)
Cladding Tube Deformation Test for Stress Reorientation of Hydrides
The phenomenon of stress reorientation of hydrides in fuel cladding tubes was studied with the help of a test technique named as the “cladding tube deformation test” (CTDT). Unirradiated Zircaloy-2 (Zry-2) cladding tube specimens charged with 250 ppm of hydrogen were tested. The test consisted of heating specimens to 400°C and then cooling them down to room temperature at cooling rates of 0.7°C/min and 2°C/min. During the cooling phase, a constant tensile load was applied to specimens with the help of two inner cylinder halves. The stress-strain fields developed in the tube specimen were calculated with the help of the finite element method (FEM). Post-testing metallographic observations revealed considerable amount of hydrides oriented radially as a result of loading. Lengths and relative position of all hydride bands were determined through a semi-automatic image processing technique. Mapping of FEM calculated stress field on the metallographic section helped to determine a stress threshold value of 72 MPa. Tests performed with a cooling rate 2°C/min showed a considerably lower extent of hydride reorientation. The above test technique was validated by performing internal pressurization tests using pre-hydrided Zry-2 cladding tubes. The external diameter of the tube was tapered by fine turning in order to exert different uniform hoop stress values on different cross sections of the tube. Internal pressurization tests were performed with a maximum temperature of 400°C and a cooling rate of 0.5°C/min. Post-testing metallographic observations on several cross sections of the specimen revealed a stress reorientation threshold of 65 MPa. Percentage of radially oriented hydrides as well as their lengths increased with the stress level until it reached a plateau. Further tests were carried out with the CTDT technique on specimens with H2 content ranging from 560 ppm to 750 ppm. These results revealed a reorientation stress threshold of around 120 MPa for higher hydrogen content.