Crack Resistance Determination of Irradiated Fuel Cladding using the Cladding Tensile Fracture Test (CTFT)

摘要

The integrity of fuel claddings as first barrier against the loss of fuel or fuel particles into the reactor circuit or towards the environment is mandatory. The resistance of fuel cladding against failure can be quantified in the form of resistance against rupture or crack propagation. This resistance is of relevance during the service in the reactor, during unloading, transportation and — especially - under intermediate dry storage conditions. The mechanical resistance can be expressed as fracture toughness; however, the testing of thin walled cladding tubes is far away from any standard requiring a certain specimen size or wall thickness. Therefore the measurement of crack resistance in fuel cladding is strongly dependent on the test design. At the Paul Scherrer Institut the fracture toughness testing of fuel claddings is performed using the Cladding Tensile Fracture Test (CTFT). Two half-cylinders are fitted into an axially notched and pre-fatigued tube section and then pulled apart; the load is induced orthogonally to the axial direction of the tube. During the displacement controlled tests the crack propagation is optically recorded. In this work, tube sections from a 7 cycles Zircaloy-2 (LK3/L, Westinghouse) fuel rod from the Swiss BWR in Leibstadt (KKL) with an average burn-up of about 60 Gwd/tU and an H content between 200 and 300 ppm as well as un-irradiated reference material have been tensile tested at 300°C. The irradiated Zircaloy-2 cladding fails in a much more brittle way than the un-irradiated cladding. Additionally, the results of the tensile tests with the un-irradiated Zircaloy-2 are compared with those of un-irradiated Zircaloy-4. While Zircaloy-4 shows a relative strong resistance against starting cracks, the un-irradiated Zircaloy-2 exhibits a weaker resistance and a late formation of plastic zone in front of the crack tip. For each sample a complete crack resistance (J-R) curve could be generated and characteristic values as Jq (crack onset) or the slope of the J—R curve have been determined. Additionally, the crack flanks of an irradiated specimen, tensile tested at room temperature and therefore exhibiting very brittle failure, have been investigated with scanning electron microscopy and backscattered electrons for potential hydrides detection.