Modification of ring tensile test for LWR fuel cladding

High burn-up fuel cladding mechanically interacts with expanded fuel pellets (PCMI: pellet/cladding mechanical interaction) under reactivity initiated accident (RIA) conditions. Mechanical property data such as stress-strain curves in the hoop direction are required in order to predict high burn-up fuel behavior and to evaluate the cladding failure limit in terms of PCMI loading at an RIA. Investigations to modify ring tensile test have been performed to measure the hoop mechanical properties of the Zircaloy cladding tube properly. The Japan Atomic Energy Research Institute (JAERI) developed the test method and geometry which minimize undesirable effects from friction and bending moment in the ring specimen. The optimized specimen has one gauge section and is stretched with the two-piece tooling consisting of half-mandrels. The gauge section of the specimen is set on the top of a half-mandrel. Dimensions of the gauge section are from 3 to 4 mm in length and 2 mm in width. Using the modified test method, mechanical properties of unirradiated Zircaloy-4 cladding were evaluated as functions of hydrogen concentration and temperature of the specimen. The cladding ductility generally increases as the temperature increases from 300 to 573 K. For hydrogen concentrations above 700 ppm, obvious increase of ductility is observed with the temperature increase from 300 to 473 K. For hydrogen concentrations below 500 ppm, on the other hand, temperature dependence of the ductility is relatively small in the present temperature range from 300 to 573 K. Accordingly, RIA-simulation experiments performed at room temperature in the NSRR is not excessively conservative from the view point of cladding ductility. (author)