Effects of neutron irradiation and iodine as a corrosive fission product on the fatigue behavior of Zircaloy-2 fuel cladding tubes were investigated using two different types of test specimens in order to evaluate the fatigue strength of BWR fuel subjected to such variable loading conditions as load following or automatic frequency control operations.
An in-cell fatigue tester was developed that enabled tests on the C-shaped specimen machined from irradiated fuel cladding tubes. Unirradiated Zry-2 plate specimens were also fatigue tested for reference. In order to obtain the effects of iodine on fatigue life, the tests using plate specimens were undertaken at iodine partial pressures from 0.5 to 140 Pa as well as in an inert gas atmosphere. Experimental results from fully reversed bending test at 623 K for unirradiated and irradiated fuel cladding tubes up to 6 × 1025 n/m2 (E > 1 MeV) are summarized as follows.
Fatigue life had a tendency to drop with increasing iodine partial pressure, reaching a saturation value about one tenth of that in an inert gas atmosphere.
Minimum iodine partial pressure affecting the fatigue behavior of fuel cladding tubes was estimated to be 0.1 Pa. This value was much higher than the calculated equilibrium vapor pressure of iodine in fuel rods, indicating that effects of iodine on the fatigue life would be very small or negligible during variable loading conditions.
Neutron irradiation was found to increase the fatigue life of cladding tube for the total strain amplitude above 0.3% and decrease it below 0.3%. The increase or decrease in fatigue cycles was attributed to the hardening effect or localized deformation in the irradiated material, respectively.
Fatigue limit of unirradiated Zry-2 tubes was shown to be 0.22%, and neutron irradiation reduced the value to 0.18%. The total strain amplitude of 0.18% was found to coincide with the elastic strain at the proportional limit under the uniaxial tensile test of irradiated Zry-2.
Both for unirradiated and irradiated specimens, transgranular fracture surfaces were induced by the bending. Ductile fracture surfaces were observed for unirradiated material, and neutron irradiation changed this surface into a typical brittle transgranular one.