IFPE/IFA-585, In-Reactor Creep Behaviour of Zircaloy-2 and Zircaloy-4 under Variable Loading Conditions

1 - Description: The test assembly IFA-585 was designed to study in-reactor creep behaviour of Zircaloy cladding under variable stress conditions. The first loading of the assembly contained two short rods, one fabricated from pre-irradiated Zr-2 (the BWR rod) and the other from unirradiated Zr-4 (the PWR rod). The diameter gauge on the PWR rod ceased functioning after about 8500 hours, but that on the BWR rod continued to give results until the end of the first loading in October 1995. Diameter change measurements were collected regularly from the BWR rod over its 14,400 hours dwell in reactor. During this time the fast neutron flux averaged 3.2 10 E+13 n/cm2/s whilst the cladding temperature stayed between 300 and 320 deg. C. The applied hoop stress was changed nine times within the range -160 to +130 MPa, from compression to tension and vice versa. From the measured variation in rod diameter with time at power, the following parameters have been determined for each stress level: elastic loading strain, primary creep strain and secondary creep rate. Although sometimes difficult to detect, primary creep is exhibited after each stress change resulting in a new secondary creep rate. Total primary strain has been shown to be proportional to the preceding change in stress and to exhibit no asymmetry between tension and compression. The stress dependence of the secondary creep rate is nearly linear and it is observed that the creep rate in tension is generally higher than that in compression for a given stress. The loading strain conforms to elastic behaviour and has been removed from the total diameter change measurements in order to obtain inelastic (creep) strain allowing comparison of the experimental results to existing creep models. Comparison is made to two creep models for Zircaloy, one based on in-reactor results, the other thermal creep results. In both cases the agreement is quite good. Operating conditions: Upper Zr-4 Tube Lower: Peak power = 400 W/cm, peak power at lower end, max-to-min power ratio = 1.15; Cladding temperature at O.D. = 350 deg. C; 305 deg. C inlet; 325 deg. C outlet; Coolant flow velocity = 1.8 m/s; Coolant Pressure = 162 bar; Fast Flux = 5.10 E+13 n/cm2s (>1 MeV); Zr-2 Tube: Peak power = Zero; Cladding temperature at O.D. = 305 deg. C