Factors and mechanisms of elongation of VVER-1000 fuel rods during thermal tests simulating dry storage modes

To prove safety of dry storage conditions, thermal tests of the VVER-1000 fuel rods were performed in electrically heated furnaces in helium gas environment under stationary conditions (they were held at Т = 380 °С for 468 days) and thermal cycling ones (48 temperature cycles in a temperature range of 20 to 380 °С) with duration of cycles varying from 1 to 10 days and the total time of thermal testing of 427 days). The effect of thermal testing on elongation of fuel rods, which were under operation in the VVER-1000 reactors for one year (burnup was ~ 20 (MW⋅days)/kgU) and for six years (burnup was ~ 70 (MW⋅days)/kgU), was studied. It was revealed that high-burnup fuel rods elongated less compared to low burnup fuel rods. The highest elongation values were obtained for the low-burnup fuel rods: it made up 6.0 mm under the stationary operating conditions and 9.5 mm under thermal cycling. Hoop strain of fuel rods increased insignificantly. As there was gas pressure under the cladding, it induced tensile stresses in it at a temperature of 380 °С and thus, stimulated thermal creep. Lower burnup fuel rods could preserve their fuel-cladding gap throughout the entire height of their fuel columns. A model of gas-filled tubular specimen with a textured cladding in which the effective axial stress is enhanced by additional tensile load due to the pressure from fuel column on the spring plunger and a greater recovery of yield strength in the longitudinal direction compared to the transverse one, is applicable to these fuel rods. For high-burnup fuel rods, this model is applicable to for that part of the cladding where it is not in contact with the fuel, i.e. for a shorter length compared to low-burnup fuel rods. Thus, the idea was proved according to which the main mechanism of fuel rod elongation is the thermal creep of the cladding induced by enhanced (due to anisotropic recovery of the yield strength) effective axial stresses resulted from the fuel gas and fuel column pressure on the spring plunger. Thermal cycling leads to greater elongation of fuel rods due to a higher creep rate of the cladding at the unsteady-state stage during short cycles.