Changes in the Microstructure and Mechanical Properties of Zr-1% Nb (E110) Alloy Cladding during Thermal Tests of VVER-1000 Fuel Elements Simulating Dry Storage Conditions

Thermal tests of two types were performed in helium to simulate dry storage conditions for fuel rods with claddings made of Zr-1% Nb (E110) alloy which had been operated in the VVER-1000 reactors up to the average calculated fuel burnup values of 20, 63 and 70 MWd/kg U. The first type of tests was performed under two conditions: steady-state (holding at T = 380°C for 468 days) and thermal cycling (48 thermal cycles at 90°С/380°C with the different duration of cycles from 1 to 10 days and total time of testing was 427 days). The second type of tests had two stages. At the first stage, two modes were achieved: vacuum drying (step-by-step heating and holding at each stage for a day at temperatures of 200, 400, and 440°C) and vacuum drying together with a simulated design basis accident (there was a step-by-step temperature reduction from 440 to 380 and to 360°С after vacuum drying and the time of holding was 14 and 168 hours, respectively). Fuel rods were held at T = 350°С for 404 days at the second stage. The thermal impact caused a decrease in the density of radiation-induced 〈a〉-type dislocations, the ratio of Nb atoms in β-Nb particles, and the size of the particles. Radiation-induced defects in the structure annealed more severely as a result of thermal tests of the first type as compared to the second type. Partial annealing of the radiation-induced defects led to a partial regain of the yield strength of the claddings to the original values to a greater extent in the longitudinal direction rather than in the transverse one. The better recovery of yield strength as a result of thermal tests of the first type rather than of the second type is due to the higher efficiency of temperature impact as a function of time on the microstructure of the E110 alloy that is estimated as the relevant cumulative annealing parameter. Different conditions of each thermal test did not lead to a significant difference in the yield strength recovery. Tensile stresses that occurred in the claddings at high temperatures during thermal tests of both types contributed to the reorientation of zirconium hydrides as the radial orientation factor F_n increased to 0.3 with a minor decrease in their specific length. It did not cause a reduction in the plasticity of the claddings because of the low degree of hydrogenation.