Corrosion, Dimensional Stability and Microstructure of VVER-1000 E635 Alloy FA Components at Burnups up to 72 MWday/kgU

摘要

The use of irradiation-resistant E635 alloy for fuel rod cladding and skeleton components of the VVER-1000 fuel assemblies advanced (FAA) has ensured the stability of the fuel assembly (FA) geometrical dimensions, minimized their bending and distortion, and increased the resistance of fuel cladding to shape changes at burnups to 72 MWday/kgU after 6 years of operation. Post-irradiation investigations of the VVER-1000 FAA components (fuel rod cladding, guide thimbles, central tube, and rigid angles) of E635 alloy show that, in terms of their major operational characteristics, additional margin to the design limits remain following six 1-year cycles. The geometrical parameters, oxidation, hydrogen absorption, tensile properties, and microstructural state of the components did not reach values that would inhibit their further performance. The oxide film thickness and the hydrogen content of E635 alloy are correlated along the core length, increase with burnup, and are a function of both temperature and neutron fluence. The oxide film thickness increases up to 80 μm at an assembly height of ～3100 mm for cladding and is less than 42 μm on the outer surface of the guide thimbles and rigid angles. Hydrogen content in the cladding is less than 0.03 % and reaches 0.06 % around the bend of the rigid angles. Transmission electron microscopy (TEM) studies of dislocation structure, and chemical and phase composition of E635 alloy components revealed microstructural characteristics influenced by temperature and neutron irradiation. The Laves phase precipitates and the matrix composition change caused by depletion of iron from SPPs in the irradiated fuel cladding, guide thimbles, central tube, and rigid angles as a result of changes in temperature and neutron fluence, and determine their mechanical properties and shape changes resistance.