Volume 8, Issue 1 (January 2011)
Radiation Damage of E635 Alloy Under High Dose Irradiation in the VVER-1000 and BOR-60 Reactors
Alloy E635 is used as a material for some parts of the water-water energy reactor (VVER)-1000 fuel assemblies (FAs). The evolution of structural components and redistribution of the elementary composition that occurred between the solid solution and phase precipitates in the E635 alloy were analyzed. The correlation between these changes and the irradiation-induced deformation of FA parts was determined. The common features of the E635 alloy’s irradiation damageability was established as a result of the irradiation of model samples in the test reactor BOR-60 and the operation of products in the VVER-1000 reactor. The peculiarities of E365 alloy’s irradiation damageability under high dose irradiation were revealed. The Laves phase (Zr(Nb,Fe)2) particles were found to be the main type of secondary phase precipitates observed in the E635 alloy products with completely or partially recrystallized structures. Under irradiation, iron released from this phase causes a transformation of its crystalline structure, i.e., it changes from hexagonal-close-packed into body-centered-cubic with an Fe-enriched β-Nb phase. The Fe content in the particles decreases as the dose rises. The above transformations are not observed on those FA areas where the damage dose is low. Some bigger (up to 0.50μ) precipitates of the (Z,Nb)2Fe phase with face-centered-cubic lattices were found in the material structure. Irradiation up to high damage doses results also in the appearance of secondary fine-dispersed (up to ∼5 in size) irradiation-induced precipitates. Practically no niobium is observed in the matrix, while all tin is in the solid solution and the Fe fraction in the matrix rises as the fluence becomes higher. The generation and formation of ⟨c⟩-dislocations occur only near the β-Nb precipitates (former Laves phases); ⟨a⟩-dislocations in the form of dislocation loops 10 nm in size and black dots are observed over the whole volume of recrystallized grains. On the whole, the changes in the structural-phase state of the FA parts tested in the VVER-1000 reactor correspond to the ideas about irradiation-induced damage in E635 alloy gained in experiments on irradiation of model samples in the BOR-60 reactor.