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Investigating Irradiation Creep of Zircaloy-4 Using In-Situ Proton Irradiation and Transmission Electron Microscopy

Zirconium alloys are used in civil nuclear reactors as fuel cladding, due to their relatively low neutron absorption cross section and being ability to maintain integrity during operation in a challenging environment – under elevated temperature, in corrosive water, under stress, and while being bombarded with different types of irradiation. In anisotropic materials, such as the hexagonal close packed Zr crystal structure, irradiation-induced defects diffuse at different rates in different crystallographic directions.

Product Number: Ed22-18344-SG
Author: Conor Gillen, Graeme Greaves, Rhys Thomas, Adeel Shaikh, Robert Harrison, Alistair Norris, Alistair Garner, Aidan Cole-Baker
Publication Date: 2022
$20.00
$20.00
$20.00

Under normal operating conditions, zirconium alloys in Pressurised Water Reactors (PWRs) can plastically deform in the presence of irradiation at stresses lower than the yield stress, resulting in component distortion and having a cascading affect upon other degradation mechanisms. In this feasibility study, dog-bone recrystallised annealed (RXA) Zircaloy-4 specimens were strained while under transmission electron microscopy (TEM) observation, producing point defects and dislocations. Repeated stop-go irradiation of 700 KeV Kr2+ ions at a flux of 1 x 1016 ions / m2s was found to impact dislocation movement, as is postulated in several mechanisms of irradiation creep. By comparing stills taken from video footage of dislocations during irradiation, the comparative speeds of individual dislocations, as well as general areas of crystallographic defects may be examined. By measuring the relative movement of features surrounding an electropolishing-induced hole at the centre of the gauge length, the effect of irradiation on the local strain can be analysed, with the results informing discussion around previously proposed creep mechanisms, such as I-Creep and the possibility of irradiation cascades causing repeated dislocation unpinning. Results suggest the impact of dislocation movement upon irradiation creep could be significant, although the contribution of mass transport mechanisms remain unclear.

Under normal operating conditions, zirconium alloys in Pressurised Water Reactors (PWRs) can plastically deform in the presence of irradiation at stresses lower than the yield stress, resulting in component distortion and having a cascading affect upon other degradation mechanisms. In this feasibility study, dog-bone recrystallised annealed (RXA) Zircaloy-4 specimens were strained while under transmission electron microscopy (TEM) observation, producing point defects and dislocations. Repeated stop-go irradiation of 700 KeV Kr2+ ions at a flux of 1 x 1016 ions / m2s was found to impact dislocation movement, as is postulated in several mechanisms of irradiation creep. By comparing stills taken from video footage of dislocations during irradiation, the comparative speeds of individual dislocations, as well as general areas of crystallographic defects may be examined. By measuring the relative movement of features surrounding an electropolishing-induced hole at the centre of the gauge length, the effect of irradiation on the local strain can be analysed, with the results informing discussion around previously proposed creep mechanisms, such as I-Creep and the possibility of irradiation cascades causing repeated dislocation unpinning. Results suggest the impact of dislocation movement upon irradiation creep could be significant, although the contribution of mass transport mechanisms remain unclear.