Atom Probe Tomography characterization of the microstructural evolution of a low copper reactor pressure vessel steel under neutron irradiation

Abstract

A low copper reactor pressure vessel steel was characterised by atom probe tomography after neutron irradiation at different fluences. The specimens were irradiated within the frame of the Surveillance Program of a production reactor. Roughly spherical clusters enriched in nickel, manganese, silicon and, in a lesser extent, phosphorus and copper were observed at all fluences. The chemical composition of these clusters shows no evolution with fluence, as well as their diameter, close to 3 nm. Their number density increases linearly with the neutron fluence. A continuous segregation of the elements found in the clusters is also observed along dislocation lines, with similar enrichments.

The Dominating Mechanisms for the Formation of Solute-Rich Clusters in Steels under Irradiation

“…This clearly supports the hypothesis that solute atoms are continuously dragged by single point defects towards point-defect sinks, irrespective of the magnitudes of solute-solute interactions. Next, a large number density (~10 22 m -3 to 10 2 3 m -3 ) of roughly spherical NSRC is systematically reported in nearly all APT studies on irradiated steels [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. A typical image obtained from APT is shown in Fig.…”

“…In this view, NSRC formation in RPV steels would essentially be an irradiation-enhanced process. In contrast, other researchers have suggested that the evolution of the system is dominantly driven by mechanisms specific to irradiation [16,21,46,47], i.e. considering as crucial the role of self-interstitial atoms (SIA).…”

“…Due to their small size and coherence with the matrix (body centred cubic -bcc -structure), they are not easily resolvable to conventional transmission electron microscopy (TEM). Their existence has been revealed mainly through other microstructural characterization techniques, such as atom probe tomography (APT) [10][11][12][13][14][15][16][17][18][19][20][21]32] and small angle neutron scattering (SANS) [24][25][26][27][28]. Recently, though, modern scanning TEM (STEM) techniques coupled with energy dispersive spectroscopy (EDS) allowed these clusters to be resolved as well [33].…”

Nanostructure Variations and Their Effects on Mechanical Strength of Ni-17Mo-7Cr Alloy under Xenon Ion Irradiation

“…The mechanism of the formation of these solute clusters and dislocation loops may be the radiation-induced segregation (RIS) of solutes to point defects clusters. 25,26) The formation of solute clusters and dislocation loops depend on mainly the original shape of point defect clusters.…”

Current understanding of radiation-induced degradation in light water reactor structural materials

“…Recent 3DAP observations of surveillance materials have provided key data for understanding microstructural evolution in actual vessel materials under operating conditions. In surveillance specimens (16MND5 steels with 0.09 and 0.044%Cu) irradiated in French PWRs at 2 × 10 11 n/cm 2 /s, fine clusters (3-4 nm in diameter) were detected and the cluster density increased with fluence up to 1.7 × 10 20 n/cm 2 [51,52]. The averaged composition in the 0.044%Cu steel was Fe-6Mn-10Ni-8Si in at% and did not change with fluence up to 7.6 × 10 19 n/cm 2 .…”