Improving the safety and reliability of reactor pressure vessel steels
Keywords: Steel
Atom Probe
Nuclear materials

The mechanical properties of reactor pressure vessel (RPV) steels are known to degrade during operation. Hardening embrittlement, due to solute cluster/precipitate formation, and non-hardening embrittlement, as a result of solute segregation to internal interfaces, are a major concern. The ability to accurately predict changes in mechanical properties as a result of these phenomena is highly desirable within the nuclear industry, both from a safety and economic perspective. Reliable predictions of the safe operational-lifetime of the RPV are required to minimise the likelihood of failure during operation. Meanwhile, shutting down a reactor prematurely is associated with substantial loss in potential revenue. This, combined with the high initial costs associated with nuclear power plant construction, mean that extending the operational life of current power plants and designing future reactors with longer lifetimes is economically desirable. However, this requires detailed understanding of how the material’s microstructure develops during operation and models must also be informed and validated via comparison with experimental data.
To study the effect that composition has on RPV steel response to thermal ageing, 19 alloys with systematic variations in the levels of Cu, Ni, Mn, Si, and P levels were thermally-aged at 330°C,365°C, and 405°C for over 30,000 hours. Results from Vickers hardness tests revealed that an alloy containing 0.3 at.% Cu displayed a significant increase in hardness; atom probe tomography(APT) experiments revealed Cu-rich precipitate formation in the matrix and at heterogeneous nucleation sites. Ageing temperature was observed to affect the measured precipitate number densities, sizes, and volume fractions, with higher number densities of smaller precipitates present after thermal ageing at lower temperatures. Meanwhile, a low-Cu alloy with elevated Ni, Mn, and Si levels displayed no significant hardness increase after 20,733 hours of ageing. However, a low number density of Mn-Ni-Si-rich precipitates were observed to have formed on heterogeneous nucleation sites in this alloy. Ageing temperature was observed to influence precipitate composition; precipitates analysed in the 405°C specimens contained significantly less Si than precipitates that were detected at lower ageing temperatures.
Four alloys that had been neutron-irradiated to a fluence of 1.4×1020n/cm2 at 290°C and a flux of 3.6×1012n/cm2s were characterised using APT. Precipitate formation was observed on homogeneous and heterogeneous nucleation sites in all four alloys, including an alloy that contained low nominal levels of Cu (0.02 at.%), Mn (0.04 at.%), and Si (0.06 at.%). The nominal alloy composition was observed to affect precipitate composition, with lower nominal levels of Ni or Mn resulting in precipitates that contained decreased levels of Ni or Mn, respectively. Precipitate nucleation site in the matrix was not observed to affect precipitate composition. However, precipitates which were detected at carbide-matrix interfaces contained less Mn than precipitates detected in the matrix. The average compositions of the precipitates detected in the matrix were observed to be different to those predicted by thermodynamic models.
APT is well-suited to investigate solute cluster formation and grain boundary segregation in RPV steels due to its unique combination of high spatial and chemical resolution. However, the analysis and interpretation of APT data is complex, particularly when studying the early stages of cluster formation and grain boundary segregation in irradiated steels. Failure to identify and account for clusters that are intersected by the edge of the analysis volume may reduce the accuracy of size and number density measurements. A novel data analysis technique has been developed which automatically and reproducibly identifies edge clusters. After implementing this method, measurements of cluster number density, size, and composition demonstrated improved accuracy.
In selected samples, the use of transmission Kikuchi diffraction (TKD) and an original analysis method has permitted the five macroscopic degrees of freedom of a grain boundary to be determined. Meanwhile, subsequent APT analysis of the same feature enabled characterisation of the segregation behaviour of solutes to the grain boundary. Combining information from both of these techniques has resulted in a more holistic description of the grain boundary. The segregation behaviour observed in neutron-irradiated alloys was complex and non-uniform across the interface, this highlighted the limitations of traditional methods for reporting grain boundary segregation in APT measurements. The causes of these limitations are discussed and alternative methods applied to a grain boundary from a neutron-irradiated steel that was characterised using TKD and APT.