Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels

Abstract

Massive, thick-walled pressure vessels are permanent nuclear reactor structures that are exposed to a damaging flux of neutrons from the adjacent core. The neutrons cause embrittlement of the vessel steel that growswith dose (fluence), as manifested by an increasing ductile-to-brittle fracture transitiontemperature. Extending reactor life requires demonstrating that large safety margins against brittle fracture are maintained at the higher neutron fluence associated with beyond 60years of service.Here synchrotron-based x-ray diffraction and small angle x-ray scattering measurements are used to characterize highly embrittling nm-scale Mn-Ni-Si precipitates that develop in the irradiated steels at high fluence. These precipitates lead to severe embrittlement that is not accounted for in current regulatory models. Application of the complementary techniques has, for the very first time, successfully identified the crystal structures of the nanoprecipitates, while also yielding self-consistent compositions, volume fractions and size distributions. I. INTRODUCTIONReactor pressure vessels (RPVs) are the primary permanent component of light water reactors (LWRs). RPVs experience irradiation embrittlement that increases with neutron fluence (see Refs[1-3] for overviews of the embrittlement phenomena and underlying mechanisms). Ensuring that large safety margins are maintained in the face of embrittlement is required to extend LWR service life to beyond 60 years. Embrittlement is marked by increases in the ductile-to-brittle transition fracture temperature (ΔT) of RPV steels. The ΔT is primarily caused by irradiation hardening (Δσ y ) associated with the formation of nm-scale precipitates and solute-defect complexes that act as obstacles to dislocation glide.At low to intermediate neutron fluence, significant hardening and embrittlement is primarily caused by the formation of coherent, transition phase copper rich precipitates (CRP). Trace impurity amounts ofCu(< 0.35 at.%) are insoluble in steels and rapidly phase separate due to radiation enhanced diffusion (RED)at © 2015. This manuscript version is made available under the Elsevier user license

Magnetic small-angle neutron scattering

“…These clusters are presumably related to the formation of "late blooming" MNPs at high fluences (Odette, 1995). Applying a combination of methods, Sprouster et al (2016) confirmed that the MNPs identified after high-fluence irradiations are welldefined phases. Böhmert et al (2004) and Wagner et al (2012) reported pronounced linear correlations of the square root of precipitate volume fraction and irradiation hardening of RPV steels.…”

On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions

“…MNSPs, have been called late blooming phases (LBP), due to their slow nucleation and growth rates compared to CRPs [4][5][6][9][10][11][18][19][20][21]. However, MNSPs are now widely observed [25][26][27][28][29][30][31][32][33][34][35]. Further, recent models [36][37][38][39] support the early predictions of their evolutions as intermetallic phases.…”

The mechanistic implications of the high temperature, long time thermal stability of nanoscale Mn-Ni-Si precipitates in irradiated reactor pressure vessel steels

“…The underprediction of embrittlement at high fluence is primarily attributed to the delayed formation of large volume fractions (f) of Mn-Ni-Si precipitates (MNSPs), which are not accounted for in current regulatory models [4][5][6][7][8]. The existence of MNSPs has been demonstrated in both surveillance and test reactor irradiations [4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22], but their detailed character and formation mechanisms is one of the most highly debated issues in the field of radiation effects [4,14,[23][24][25][26]. The central question relates to the driving force for MNSP formation.…”