Microstructural Processes in Irradiated Materials: Reactor Pressure Vessel Steels
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee
Program Organizers: Thak Sang Byun, Pacific Northwest National Laboratory; Chu-Chun Fu, Commissariat à l'énergie atomique et aux énergies alternatives (CEA); Djamel Kaoumi, University of South Carolina; Dane Morgan, University of Wisconsin-Madison; Mahmood Mamivand, University of Wisconsin-Madison; Yasuyoshi Nagai, Tohoku University

Monday 2:00 PM
February 27, 2017
Room: Del Mar
Location: Marriott Marquis Hotel

Session Chair: Peter Wells, University of California-Santa Barbara; Peter Hosemann, University of California-Berkeley

2:00 PM  Invited
A Summary of ATR–2 Reactor Pressure Vessel Steel High Fluence Irradiation: Some New Science and Implications to Extended Reactor Life: G. Robert Odette1; Peter Wells1; Takuya Yamamoto1; Nathan Almirall1; Randy Nanstad2; 1University of California Santa Barbara; 2Oak Ridge National Laboratory
    The UCSB ATR-2 program’s goal is to support the development of new, physical RPV embrittlement models for high extended-life fluence. ATR-2 contains 1625 specimens of various types, composed of 172 alloys, irradiated over a range of flux, fluence and temperature. Initial results show current regulatory models generally under predict hardening at 1.4x1020 n/cm2 and 290°C. APT, SANS and SAXS show that the excess hardening is due to Mn-Ni-Si precipitates (MNSP) that emerge at high fluence, and for all Cu, and a wide range of Ni, levels. The ATR-2 flux is ≈ 10-70 times higher than in service. ATR-2 will bridge existing low flux surveillance to high flux test reactor databases. The combined databases will help to quantify the “when (flux/fluence), where (alloy compositions) and how much (hardening/shift contributions)” of MNSP. Notably, the new ATR-2 microstructural and mechanical property data are in remarkable agreement with a previously developed physically based model.

2:30 PM  
Structural Characterization of Precipitates in Neutron Irradiated Surveillance Reactor Pressure Vessel Steels: David Sprouster1; E Dooryhee1; S Ghose1; M Elbakhshwan1; P Wells1; T Stan1; N Almirall2; G. R. Odette2; M. Sokolov3; R. Nanstad3; L Ecker1; 1Brookhaven National Laboratory; 2Materials Department, University of California, Santa Barbara; 3Oak Ridge National Laboratory
    Reactor pressure vessels, the primary permanent component of Light Water Reactors (LWRs) are exposed to intense neutron radiation fields at service temperatures ≈ 290°C, leading to irradiation hardening and embrittlement. This is primarily due to nano-scale precipitates, including functionally significant volume fractions of Cu and Mn-Ni-Si phases. The latter more likely emerge at high fluences that are associated with extended plant life. Here, we describe recent efforts employing synchrotron-based techniques (X-ray diffraction, and small-angle X-ray scattering) to quantitatively determine the size, volume fraction and microstructural properties of the radiation induced precipitates in several reactor pressure vessel materials that have been irradiated in test reactor up to 1× 1020 n/cm2. We will discuss our results and compare to complementary characterization methods. Our multi-characterization approach will better enable quantification of the compositions and structures of the crystallographic phases that develop during irradiation.

2:50 PM  
Modeling Cu-Mn-Ni-Si Precipitation in Reactor Pressure Vessels: Mahmood Mamivand1; Huibin Ke1; Peter Wells2; George Odette2; Dane Morgan1; 1University of Wisconsin-Madison; 2University of California-Santa Barbara
    Reactor pressure vessels (RPVs) are critical structural elements in nuclear reactors and regulations require a very low probability for RPV failure. RPVs experience embrittlement under neutron irradiation, which could lead to the cleavage fracture of the structure. Quantification and prediction of RPVs embrittlement are necessary to prove the safe operation of RPVs under extended life conditions. One major source of RPV embrittlement is the formation of Cu-Mn-Ni-Si (CMNS) precipitates. We developed a CALPHAD informed Cluster Dynamics model to capture the CMNS precipitation in RPVs under irradiation. We benchmarked the model against available experimental data from test reactors and then use the validated model to gain insight into RPV embrittlement under light water reactor (LWR) extended life conditions.

3:10 PM  
Kinetic Monte Carlo Modeling of CuMnNiSi Precipitation in Reactor Pressure Vessel Steels: Shipeng Shu1; Dane Morgan1; Peter Wells2; Nathan Almirall2; Robert Odette2; 1University of Wisconsin-Madison; 2University of California, Santa Barbara
    In reactor pressure vessel (RPV) steels, significant embrittlement can be caused by the formation of nanometer-scale Mn-Ni-Si precipitates (MNSPs). Here a kinetic Monte Carlo (KMC) model parameterized using CALPHAD and recent APT data is used to simulate the precipitation processes in RPV steels. The model predicts a temperature dependent composition of MNSPs, which is consistent with experimental observations. It is proposed that due to the lack of mobility inside the MNSPs, the diffusion limited-growth of MNSPs can lead to this temperature dependent precipitate composition. The KMC model has also been used to investigate the morphology of the Cu-MNSP precipitates formed during irradiation and annealing. A kinetically limited growth mechanism is proposed to explain the fact that MNSP forms as an appendage on Cu clusters, rather than a shell coating the Cu core.

3:30 PM  
Phase-field Modelling of Gamma-precipitate Behaviour in RPV Steel: Kunok Chang1; Junhyun Kwon1; 1Korea Atomic Energy Research Institute
    We performed the phase-field modelling of gamma-precipitate behaviour in RPV steel. We adopted the phase stability and solute mobility of Fe-Cu-Mn-Ni quaternary system, which was assessed by T. Koyama et al. Additionally, we incorporated inhomogeneous elasticity due to the misfit strain between alpha and gamma phases and interstitial /dislocation loop. We found that the particle morphology evolves from spherical to the cuboidal and cubical particle as the misfit strain increases and the morphological evolution heavily affects the precipitate kinetics of the gamma phase. We also investigated a role of the external source of the inhomogeneous elasticity, such as dislocation and interstitial loop, which is generally produced by the neutron irradiation.

3:50 PM Break

4:05 PM  Invited
Effect of Heat Load on Microstructural Development in Irradiated Steels: Naoyuki Hashimoto1; Eriko Suzuki2; 1Hokkaido University; 2Japan Atomic Energy Agency
    Thermal neutron induces cascade damage and defect clusters would form in structure materials. When the severe accident or some crucial troubles happens and results in the stop of coolant system, it is predicted that temperature would rise up in a short time and then cooled down rapidly or slowly. In such heat load might affect the mechanical property of structural materials. In order to investigate the influence of heat load on mechanical properties, this study is focused on the change of microstructure such as the size and the density of loops, line dislocations and voids in electron- and ion-irradiated steels. The heat treatment at several heating and cooling rates after irradiation revealed that irradiation-induced microstructure would be recovered after heat treatment, especially at early stage of irradiation. On the other hand, in case of higher irradiation, longer cooling could proceed microstructural evolution.

4:35 PM  
Instrumental Methodology at the Atomic Scale to a Better Understanding of Grain Boundary Segregation Mechanisms in Steels: Alfiia Akhatova1; Bertrand Radiguet1; Fabien Cuvilly1; Emmanuel Cadel1; Auriane Etienne1; Laurence Chevalier1; David Gibouin1; Philippe Pareige1; 1GPM, University of Rouen
    Intergranular segregation of phosphorous could contribute to long term embrittlement of RPV steels and dramatically degrade them. The segregation intensity is dependent on material composition, ageing temperature, irradiation conditions and grain boundary nature determined by misorientation between neighboring grains and the grain boundary plane. However, the last item is poorly covered in the literature. This talk will report on the characterization of intergranular segregation after self-ion irradiation and thermal aging of Fe-P model alloys with different bulk compositions. The methodology, combining different techniques such as Atom Probe Tomography, Scanning Transmission Electron Microscopy, Nanoscale Secondary Ions Mass Spectrometry and Scanning Electron Microscopy/Focused Ion Beam/Transmission Kikuchi Diffraction to get accurate and representative information about intergranular segregation will be described. Influence of grain boundary nature on the level of equilibrium segregation after annealing at 450°C and non-equilibrium segregation after self-ion irradiation (Fe5+, up to 1dpa, 450°C) will be discussed.

4:55 PM  
Hardening Mechanism of a Neutron Irradiated Reactor Pressure Vessel Steel Studied by APT, PAS and WB-STEM: Masaki Shimodaira1; Takeshi Toyama1; Kenta Yoshida1; Koji Inoue1; Yasuyoshi Nagai1; Toshimasa Yoshiie2; Milan Konstantinovic3; Robert Gerard4; 1Tohoku University; 2Kyoto University; 3SCK-CEN; 4Tractebel ENGIE
    Reactor pressure vessel steels with Cu content of 0.04wt% irradiated to the fluence of 1 × 1020 n/cm2 in the maximum were investigated by atom probe tomography (APT), positron annihilation spectroscopy (PAS) and weak beam scanning transmission electron spectroscopy (WB-STEM). The fluence dependency of the irradiation hardening due to solute cluster (SC) formation estimated from the number density, the average size and the volume fraction of the SCs obtained by APT was different from the trend of the measured micro-Vickers hardness. It is consistently explained that the difference can be caused by the defect formation observed by PAS and WB-STEM.

5:15 PM  
Chemistry Factor Development for Prediction of Reactor Pressure Vessel Embrittlement: Peter Wells1; Takuya Yamamoto1; Huibin Ke2; Nathan Almirall1; Dane Morgan2; G Odette1; 1UC Santa Barbara; 2University of Wisconsin, Madison
    Embrittlement of reactor pressure vessel (RPV) steels may limit the lifetime of light water nuclear reactors. To safely operate our nation’s fleet of reactors for an extended lifetime, robust embrittlement prediction models must be developed and validated. One particular challenge with predicting embrittlement is that no two RPVs are exactly the same, both in terms of service conditions and especially their alloy chemistries. Thus, individual and synergistic effects of solutes, like Cu, Mn, Ni and Si, must be quantified to accurately predict embrittlement. Data will be presented for a matrix of alloys, with systematic variations in composition, to determine an embrittlement chemistry factor defined in terms of the volume fraction of precipitates at a given irradiation condition. In addition, empirical chemistry factors will be presented and compared with advanced thermodynamic models to better understand the synergistic relationship among the various elements.

5:35 PM  
Computer Simulation of Defect-free Channel Formation by the Monte Carlo Method: Peter Doyle1; Kelsa Benensky1; Steven Zinkle1; 1University of Tennessee, Knoxville
    Defect-free dislocation channel formation promotes plastic instability due to localized plastic flow and is observed in many materials irradiated at low temperatures. Channel formation is manifested by pronounced loss of ductility and strain hardening; therefore it is important for predicting safe operating conditions of nuclear reactor structures. The present work uses a Monte Carlo method to model the channel formation process via a simple stochastic process dependent upon grain size, defect density, and defect size. Based on MD simulations and in-situ experimental observations, each dislocation encounter with a loop or SFT is assumed to cause absorption of the defect cluster, producing a jog up or down of height d/2 where d is the loop diameter. The model predicted rapid channel formation and comparison of predicted channel sizing to existing TEM data yielded good agreement, indicating channel formation is predominately dependent on simple stochastic processes, with energetics being a minor effect.