Low Copper Steels Research Articles

Predictive reactor pressure vessel steel irradiation embrittlement models: Issues and opportunities

Nuclear plant life extension to 80 years will require accurate predictions of neutron irradiation-induced increases in the ductile-brittle transition temperature (ΔT) of reactor pressure vessel steels at high fluence conditions that are far outside the existing database. Remarkable progress in mechanistic understanding of irradiation embrittlement has led to physically motivated ΔT correlation models that provide excellent statistical fits to the existing surveillance database. However, an important challenge is developing advanced embrittlement models for low flux-high fluence conditions pertinent to extended life. These new models must also provide better treatment of key variables and variable combinations and account for possible delayed formation of “late blooming phases” in low copper steels. Other issues include uncertainties in the compositions of actual vessel steels, methods to predict ΔT attenuation away from the reactor core, verification of the master curve method to directly measure the fracture toughness with small specimens and predicting ΔT for vessel annealing remediation and re-irradiation cycles.

Prediction of radiation hardening in reactor pressure vessel steel based on a theoretical model

A theoretical model is presented that calculates the amount of hardening in reactor pressure vessel (RPV) steels under neutron irradiation. A reaction rate theory is used to describe the evolution of radiation-induced microstructures. The point defect clusters (PDC) and copper-rich precipitates (CRP) have been recognized as contributing to radiation hardening in irradiated alloys, which act as barriers to the dislocation motion on the slip plane. The model includes a detailed description of PDC and CRP, and estimates the increase in yield strength due to these microstructures. The yield strength changes predicted by the model calculation are compared with the measured yield strengths, which were obtained from a surveillance program in French pressurized water reactors (PWR) and from post-irradiation tests performed at a high-flux advanced neutron application reactor (HANARO) in Korea. A fair agreement was found between estimated and observed hardening of irradiated RPV steels in spite of uncertainty about a broad range of irradiation and material parameters. The calculation results imply that the inclusion of CRP tends to overestimate the amount of radiation hardening in low-copper RPV steels. Therefore, the PDC evolution is more important for predicting the amount of radiation hardening in low-copper steels.

Microstructural characterization of irradiation-induced Cu-enriched clusters in reactor pressure vessel steels

The effect of irradiation on microstructure of four irradiated reactor pressure vessel steels (a low copper A533B-1 plate, a low copper A508-3 forging, a high copper Linde 80 flux weld and a high copper Linde 1092 flux weld) was determined by using complementary microstructural techniques such as optical position-sensitive atom probe (OPoSAP), field emission gun scanning transmission electron microscopy (FEGSTEM) and small angle neutron scattering (SANS). In the low copper steels, irradiation resulted in small shifts in transition temperature and small changes in hardness increments. The microstructural analyzes showed that this response was dominated by matrix damage. In contrast, both copper-enriched clusters and matrix damage formed in the high copper welds. This information was then used as input to the Russell–Brown model to predict the change in hardness resulting from copper-enriched clusters. The calculated hardness increments were found to be consistent with the experimental data.

Recent progress in understanding reactor pressure vessel steel embrittlement

This paper reviews the current understanding of the basic mechanisms of irradiation embrittlement in reactor pressure vessel steels. Radiation enhanced diffusiona at operating temperatures around 290°C leads to the formation of various ultrafine scale hardening phases, including copper rich and copper catalysed manganese-nickel rich precipitates. Other nanofeatures that do not require copper, so-called matrix defects, include alloy phosphides and carbonitrides as well as defect cluster-solute complexes. Matrix defects that are thermally unstable (anneal) under irradiation play a very important role in mediating flux and temperature effects. The balance of features depends on the composition of the steel and the irradiation conditions. Copper enriched phases, which are the dominant embrittling feature in alloys containing significant trace quantities of this element, are fairly well understood. In contrast, the detailed identity and etiology of the matrix defects and manganese-nickel rich phases that may form in very low copper steels has not yet been established. Embrittlement of typical (Mn-Mo-Ni) pressure vessel steels, manifested as shifts in Charpy V-notch transition temperature, can generally be related to yield stress increases. Yield stress increases from copper rich precipitates are consistent with predictions using defect-obstacle interaction theory coupled with a new model for superposition of the hardening from both pre- and post-irradiation sources of strength. Details of the strengthening contributions from the other irradiation features are not as well established, but appear to be reasonably consistent with theory. These concepts have led to the development of thermodynamic-kinetic-micromechanical models that are broadly consistent with experiment, and rationalize the highly synergistic effects of important irradiation (e.g., temperature, flux, fluence) and metallurgical (e.g., copper, nickel, manganese, phosphorous and heat treatment) variables on both irradiation hardening and hardening recovery during post-irradiation annealing. Open questions can be addressed with a hierarchy of new theoretical and experimental tools, which range from atomistic modeling to tomographic methods of observing the sequence-of-events leading to fracture. Advanced microstructural evolution, microstructure-property and micromechanical models, validated and calibrated by well designed experiments, will greatly enhance our ability to predict pressure vessel embrittlement and to resolve out-standing technical issues.

Kinetic Lattice Monte Carlo Simulations of Cascade Aging in Iron and Dilute Iron-Copper Alloys

AbstractNeutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster- complexes constitute both thermally stable and unstable matrix defect features.

Kinetic Lattice Monte Carlo Simulations of Cascade Aging in Iron and Dilute Iron-Copper Alloys

AbstractNeutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster-complexes constitute both thermally stable and unstable matrix defect features.

Analysis of flow oscillations in a natural circulation system taking account of non-equilibrium boiling and void flashing

Pressure vessel steels used in PWRs are known to be prone to hardening and embrittlement under neutron irradiation. The changes in mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and copper-rich precipitates. However, the real nature of the irradiation induced damage, in these particularly low copper steels (< 0.1 wt%), has not been clearly identified yet.A new experimental work has been carried out thanks to atom probe and field ion microscopy (APFIM) facilities and, more particularly with a new generation of atom probe recently developed, namely the tomographic atom probe (TAP), in order to improve:-the understanding of the complex behavior of copper precipitation which occurs when low-alloyed FeCu model alloys are irradiated with neutrons;-the microstructural characterization of the pressure vessel steel of the CHOOZ A reactor under various fluences (French Surveillance Programme).The investigations clearly reveal the precipitation of copper-rich clusters in irradiated FeCu alloys while more complicated Si, Ni, Mn and Cu-solute ‘clouds’ were observed to develop in the low-copper ferritic solid solution of the pressure vessel steel.

Clustering of Point Defects Under Electron Irradiation in Dilute Iron Alloys and an Iron Manganese Nickel Alloy

AbstractIn low copper steels for nuclear reactor pressure vessel, point defect clustering seems to play an important role in hardening. In order to study the hardening component which results from the clustering of freely migrating point defects, we irradiated, in a high voltage electron microscope, Fe, the alloys Fe0.13%Cu and Fe0.014%P, alloys and the alloy Fel.5%Mn0.8%Ni0.1%Cu0.01%P, the composition of which is close to the matrix of pressure vessel steels. We studied the nucleation of dislocation loops and their growth velocity. We find out that copper and phosphorus have no effect on the vacancy migration energy but that this parameter decreased significantly in the complex alloy. The main point is certainly that loops are nucleated in the complex model alloy up to 500°C while no loop appears above 300°C in Fe and in FeCu. FeP shows an intermediate behavior.

APFIM investigation of clustering in neutron-irradiated Fe [sbnd]Cu alloys and pressure vessel steels

Pressure vessel steels used in PWRs are known to be prone to hardening and embrittlement under neutron irradiation. The changes in mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and copper-rich precipitates. However, the real nature of the irradiation induced damage, in these particularly low copper steels (< 0.1 wt%), has not been clearly identified yet. A new experimental work has been carried out thanks to atom probe and field ion microscopy (APFIM) facilities and, more particularly with a new generation of atom probe recently developed, namely the tomographic atom probe (TAP), in order to improve: - the understanding of the complex behavior of copper precipitation which occurs when low-alloyed Fe Cu model alloys are irradiated with neutrons; - the microstructural characterization of the pressure vessel steel of the CHOOZ A reactor under various fluences (French Surveillance Programme). The investigations clearly reveal the precipitation of copper-rich clusters in irradiated Fe Cu alloys while more complicated Si, Ni, Mn and Cu-solute ‘clouds’ were observed to develop in the low-copper ferritic solid solution of the pressure vessel steel.

Radiation Induced Microstructural Evolution in Reactor Pressure Vessel Steels

AbstractThe evolution of the fine scale microstructural features leading to irradiation embrittlement of reactor pressure vessel steels is described. Copper rich phases undergo accelerated precipitation from supersaturated solution due to radiation enhanced diffusion. In steels with significant trace quantities of copper the precipitates, characterized by high concentrations and small sizes, are the dominant embrittling feature. Precipitate concentrations, sizes, volume fractions and compositions are consistent with thermodynamic and kinetic models that rationalize the effects of a number of irradiation and metallurgical variables. Phosphide and carbonitride phases may also develop along with new manganese nickel rich precipitates, promoted by high nickel contents. These features may lead to severe embrittlement at high fluence even in low copper steels. While their detailed identity and characteristics are not known, defect cluster-solute complexes with a range of thermal stability are important both directly and indirectly; for example, in mediating flux and temperature effects. In conjunction with the application of state-of-the-art characterization methods, development of advanced modeling tools will be needed to address a number of outstanding issues.