F. Bergner

Flux dependence of cluster formation in neutron-irradiated weld material

The effect of neutron flux on the formation of irradiation-induced clusters in reactor pressure vessel (RPV) steels is an unresolved issue. Small-angle neutron scattering was measured for a neutron-irradiated RPV weld material containing 0.22 wt% impurity Cu. The experiment was focused on the influence of neutron flux on the formation of irradiation-induced clusters at fixed fluence. The aim was to separate and tentatively interpret the effect of flux on the characteristics of the cluster size distribution. We have observed a pronounced effect of neutron flux on cluster size, whereas the total volume fraction of irradiation-induced clusters is insensitive to the level of flux. The result is compatible with a rate theory model according to which the range of applied fluxes covers the transition from a flux-independent regime at lower fluxes to a regime of decelerating cluster growth. The results are confronted with measured irradiation-induced changes of mechanical properties. Despite the observed flux effect on cluster size, both yield stress increase and transition temperature shift turned out to be independent of flux. This is in agreement with the volume fraction of irradiation-induced clusters being insensitive to the level of flux.

Mechanical Properties of Spark Plasma Sintered Fe-Cr Compacts Strengthened by Nanodispersed Yttria Particles

Abstract Oxide dispersion strengthening of high-Cr steels is a well-recognized way to extend the application window including nuclear applications for this class of materials. The experimental investigation of model alloys of less complexity is important in order to separate individual influence factors and to understand the irradiation behaviour. The present work is devoted to the mechanical properties of ODS Fe-9wt%Cr alloys produced by means of spark plasma sintering. The range of material conditions covers contents of nanodispersed yttria of 0 (reference), 0.3 wt%, and 0.6 wt% as well as variations of the milling time. Results obtained for the density, elastic properties, hardness, tensile behaviour, and brittle-ductile transition are reported, and the effect of ODS content and PM process parameters is discussed.

The Influence of Helium and ODS on the Irradiation-Induced Hardening of Eurofer97 at 300°C

The influence of helium on the mechanical properties of reduced-activation ferritic/martensitic Cr-steels under fusion-relevant irradiation conditions is still a concern. While the fact that He can influence the mechanical properties is well established [1,2], the underlying mechanisms are not fully understood [1,2]. In this work the effect of He and displacements per atom (dpa) on the irradiation-induced hardening of Eurofer97 at 300°C was studied. Self-ion irradiation was applied to simulate the neutron-irradiation-induced damage. Helium was implanted prior to (pre-implantation), simultaneously (dual-beam irradiation) or following the (post-implantation) self-ion irradiation to investigate the He effect. Nanoindentation was used in order to characterize the damage layer. Under the present conditions (300°C, 1 dpa, 10 appmHe) the observed hardening increased in the following order: single-beam Fe-ion irradiation/pre-implantation < simultaneous implantation < post-implantation. We conclude, that there is a significant interaction between damage and He. Additionally, Eurofer97 and ODS-Eurofer were irradiated with Fe ions up to 1 and 10 dpa to study the effect of the oxide particles on the irradiation-induced hardening. We have found a higher irradiation-induced hardening at 1 dpa for ODS-Eurofer but a steeper hardness increase per dpa up to 10 dpa for Eurofer97.

Vibrational contribution to the thermodynamics of nanosized precipitates: vacancy–copper clusters in bcc-Fe

The effects of lattice vibration on the thermodynamics of nanosized coherent clusters in bcc-Fe consisting of vacancies and/or copper are investigated within the harmonic approximation. A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by molecular dynamics is applied to obtain the most stable cluster configurations at T = 0 K. The most recent interatomic potential built within the framework of the embedded-atom method for the Fe-Cu system is used. The total free energy of pure bcc-Fe and fcc-Cu as well as the total formation free energy and the total binding free energy of the vacancy-copper clusters are determined for finite temperatures. Our results are compared with the available data from previous investigations performed using many-body interatomic potentials and first-principles methods. For further applications in rate theory and object kinetic Monte Carlo simulations, the vibrational effects evaluated in the present study are included in the previously developed analytical fitting formulae.

Alternative Fabrication Routes toward Oxide-Dispersion-Strengthened Steels and Model Alloys

The standard powder metallurgy (PM) route for the fabrication of oxide-dispersion-strengthened (ODS) steels involves gas atomization to produce a prealloyed powder, mechanical alloying (MA) with fine oxide powders, consolidation, and finally thermal/thermomechanical treatment (TMT). It is well established that ODS steels with superior property combinations, for example, creep and tensile strength, can be produced by this PM/MA route. However, the fabrication process is complex and expensive, and the fitness for scaling up to the industrial scale is limited. At the laboratory scale, production of small amounts of well-controlled model systems continues to be desirable for specific purposes, such as modeling-oriented experiments. Thus, from the laboratory to industrial application, there is growing interest in complementary or alternative fabrication routes for ODS steels and related model systems, which offer a different balance of cost, convenience, properties, and scalability. This article reviews the state of the art in ODS alloy fabrication and identifies promising new routes toward ODS steels. The PM/AM route for the fabrication of ODS steels is also described, as it is the current default process. Hybrid routes that comprise aspects of both the PM route and more radical liquid metal (LM) routes are suggested to be promising approaches for larger volumes and higher throughput of fabricated material. Although similar uniformity and refinement of the critical nanometer-sized oxide particles has not yet been demonstrated, ongoing innovations in the LM route are described, along with recent encouraging preliminary results for both extrinsic nano-oxide additions and intrinsic nano-oxide formation in variants of the LM route. Finally, physicochemical methods such as ion beam synthesis are shown to offer interesting perspectives for the fabrication of model systems. As well as literature sources, examples of progress in the authors’ groups are also highlighted.

Cluster Dynamics Simulation of Reactor Pressure Vessel Steels under Irradiation

The coupling between copper rich precipitates (CRP) and point defects in neutron irradiated iron alloys and VVER steels was investigated by means of cluster dynamics (CD) simulations. The consideration of the strain energy effect on CRP kinetics as well as the application of the regular solution model for the case of different fixed copper contents of CRP provides a good agreement between the simulation results and experimental data for complex iron based alloys with small (0.015 wt%) and high (0.42 wt%) copper content. It was found that the CD simulation is applicable to irradiated VVER steel with 0.07 wt% of copper.

JRQ and JPA irradiated and annealed reactor pressure vessel steels studied by positron annihilation

ABSTRACT The paper is focused on a comprehensive study of JRQ and JPA reactor pressure vessel steels from the positron annihilation lifetime spectroscopy (PALS) point of view. Based on our more than 20 years’ experience with characterization of irradiated reactor steels, we confirmed that defects after irradiation start to grow and/or merge into bigger clusters. Experimental results shown that JPA steel is more sensitive to the creation of irradiation-induced defects than JRQ steel. It is most probably due to high copper content (0.29 wt.% in JPA) and copper precipitation has a major impact on neutron-induced defect creation at the beginning of the irradiation. Based on current PALS results, no large vacancy clusters were formed during irradiation, which could cause dangerous embrittlement concerning operation safety of nuclear power plant. The combined PALS, small angle neutron scattering and atomic probe tomography studies support the model for JRQ and JPA steels describing the structure of irradiation-induced clusters as agglomerations of vacancy clusters (consisting of 2–6 vacancies each) and are separated from each other by a distribution of atoms.

Damage accumulation in pure iron and high concentrated Fe–12.5 at% Cr alloy: comparison between object kinetic Monte Carlo and cluster dynamics

A comparison between object kinetic Monte Carlo (OKMC) and cluster dynamics (CD) simulations of damage accumulation has been made in the domain of low (77–150 K) temperatures for electron-irradiated and then annealed pure iron as well as at 573 K temperature for neutron-irradiated high concentrated Fe–12.5 at% Cr alloy. Findings indicate that an increase in the size of the simulation box up to 300 nm×300 nm×300 nm in OKMC simulation and direct integration of master equation of CD provide the quantitative agreement results of these methods among themselves as well as with the experimental data of microstructure evolution of Fe–12.5 at% Cr alloy and qualitative agreement for the case of pure iron electron irradiated at 77 K and then annealed at temperatures between 77 and 150 K.

Irradiation Damage and Embrittlement in RPV Steels Under the Aspect of Long Term Operation: Overview of the FP7 Project LONGLIFE

The increasing age of the European NPPs and envisaged lifetime extensions up to 80 years require an improved understanding of RPV irradiation embrittlement effects connected with long term operation (LTO). Phenomena which might become important at high neutron fluences (such as late blooming effects and flux effects) must be considered adequately in the safety assessments. Therefore the project LONGLIFE was initiated within the 7th Framework Programme of the European Commission. The project aims at: i) improved knowledge on LTO phenomena relevant for European reactors; ii) assessment of prediction tools, codes, standards and surveillance guidelines. In the paper, we give an overview of the project structure and the related tasks. Furthermore we present two examples for the experimental evidence of LTO relevant phenomena: the first example is related to the flux dependence of defect cluster formation in a neutron irradiated weld material. We have found that the size of the irradiation induced defects exhibits a flux effect whereas the mechanical properties are almost independent of the flux. The second example refers to the acceleration of irradiation hardening after exceeding a threshold fluence. This effect was observed by means of both small angle neutron scattering (SANS) and tensile testing for low Cu RPV steels irradiated at a temperature of 255 °C. These examples demonstrate that LTO irradiation effects have to be investigated in more detail to guarantee the applicability of the embrittlement surveillance guidelines beyond 40 years of operation.Copyright

Nanoindentation of ion-irradiated reactor pressure vessel steels – model-based interpretation and comparison with neutron irradiation

Abstract Ion-irradiation-induced hardening is investigated on six selected reactor pressure vessel (RPV) steels. The steels were irradiated with 5 MeV Fe2+ ions at fluences ranging from 0.01 to 1.0 displacements per atom (dpa) and the induced hardening of the surface layer was probed with nanoindentation. To separate the indentation size effect and the substrate effect from the irradiation-induced hardness profile, we developed an analytic model with the plastic zone of the indentation approximated as a half sphere. This model allows the actual hardness profile to be retrieved and the measured hardness increase to be assigned to the respective fluence. The obtained values of hardness increase vs. fluence are compared for selected pairs of samples in order to extract effects of the RPV steel composition. We identify hardening effects due to increased levels of copper, manganese-nickel and phosphorous. Further comparison with available neutron-irradiated conditions of the same heats of RPV steels indicates pronounced differences of the considered effects of composition for irradiation with neutrons vs. ions.