Reduced Activation Steels Research Articles

Retraction Note: Nano-Scale Precipitates of Reduced Activation Steels for the Application of Nuclear Fusion Reactors

Retraction Note: Nano-Scale Precipitates of Reduced Activation Steels for the Application of Nuclear Fusion Reactors

Constraint loss correction: Enhancing transferability of fracture toughness of Eurofer97 compact tension specimens between specimen sizes

This paper proposes and implements a constraint loss correction as a supplement to the ASTM correction factor for the master curve method (ASTM-E1921) for Eurofer97 reduced activation steel in order to enhance the transferability of results between compact tension (CT) specimen sizes. To facilitate the acquisition and scalability of reliable fracture toughness data from one specimen size to another and to apply the Master-Curve method with confidence when using small specimens, the difference in the structure of the crack tip stress field between large and small specimens was modeled. The stress intensity factors and critical volumes/stresses of two specimen sizes (1 T and 0.18 T) were calculated with 3D finite element simulations run over a large displacement range and at a temperature range of −120 to −80 °C and the results were compared with real data from a database of toughness tests of Eurofer97 steel. The K values from a miniaturized specimen without or with ASTM correction and with the proposed combined correction with constraint loss factor (which is variable over the entire loading range) were compared with simulated CT-1 T stress intensity values. It was found that this new correction factor was accurate up to a target toughness of 200 MPa.m1/2, which represents a major improvement on the current state of the art.

Multipurpose ion beam analysis chamber for fusion material science applications

In the fusion industry, there is a need for the development of new steel alloys, which could fulfil the requirements that are, in some cases, very much different from those of other fields of industry. High temperature and radiation levels alone are reasons why the currently available alloys could hardly be used in certain areas of a (e.g.) tokamak. In future fusion devices the utilization of novel steel compounds, such as Oxide Dispersion-Strengthened (ODS) alloys or reduced activation steels, would have considerable advantages over conventional steels. Diffusion bonding of ODS steels is a focus-area of nowadays research. The quality of the bonds can be investigated with non-destructive nuclear methods, such as RBS, PIXE and PIGE.The Centre for Energy Research has developed an ion beam analysis chamber, which provides the possibility to perform multiple nuclear measurements simultaneously on small-sized samples.This paper outlines the design driver considerations and an overview of the design is also given.

Unsteady state precipitation of M23C6 carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

Unsteady state precipitation of M23C6 carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

Enhanced fatigue property by fabricating a gradient nanostructured surface layer in a reduced-activation steel

Reduced activation ferritic/martensitic (RAFM) steels have been selected as candidate structural materials for future advanced nuclear power systems. In the present work, the influence of a gradient nanograined surface layer on the fatigue properties of RAFM steels was studied. A gradient nanostructured (GNS) surface layer with a thickness of ∼85 ​μm was prepared on RAFM steel utilizing surface mechanical rolling treatment (SMRT). The mean grain size was approximate 43 ​nm at the topmost surface and increased gradually with depth. The results of the stress-controlled tension-compression fatigue experiments showed that the fatigue life enhanced approximately 6 times in the SMRT samples compared to the corresponding base metal counterparts. The relationship between the applied stress amplitude and the fatigue lifetime, and the fracture morphology showed that the surface strengthening and strain delocalization were caused by GNS, which suppressed surface crack initiation process, and hence the fatigue properties of RAFM steels improved. In addition, the deformation compatibility in GNS and coarse-grained boundaries leading to more dislocation interactions and accumulation during the cyclic process, also plays a crucial role in enhancing the fatigue properties of RAFM steel.

Nanoindentation Response of Ion-Irradiated Fe, Fe-Cr Alloys and Ferritic-Martensitic Steel Eurofer 97: The Effect of Ion Energy

Nanoindentation of ion-irradiated nuclear structural materials and model alloys has received considerable interest in the published literature. In the reported studies, the materials were typically exposed to irradiations using a single ion energy varying from study to study from below 1 MeV to above 10 MeV. However, systematic investigations into the effect of self-ion energy are still insufficient, meaning that the possibilities to gain insight from systematic energy variations are not yet exhausted. We have exposed pure Fe, ferritic Fe-9Cr, martensitic Fe-9Cr and the ferritic-martensitic reduced-activation steel Eurofer 97 to ion irradiations at 300°C using 1, 2 and 5 MeV Fe2+ ions as well as 8 MeV Fe3+ ions and applied nanoindentation, using a Berkovich diamond indenter, to characterize as-irradiated samples and unirradiated references. The effect of the ion energy on the measured nanoindentation response is discussed for each material. Two versions of a primary-damage-informed model are applied to fit the measured irradiation-induced hardening. The models are critically compared with the experimental results also taking into account reported microstructural evidence. Related ion-neutron transferability issues are addressed.

Strengthening Effect of Multiscale Second Phases in Reduced Activation Ferrite/Martensitic Steel

Reduced activation ferrite/martensitic steels containing yttrium (Y), titanium (Ti), and zirconia (Zr) are melted using vacuum induction melting–electroslag remelting (ESR) to study the effects of second phases on the microstructure, tensile properties, and impact toughness of the alloys. In addition, two heat treatment processes are used to obtain the dispersed distribution of fine M23C6 carbides in reduced activation steel. The results show that ESR not only refined micron inclusions but also increase the number of submicron inclusions in the alloys. The number of submicron inclusions per unit volume of the ESR ingots is 1.11–1.43 × 1019 m−3. These inclusions not only reduced the austenite grain size through pinning but also effectively pinned dislocations to improve the mechanical properties of the steel. MX carbonitrides rather than M23C6 precipitated first in the steel with A–A–T heat treatment at 920 °C, decreasing the mean size of M23C6. The refined grain size, martensitic laths, and M23C6 are beneficial to the mechanical properties of the alloys. The yield strengths of the E–Y–Ti and E–Y–Zr alloys are 659 and 657 MPa, and their ductile–brittle transition temperatures are −93 and −86 °C, respectively.

Scaling of deuterium retention in

In continuation of earlier work on 3 MeV proton-damaged tungsten and reduced-activation steels we present new results on Eurofer97, Beryllium and W-5%Re sintered alloy irradiated <400 K. Methodical improvements result in largely reduced uncertainties. Beryllium is loaded using a 5 kV D2 + ion-source to 6.3*1021 D m−2 at 300 K. Eurofer97 and W-5Re are loaded in PSI-2 to 3*1025 D m−2. Irradiation and D-loading are conducted at ∼400 K. The D retention is measured by 3He μ-NRA. An exponential saturation fits the W-5Re D-retention data with R2 = 0.99 . The retention increases by a factor 10.3 in W-5Re, similar as in W, but on a ~7 times lower level. Within ±25% uncertainty D Retention in Eurofer97 proves to be independent of displacement damage up to 6.3 DPA. Beryllium shows increased retention by a factor 3 up to the tested maximum of 0.08 DPA. The retention in beryllium starts saturating, but the limited DPA range allows fitting the data with exponentials and power-laws.

COMPARISON OF WASTE DUE TO IRRADIATED STEELS IN THE ESFR AND DEMO

For either nuclear fusion or generation IV fission reactors to be viable as a commercial energy source the decommissioning and waste disposal solutions must be considered during the design. A multi-step simulation process combining Monte Carlo Neutron Transport simulations with inventory simulations have been performed to estimate the activation of steels in key reactor components of the European Sodium-cooled fast Reactor (ESFR). Waste classifications based on UK waste disposal regulations have been applied to the key components to estimate the expected masses of low level and intermediate level waste. The use of reduced activation steels, EUROFER and F82H, in reactor components external to the core results in a factor of 10 reduction in the percentage of waste classified as Intermediate Level Waste (ILW). Waste estimates are compared to existing waste estimates for the European Demonstration fusion reactor (DEMO). The ESFR has a lower percentage of ILW per total reactor mass due to irradiated steels compared to DEMO. However, there is no Higher Activity Waste (HAW) associated with DEMO, compared with arisings from the ESFR spent fission fuel.

The influence of temperature on the elastic properties of body-centered cubic reduced activation steels

A first-principles based modeling approach to the effect of temperature on the isothermal single-crystal and polycrystalline elastic parameters of Fe-rich solid solutions is reported. The approach integrates alloy theory for chemical and magnetic disorders with accessible experimental data for the equilibrium volume and ferromagnetic phase transition, and is adopted to predict the temperature-dependent elastic parameters of the body-centered cubic phase of three reduced activation steels, CLAM/CLF-1, F82H, EUROFER97, considered as high-temperature material in power reactors. The predictions are assessed based on available experimental data for a reduced activation steel and both experimental and theoretical data for pure Fe. Alloying effects on the elastic constants relative to pure Fe are found to differ in the magnetically ordered and disordered phases. Contributions due to loss of long-range magnetic order, volume expansion, and entropy are important in determining the temperature dependence of the elastic parameters in all investigated materials. A previously reported, peculiar magneto-volume phenomenon on the equation of state in pure Fe is gradually removed by alloying and magnetic disordering, which requires particular attention when describing the thermo-chemical effects derived from the equation of state in Fe-rich solid solutions.

Effect of thermal cycles on microstructure of reduced activation steel fabricated using laser melting deposition

Reduced activation steel was successfully fabricated by laser melting deposition employing a Gaussian and a ring-shaped laser. The microstructure evolution of the reduced activation steel was investigated using the scanning electron microscope, transmission electron microscope and electron backscatter diffraction. The experimental results showed that the grains close to the substrate were smaller than the grains in the upper layers. Compared to those deposited using a Gaussian laser, the samples deposited using a ring-shaped laser showed a more homogeneous microstructure. Furthermore, a finite element analysis (FEA) model was applied to reveal the thermal history during laser melting deposition. The simulation results were well validated with the experimental results. FEA results indicate that the peak temperature increases and the cooling rate decreases, as the layer gets further from the substrate. Additionally, the temperature and the cooling rate resulting from the Gaussian laser model were higher at the midline of the samples and lower around the edges, whereas those of the ring-shaped laser model were consistent with both at the center and around the edges.

Influence of NbC-doping on the microstructure and thermo-mechanical properties of tungsten coating fabricated by supersonic atmospheric plasma spraying

In this study, NbC-free and NbC-doped tungsten powders were thermally sprayed via a supersonic atmospheric plasma spraying (SAPS) onto reduced activation steel substrates to form coatings. Scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectrometry analyses, as well as thermal diffusivity, thermal conductivity, hardness and Young's modulus measurements, were used to characterize the influences of NbC-doping on the microstructure and thermo-mechanical properties of tungsten coating. The results demonstrated that the coatings all had a typical thermal spraying microstructure characterized by the lamellar structure, the splat boundary, and the pores or holes. NbC-doping decreased the porosity and increased the lamellar thickness of tungsten coating. WO3 and (Nb, W)2O5 produced by inevitable metallurgical physical chemistry reactions in SAPS process existed in the NbC-free and NbC-doped coatings, respectively. Those oxides generated by metallurgical physical chemistry reactions were changed from ellipsoid morphology with sharp poles and distributed on the grain boundaries in the NbC-free coating into quasi-spherical morphology and distributed mainly in grains in the NbC-doped coating. Thermo-mechanical properties such as thermal diffusivity, thermal conductivity, hardness, Young's modulus, and the ability to resist cracking of tungsten coating were improved after the doping of NbC.

Deuterium retention in tungsten and reduced activation steels after 3 MeV proton irradiation

Abstract Nuclear fusion plasma-facing materials (PFM) will suffer from irradiation, leading to significant changes in the material properties. This study investigates the impact of displacement damage on the deuterium retention near room temperature. ITER grade tungsten, Eurofer-97, and HiperFer 17Cr5 steel samples are irradiated with a tandem accelerator with ~3 MeV protons at currents of 100–600 nA on 250–550 µm spots at 320±10 K. In total 33 spots from 0 to 0.9 displacements per atom (DPA) at 0–4 µm depth are irradiated on 5 samples. After irradiation, the samples are exposed to D2 plasmas with a peak ion-flux of 2.1 × 1021 D/m²s for 4 h at The long-term D retention in both W and steel increases with DPA with a saturation starting around 0.2 DPA. Retention in W increased by a factor 12 with up to 3.2 at.% D, while in steel increases up to 180 times with up to 0.08 at.% D were observed. The results highlight the importance of using steels also in PFMs. Compatibility of the results with heavy ion irradiations boosts the confidence in inter-comparability between different ion types, but also between ions and neutrons.

Microstructure evolution and mechanical properties of reduced activation steel manufactured through laser directed energy deposition

A new laser metal desposition process based on an inside-laser coaxial powder feeding system was successfully applied to manufacture reduced activation steel, which is featured with fine microstructure and excellent mechanical properties. In addition, infrared thermal imaging experiments and Abaqus numerical simulation were conducted to characterize the complex thermal history during the laser metal deposition process. The microstructure and mechanical properties of reduced activation steel were systematically investigated in the as-fabricated and heat-treated samples. The results indicat that the peak temperature increased and the cooling rate decreased in the melt pool when the additional layers were deposited as a result of a cumulative effect of heat in the fabricated thin wall samples. The reduced cooling rate directly contributed to the decreased heterogeneous nucleation rate and the coarsening of austenite grains in the top domain. The differences in terms of microstructure and hardness of the as-fabricated samples along the building direction were also in a good agreement with the evolution of temperature field. The thermal cycling experimental and cyclic heat treatment results confirmed that in-situ thermal cycles were unable to trigger recrystallization because the stored strain energy was insufficient to induce nucleation of new austenite grains during laser directed energy deposition.

Microstructure and mechanical properties of thick CLF-1 reduced activation steel weld joint made by electron beam welding joint

The effect of post-welded heat treatment (PWHT) on microstructure and mechanical properties of electron beam welded joint of 35 mm thick China low-activation ferrite steel CLF-1 was studied. The results showed that the hardness and microstructure of the joint for both the as-welded and the PWHT were uniform along the thickness direction, and the hardness behaved a W-shaped distribution. The joint microstructure was composed of tempered martensite and precipitated phases which were mainly chromium and tungsten carbides. A small amount of δ-ferrite existed near the fusion line in the fusion zone. With the increase of tempering temperature, the microstructure of joint and base metal didn’t change significantly. With the extension of tempering time, the precipitated phases of base metal grew and the joint microstructure didn’t change significantly. The suitable PWHT was suggested as tempering at 1013 K for 2∼4 h, which not only ensured the strength of base metal, but also greatly improved low-temperature impact energy of the joint at 253 K.

Thermodynamic Analysis for the Magnetic-Field-Induced Precipitation Behaviours in Steels

Alloy carbide M23C6 plays a significant role in the creep strength of reduced activation steels. Experiments have proven that a magnetic field accelerates the precipitation of M23C6 at intermediate temperature. A scheme that combines first-principle calculations, Weiss molecular field theory and equilibrium software MTDATA is proposed to investigate the thermodynamic features of magnetic-field-induced precipitation. The calculated results reveal that the origin of the magnetic moment is the NaCl-like crystal structure. The magnetic field enhances the exchange coupling and stabilizes the ferromagnetic phase region. The external field influences the Curie temperature, thereby changing the magnitude and position of the maximum magnetic heat capacity, magnetic entropy and enthalpy. The strong magnetic field improves the stability of M23C6, and the theoretical results agree well with the previous experiment. The study provides a theoretical basis for the magnetic-field-induced precipitation behaviours in steels.

Energetics of helium-vacancy complexes in Fe-9Cr alloys from first-principles calculations

The formation of helium-vacancy complexes under irradiation is crucial for the nucleation and early-stage growth of helium bubbles in reduced activation steels. The energetics of HenVm (V represents vacancy) complexes (n and m = 0–4) in Fe-9Cr alloy models and bcc Fe were investigated by first-principles calculations. A stronger self-trapping of He in Fe-9Cr alloys than in bcc Fe is found. The existence of Cr suppresses multiple He trapping in the vacancy to some extent. Besides, alloying element Cr strengthens lattice expansion induced by He atoms. Lower formation energy of HenVm complexes and stronger binding of a He atom to HenVm complexes in Fe-9Cr alloys as compared to in bcc Fe indicate that density of these small complexes can be higher in alloys. When m/n > 1, aggregation of a vacancy to HenVm complexes becomes more difficult in Fe-9Cr alloys than in bcc Fe. It is well known that the irradiation induced void swelling is related to the aggregation of small vacancy clusters or HenVm complexes. The results indicate that the addition of Cr atoms to the Fe matrix is beneficial to the irradiation swelling resistance aided by dispersed HeV complexes.

Influence of Preliminary Irradiation by a High Heat Flux on the Re-emission and Thermal Desorption of Deuterium Implanted from Reduced Activation Steels

The deuterium release from reduced activation Eurofer steel samples is investigated by measuring the re-emission directly during ion irradiation and via thermal desorption spectroscopy without contact with air. A part of the experiments are carried out using a sample previously subjected to high-power pulsed thermal action using the QSPA-T setup, Troitsk Institute of Innovative and Thermonuclear Research. Subsequent irradiation is performed using a 5-keV D 3 + ion beam to a fluence of up to 1021 m–2 at room temperature. In all cases, a major part of the implanted deuterium is released from the sample already at the irradiation stage. A significant part of the deuterium also desorbs in the interval between irradiation and spectroscopic measurements. Deuterium re-emission from damaged samples reaches a maximum value more slowly than that from undamaged ones, and deuterium release during holding is more intense. This can be explained by the structure of the damages caused by the heat flux: the hydrogen-trap concentration grows in the material, and the surface area participating in desorption increases.

HiperFer, a reduced activation ferritic steel tested for nuclear fusion applications

Materials are the most urgent issue in nuclear fusion research. Besides tungsten, steels are considered for unifying functional and structural materials due to their cost and mechanical advantages over tungsten. However, the fusion neutrons impose a strong constraint on the ingredients of the steel in order to avoid long lasting activation, while the material has to pertain sputtering resistance, low hydrogen retention, and long-term mechanical stability. In this proof-of-principle, we demonstrate the interesting properties of the new material HiperFer (High performance Ferrite) as a material suitable for fusion applications.The investigation covers neutron activation modelled by FISPACT-II, plasma sputtering and deuterium retention experiments in PSI-2, thermo-mechanical properties and component modelling. The material was found to feature a low nuclear inventory. Its sputtering yield reduces due to preferential sputtering by a factor 4 over the PSI-2 D2 plasma exposure with possible reductions of up to 70 indicated by SD.Trim.SP5 modelling. The exposure temperature shows a strong influence on this reduction due to metal diffusion, affecting layers of 1 µm in PSI-2 at 1150 K exposure for 4 h. Deuterium retention in the ppm range was found under all conditions, together with ∼10 ppm C and N solubility of the ferritic material. The creep and cyclic fatigue resistance exceed the values of Eu-97 steel. As an all HiperFer component, heat loads in the order of 1.5 MW/m² could be tolerated using water-cooled monoblocks. In conclusion, the material solves several contradictions present with alternative reduced-activation steels, but its applications temperatures >820 K also introduce new engineering challenges.

Surface morphology of Tungsten-F82H after high-heat flux testing using plasma-arc lamps

F82H reduced activation steel coated with vacuum plasma sprayed (VPS) tungsten is a candidate as a plasma facing material for main chamber components in future fusion reactors. Due to different coefficients of thermal expansion (CTE), significant thermal stresses are expected in these bimetallic materials. Thus, a major uncertainty in the performance of W/F82H components during the operation under high-heat fluxes is the effect of CTE mismatch. In this study, a high intensity plasma-arc lamp was used for high-heat flux cycling tests of W/F82H specimens. While no surface damage was observed for specimens tested for 100–200 cycles at a heat flux of 1.4 MW/m2 pulse when the backside surface temperature was maintained below 550 °C, significant cracking occurred at higher temperatures. A simple analytical model for bimetallic materials indicated that the stress in the VPS-W layer is likely to exceed its failure stress solely due to the bilayer thermal stress. A finite element analysis of the state of stress and deformation confirmed that a significant stress also would occur at the W surface due to the rigid-body like constraint imposed by the clamp, which can be the main cause of the cracking.