Small-angle neutron scattering study on the effect of hydrogen in irradiated reactor pressure vessel steels

Hydrogen uptake can enhance the neutron embrittlement of reactor pressure vessel (RPV) steels. This suggests that irradiation defects act as hydrogen traps. The evidence of hydrogen trapping was investigated using the small-angle neutron scattering (SANS) method on four RPV steels. The samples were examined in the unirradiated and irradiated states and both in the as-received condition and after hydrogen charging. Despite the low bulk content of hydrogen achieved after charging with low current densities, an enrichment of hydrogen in small microstructural defects could be identified. Preferential traps were microstructural defects in the size range of ≈ > 10 nm in the unirradiated and irradiated samples. However, the results do not show any evidence for hydrogen trapping in irradiation defects.     

Microstructural stability of modified 9Cr–1Mo steel during long term exposures at elevated temperatures

This paper describes the stability of microstructure in normalized and tempered modified 9Cr–1Mo steel exposed to service temperatures in the range of 773–873 K for different time durations. A detailed microstructural and microchemical analysis of the secondary phases was carried out using optical and electron microscopy techniques. The microstructural observations, supported by hardness measurements showed that the lath morphology of the tempered martensite was retained even after 10 000 h of aging. The coarsening of M₂₃C₆ carbide was observed until 5000 h, when the Laves phase started appearing. The microstructural features observed are discussed in conjunction with the embrittlement observed in this steel on high temperature aging exceeding 5000 h.     

Thermal stability of hydrostatically extruded EUROFER 97 steel

EUROFER 97 steel is a candidate structural material for the future fusion power reactors, as well as for the European Test Blanket Modules (TBMs) to be tested in ITER. In the reported study, the microstructure of EUROFER 97 was modified by hydrostatic extrusion (HE) which reduced the grain size from 400 to 86 nm and that of the carbide particles from 111 to 75 nm. The changes in the microstructure significantly improved the strength of the extruded samples. However, it is important that the enhanced properties of nanostructured materials are stable over the required range of intended service temperature. The thermal stability of the nanostructured EUROFER steel was evaluated by subjecting the hydrostatically extruded samples to annealing at temperatures ranging from 473 to 1073 K (200–800 °C) for 1 h. Tensile tests and microhardness measurements with a 200 g load were carried out on the annealed samples to determine the effect of the heat treatment. The results show that the highest microhardness (403 HV_0.2) was achieved for samples annealed at 673 K. However, the tensile and yield strength decreased at the higher temperature of 873 K and the total elongation increased to 15%, compared to only 3% for as-extruded samples. The changes in the mechanical properties were rationalized by the examination of the microstructural changes. During heating the initial grain size remains virtually unchanged below a temperature of 873 K. However, above 873 K the grain size increased and it is very likely that growth will be very rapid at higher temperatures.     

Characterization of creep-fatigue in ferritic 9Cr-1Mo-V-Nb steel using ultrasonic velocity

The microstructural evolution of ferritic 9Cr-1Mo-V-Nb steel, subjected to creep-fatigue at 550 °C, was evaluated nondestructively by measuring the ultrasonic velocity. The ultrasonic velocity was strongly depended on the microstructural changes during creep-fatigue. The variation in the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region (within 0.2 N_f), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of 0.2 N_f and 0.8 N_f and a decrease in the final region. The change of the ultrasonic velocity during creep-fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.     

Monitoring of a radiation recovery in neutron-irradiated RPV steel by using the magnetic methods

The effects of a neutron irradiation on the magnetic parameters of a reactor pressure vessel (RPV) steel with different microstructures, partially due to a difference in the steel refining processes were investigated. The samples were irradiated in a research reactor with a fluence of 4.5 × 10¹⁹ n/cm² at 288 °C. The measurement of the Barkhausen noise (BN) was conducted to explore the relationship between the microstructural state and the domain wall motion. The BN profiles of the unirradiated samples showed a consistent change with the microstructure (grain size, carbide morphology, lath width), but the neutron-irradiated sample did not show a consistent change with microstructural state. The radiation hardening and recovery characteristics have been investigated by using the Vickers hardness method, and the relationship between the BN and the microhardness is obtained for four different steels. The linear relationship between mechanical and magnetic properties can be used as a nondestructive evaluation for radiation damage.     

Mechanical properties and microstructural investigations of TIG welded 40 mm and 60 mm thick SS 316L samples for fusion reactor vacuum vessel applications

The development of advanced fusion reactors like DEMO will have various challenges in materials and fabrication. The vacuum vessel is important part of the fusion reactor. The double walled design for vacuum vessel with thicker stainless steel material (40–60 mm) has been proposed in the advanced fusion reactors like ITER. Different welding techniques will have to be used for such vacuum vessel development. The required mechanical, structural and other properties of stainless steels have to be maintained in these joining processes of components of various shapes and sizes in the form of plates, ribs, shells, etc. The present paper reports characterization of welding joints of SS316L plates with higher thicknesses like 40 mm and 60 mm, prepared using multi-pass Tungsten Inert Gas (TIG) welding process. The weld quality has been evaluated with non-destructive tests by X-ray radiography and ultrasonic methods. The mechanical properties like tensile, bend tests, Vickers hardness and impact fracture tests have been carried out for the weld samples. Tensile property test results indicate sound weld joints with efficiencies over 100%. Hardening was observed in the weld zone in non-uniform manner. Macro and microstructure studies have been carried out for Base Metal (BM), Heat Affected Zone (HAZ) and Weld Zone (WZ). Scanning Electron Microscopy (SEM) analysis carried out for the impact fractured specimens show ductile fracture. The microstructural study and ferrite number data indicate the presence of high content of delta ferrite in the weld zone as compared to the delta ferrite in base metal.     

Study of the M23C6 precipitation at 550°C and 700°C in AISI 304 stainless steel by small angle neutron scattering

An investigation of the M₂₃C₆ carbide (M = metal atom) precipitation induced by ageing at 550°C and 700°C in AISI 304 stainless steel was performed using Small Angle Neutron Scattering (SANS) and Transmission Electron Microscopy (TEM). Coherent scattering cross section curves were obtained for the differently aged samples from which the total surface and total volumes of precipitates per unit volume of sample were determined. The size distribution function of M₂₃C₆ carbide was obtained for each ageing time. The SANS results are in satisfactory agreement with the localized TEM observations.     

Characterizing microstructural changes in ferritic steels by positron annihilation spectroscopy: Studies on modified 9Cr–1Mo steel

Applicability of positron annihilation spectroscopy in probing the microstructural changes in ferritic steels has been investigated with thermal treatment studies on modified 9Cr–1Mo steel, during 300–1273 K. Positron lifetime results are compared with those of ultrasonic velocity and hardness techniques with two initial microstructural conditions i.e., normalized and tempered condition as well as only normalized condition. In first case, positron lifetime is found to be sensitive to small changes in metal carbide precipitation which could not be probed by other two techniques. In later case, positron lifetime is found to be sensitive to defect annealing until 673 K and in distinguishing the growth and coarsening of metal carbide precipitation stages during 773–1073 K. The present study suggests that by combining positron lifetime, ultrasonic velocity and hardness measurements, it is possible to distinguish distinct microstructures occurring at different stages.     

Effect of flowing sodium on corrosion and tensile properties of AISI type 316LN stainless steel at 823 K

AISI type 316LN stainless steel was exposed to flowing sodium in mass transfer loop (MTL) at 823 K for 16 000 h and then examined for changes in the tensile properties due to the mass transfer and corrosion effects. Comparisons in microstructural and mechanical properties were made between annealed, thermally aged and sodium exposed materials. Microstructural examination of thermally aged and sodium exposed materials revealed precipitation of carbides at the grain boundaries. The sodium exposed samples contained a degraded layer at the surface up to a depth of around 10 μm and a surface carburized layer of about 30 μm. There was about 15% increase in yield strength and a decrease of about 20% in ductility for the sodium exposed material vis-a-vis thermally aged material and this was attributed to carburization effects and microstructural changes.     

Characteristics relevant to ductile failure of bimetallic welds and evaluation of transferability of fracture properties

Life management and structural integrity assessment of bimetallic welds in its state-of-the-art form relies on practical methods derived on the basis of years of experience in operation and simplistic strength of materials analyses. The complex conditions and properties of the weldment, as resulting from the elaborate interaction of different microstructures with gradients in material properties, have limited the ability of currently existing methods to construct the assessment on the basis of actual failure mechanisms of bimetallic welds. Current work addresses the assessment procedure by combining experimental and numerical fracture mechanics comprising a micro-mechanical evaluation of the relevant damage mechanisms. The studied dissimilar ferrite (SA508)–austenite (AISI 304) circumferencial weld is one with a Ni-enriched buttering layer.The experimental work comprises tensile and fracture mechanical characterization of the different microstructural zones of the bimetallic weld. Tensile properties are determined with microstructure specific flat bar specimens as well as round bar specimens enabling better inference of true stress–strain curves. Fracture resistance curves are established by applying small-specimen testing techniques. Different crack configurations are modeled by finite element analysis (FEA) to assess the relationships between fracture types, toughness and local near crack tip constraint parameters. Transferability and characterization question are considered by determining J–Q-trajectories and employing small-scale yielding corrections (SSYCs). On the basis of the experimental and numerical results and a fractographical investigation, the micromechanics of fracture are interpreted. Differences in strain hardening capacities of microstructural zones are found to most severely affect the toughness transitions of the weld and the associated failure modes. Two prime failure types are noted, one for cracks located at outer heat affected zone (HAZ) resulting in an unstable crack deflection towards the fusion line (FL) and another type associated with cracks positioned near the fusion line, wherein a low-toughness ductile fracture process results. Small fracture mechanics specimen is found applicable for fracture resistance determination of bimetallic weldments.     

Preliminary experiments on the corrosion of CLAM steel in flowing eutectic LiPb

China low activation martensitic steel (CLAM), which is one of the RAFMs and under development in ASIPP, is considered as the primary candidate structural material for blankets in FDS series fusion reactors and will be in contact with the liquid breeder LiPb, so its corrosion behavior is of significance for its successful application. In the thermal convection LiPb loop DRAGON-I, the corrosion experiment of CLAM in flowing LiPb at 480 °C was performed up to about 2500 h to evaluate the corrosion behavior of CLAM in LiPb. The exposed samples were observed and analyzed, respectively, by metallography and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy after 500, 1000, 1500, 2000 and 2500 h corrosion experiments, respectively. The results show that the corrosive attack started with the dissolution of the passivated oxides formed during the heat treatment on the surface of CLAM steel. Then the corrosive attack continued further into the steel matrix. The weight loss was about 0.38 mg/cm² after 2500 h exposure.     

核电站用17-4沉淀硬化不锈钢阀杆热老化微结构小角中子散射研究

17-4沉淀硬化马氏体不锈钢阀杆广泛应用于压水堆核电站中,该阀杆在高温（300～350 ℃）下长期服役时面临热老化脆化问题,影响核电站安全。本文针对核电站实际服役的阀杆样品,开展了小角中子散射实验,结合冲击试验、扫描电子显微镜和金相显微镜分析等,将严重老化与轻微老化的阀杆样品进行对比,研究了试样在长期热时效过程中内部nm结构的变化。冲击试验、断口的扫描电镜和金相组织图像显示,严重老化的阀杆发生了明显的脆化现象。利用多分散小球模型和Porod定律对小角中子散射实验数据进行拟合,结果表明,球形nm析出物直径约为1 nm,随着热老化程度的加剧,nm析出物尺寸变大,体积分数增多约19%。小角中子散射结果与材料的宏观力学性能变化有明显的关联性。     

注氘低活化马氏体钢在电子辐照下的缺陷行为

低活化铁素体/马氏体(RAFM)钢被视为国际热核聚变反应堆以及聚变反应堆的第1壁候选结构材料之一,很多国家均在研究不同的RAFM钢,中国低活化马氏体(CLAM)钢的研究亦正在进行。核聚变会产生氢、氦、氘及氚,这些气体元素与辐照缺陷结合在一起,对材料的辐照性能会产生较大影响。本文对注氘后不同温度下的辐照后微观结构进行研究。试验利用日本北海道大学的JEOL-1300高压电子显微镜研究注氘CLAM钢从室温到873K在1250keV电子辐照下的微观结构变化。研究结果表明,在电子辐照下,注氘产生的缺陷团会出现消失和长大两种现象,意味着间隙型与空位型位错环在注氘过程中同时产生。并研究了注氘产生的空洞。     

Study by EELS of helium bubbles in a martensitic steel

This work presents measurements of the helium density and pressure in small bubbles in a martensitic steel, which is a vital first step towards identifying their role in the microstructural mechanisms determining the macroscopic properties of the material. Electron Energy-Loss Spectroscopy in the Scanning Transmission Electron Microscope has been used to analyse individual bubbles. The energy shift of the 1s → 2p transition and the helium density have been measured for each bubble analysed. The pressure inside the bubbles has been calculated from the helium density using an equation of state. In these bubbles, the values for the helium pressure seem to be smaller than the equilibrium pressure, and agree in trend with the findings of previous studies, although our results extend to smaller radii and higher pressures.     

Measurement and analysis of SHCCT diagram for CLAM steel

China Low Activation Martensitic (CLAM) steel is a leading candidate material for construction of the Chinese fusion reactor Test Blanket Module. The Simulated HAZ Continuous Cooling Transformation (SHCCT) diagram is developed via physical simulation, and the effects of thermal history on the microstructure and mechanical properties of the weld coarse-grain heat-affected zone (CGHAZ) in CLAM steel are evaluated. The results of thermal cycle simulation show that grain size increases and hardness decreases gradually with increasing heat input. Under certain conditions, especially when cooling times from 800 °C to 500 °C (T8/5) are larger than 136 s, delta ferrite may form which is deleterious for the TBM application. The amounts of delta ferrite are given under different T8/5. A SHCCT diagram of CLAM steel is developed using dilatometry and it predicts the AC1, AC3 and the Ms temperatures. With decreased cooling rate (larger T8/5), martensite laths widen and carbide precipitates grow. The results indicate that welding heat input should be taken into consideration and controlled in practical CLAM steel welding process applications.     

Experimental Study of the Iranian Inertial Electrostatic Confinement Fusion Device as a Continuous Neutron Generator

Among many facilities in the field of nuclear fusion devices, inertial electrostatic confinement (IECF) device has the specific character of tendency to generate fusion products continuously. Besides the distinctive characteristics, it has become an outstanding focus of interest for many scientists because of several applications such as the ability of performing hydrogen boron fusion. This paper summarizes primary results of the design and construction of the first Iranian IECF device (IR-IECF). It consists of 13.5 cm diameter stainless steel cathode, 41 cm diameter anode with a 60 cm diameter and 60 cm height vacuum chamber. The outcomes of neutron detection represent more than 10⁷ neutron/s at the maximum biased voltage of −140 kV and 70 mA current with deuterium operational filling gas in the steady state regime.     

Thermal hydraulics and mechnical design of the blanket for the Tokamak conceptual reactor UWMAK-II

A conceptual blanket design for UWMAK-II based on breeding in LiAlO₂ and helium cooling for a D-T fusion reactor is described. The reactor is a Tokamak with 316 stainless steel as the primary structural material, a major radius of 13 m and a minor radius of 5 m. The power output is 5000 MW(th) and the maximum temperature in the stainless steel structure is 650°C. This reactor design study is one of a series performed to evaluate the merits of various fusion reactor design concepts. In this paper the mechanical and the thermal hydraulics problem associated with the blanket for this reactor is described. Special attention has been given to the need for repairing and replacing the first wall of the blanket. Other problems which may arise from such a blanket design are also discussed.     

Effect of hot rolling process on the mechanical and microstructural property of the 9Cr–1Mo steel

The effect of hot rolling on the mechanical and microstructural property has been investigated to simulate the effect of hot extrusion during the manufacturing process of the fuel cladding for sodium cooled fast reactors (SFRs). Hot rolling of modified 9Cr–1Mo steel was carried out either at 1050 °C or 950 °C upon cooling after normalizing. Continuous annealing right after the hot rolling at 950 °C for 1 h has been carried out followed by the mechanical testing and microstructural analysis. The results showed that hot rolling without any annealing or tempering treatment leaves residual stress so that it leads to the abrupt increase of material strength that would affect cladding formability. Continuous annealing right after the hot rolling process can alleviate residual stress without decreasing too much of material strength. Hot rolling either at 1050 °C or 950 °C increases the number density of the remained precipitate which leads to the precipitation hardening. Introduction of continuous annealing results in an increase in the fraction of secondary V-rich MX precipitate that leads to an increase in the stability at high temperature mechanical property.     

Design of a W/steel functionally graded material for plasma facing components of DEMO

The design of a graded transition between tungsten and steel for plasma facing components of a nuclear fusion reactor is proposed and the interest of such a transition is demonstrated by FEM calculations of the thermo-mechanical behaviour in the operating conditions of the DEMO reactor. The transition consists in stacked layers of W-WC and WC-Fe between W and Eurofer steel. The maximum surface temperature of the structural component could be maintained below 1300 °C for a very simple multilayer geometry, from FEM calculations. The maximum strains and equivalent elastic stresses could be reduced by a factor of about 3 as compared to a direct W/steel joint. Considerations about processing techniques of such a component are discussed, based on the literature background and a few preliminary tests.     

Physical and thermal properties of 8 MeV electron beam irradiated HPMC polymer films

Microstructural modification in hydroxypropyl methylcellulose (HPMC) polymer films induced by electron irradiation is studied. Irradiation was performed in air at room temperature using a 8 MeV electron accelerator at doses of 25, 50, 75 and 100 kGy. Irradiation can be used to crosslink or degrade the desired component or to fix the polymer morphology. Changes in microstructural parameters, crystallinity and thermal properties in virgin and irradiated HPMC films have been studied using wide angle X-ray scattering data and differential scanning calorimetry. The heat of fusion and the degree of crystallinity are found to be highest for unirradiated HPMC and the crystallite size is larger in virgin HPMC films.