Long-term properties of reduced activation ferritic/martensitic steels for fusion reactor blanket system

Thermal aging properties of reduced activation ferritic/martensitic steel F82H was researched. The aging was performed at temperature ranging from 400 °C to 650 °C up to 100,000 h. Microstructure, precipitates, tensile properties, and Charpy impact properties were carried out on aged materials. Laves phase was found at temperatures between 550 and 650 °C and M₆C type carbides were found at the temperatures between 500 and 600 °C over 10,000 h. These precipitates caused degradation in toughness, especially at temperatures ranging from 550 °C to 650 °C. Tensile properties do not have serious aging effect, except for 650 °C, which caused large softening even after 10,000 h. Increase of precipitates also causes some degradation in ductility, but it is not critical. Large increase in ductile-to-brittle transition temperature was observed in the 650 °C aging. It was caused by the large Laves phase precipitation at grain boundary. Laves precipitates at grain boundary also degrades the upper-shelf energy of the aged materials. These aging test results indicate F82H can be used up to 30,000 h at 550 °C.     

Behaviour of chromium steels in liquid Pb-55.5Bi with changing oxygen content and temperature

The behaviour of protective oxide layers on P122 steel and its welds and of ODS steel in liquid Pb_44.5Bi_55.5 (LBE) is examined under conditions of changing temperatures and oxygen concentrations. P122 (12Cr) and its welded joints are exposed to LBE at 550 °C for 4000 h with oxygen concentrations of 10⁻⁶ and 10⁻⁸ wt% (p(O₂) = 8.1 × 10⁻²³ bar and 5.2 × 10⁻²⁷ bar) which change every 800 h. It is found that like in case of constant oxygen concentration of 10⁻⁶ wt% a protective spinel layer (Fe(Fe_1−xCr_x)₂O₄) was maintained on P122 and also on its welded joint. Two experiments with exposure times of 4800 h are conducted on ODS steel, both with temperatures changing from 550 to 650 °C and back every 800 h, one experiment with 10⁻⁶ the other with 10⁻⁸ wt% oxygen in LBE. Both experiments show strong local dissolution attack after 4800 h which is in agreement with the behaviour of ODS in LBE at a constant temperature of 650 °C. However, dissolution attack is less in LBE with 10⁻⁸ wt% oxygen (p(O₂) = 3.0 × 10⁻²⁵ bar).     

The ternary system: silicon-uranium-vanadium

Phase equilibria in the system Si-U-V were established at 1100 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed, U₂V₃Si₄ and (U_1−xV_x)₅Si₃, for which the crystal structures were elucidated by X-ray powder data refinement and found to be isotypic with the monoclinic U₂Mo₃Si₄-type (space group P2₁/c; a = 0.6821(3), b = 0.6820(4), c = 0.6735(3) nm, β = 109.77(1)°) and the tetragonal W₅Si₃-type (space group I4/mcm, a = 1.06825(2), c = 0.52764(2) nm), respectively. (U_1−xV_x)₅Si₃ appears at 1100 °C without any significant homogeneity region at x ∼ 0.2 resulting in a formula U₄VSi₃ which corresponds to a fully ordered atom arrangement. DTA experiments clearly show decomposition of this phase above 1206 °C revealing a two-phase region U₃Si₂ + V₃Si. At 1100 °C U₄VSi₃ is in equilibrium with V₃Si, V₅Si₃, U₃Si₂ and U(V). At 800 °C U₄VSi₃ forms one vertex of the tie-triangle to U₃Si and V₃Si. Due to the rather high thermodynamic stability of V₃Si and the corresponding tie-lines V₃Si + liquid at 1100 °C and V₃Si + U(V) below 925 °C, no compatibility exists between U₃Si or U₃Si₂ and vanadium metal.     

Oxidation and creep failure of alloy 617 foils at high temperature

The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800-1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr₂O₃), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al₂O₃), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr₂O₃ oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Q_app, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.     

M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.     

Precipitate phases of a ferritic/martensitic 9% Cr steel for nuclear power reactors

One of the reasons that ferritic/martenstic steels have been considered as candidate materials for nuclear power reactors is their superior creep resistance at elevated temperature. The creep rupture strength of 9% chromium steel could be improved by a fine dispersion of secondary precipitate phase. The precipitate phases in extra-low carbon 9% chromium steel with tempered conditions were investigated by transmission electron microscope and energy-dispersive X-ray analysis. The steel specimens were normalized and then tempered at different temperatures. Niobium-rich MN nitrides (Nb_0.6V_0.3Cr_0.1)N, and two kinds of vanadium nitrides, (V_0.6Nb_0.2Cr_0.2)N and (V_0.45Nb_0.45Cr_0.1)N having a f.c.c. crystal structure, were identified in the steel specimens tempered at 600-780 °C, and 750 or 780 °C respectively. Hexagonal chromium-rich M₂N precipitate phases with different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the tempered steel specimens. The M₂N phase showed a decrease/an increase in its chromium/vanadium content as the tempering temperature was increased. The influence of precipitates and heat treatment conditions on the high temperature properties of 9% Cr steel was discussed.     

The annealing behavior of Cu nanoparticles in Ar-ion implanted spinel

Magnesium aluminate spinel crystals (MgAl₂O₄ (1 1 0)) deposited with 30 nm Cu film on surface were implanted with 110 keV Ar-ions to a fluence of 1.0 × 10¹⁷ ions/cm² at 350 °C, and then annealed in vacuum condition at the temperature of 500, 600, 700, 800 and 900 °C for 1 h, respectively. Ultraviolet-visible spectrometry (UV-VIS), scanning electron microscopy (SEM), Rutherford backscattering (RBS) and transmission electron microscopy (TEM) were adopted to analyze the specimens. After implantation, the appearance of surface plasmon resonance (SPR) absorbance peak in the UV-VIS spectrum indicated the formation of Cu nanoparticles, and the TEM results for 500 °C also confirmed the formation of Cu nanoparticles at near-surface region. In annealing process, The SPR absorbance intensity increased at 500 and 700 °C, decreased with a blue shift of the peak position at 600 and 800 °C, and the peak disappeared at 900 °C. The SPR absorbance intensity evolution with temperature was discussed combined with other measurement results (RBS, SEM and TEM).     

Direct carbothermic reduction of actinide oxalates: Example of Nd(III) oxalate-carbon mixtures conversion

Mixed uranium-plutonium monocarbide was obtained by carbothermic reduction of mixture of UO₂, PuO₂ and C at around 1500 °C under vacuum or argon for 4 h followed by sintering at around 1650 °C under mixture of Ar + 8%H₂ for 10 h. Moreover, PuO₂ was synthesized by oxalic precipitation followed by calcination at around 700 °C (PUREX process). This work deals with a process simplification, i.e. fabrication of carbide by calcination of actinide oxalate, precipitated in presence of carbon, at 1600 °C for 13 h under argon. In this preliminary study, neodymium was used as a surrogate of actinides. No carbon influence was noticed during calcination of neodymium oxalate into oxide. Carbon content was the same before and after calcination making possible conversion to carbide directly from mixture of neodymium oxalate and C.     

Erosion of carbon fiber composites under high-fluence heavy ion irradiation

The ion-induced erosion, determining by sputtering yield Y and surface evolution including structure and morphology changes of the modified surface layers, of two commercial carbon fiber composites (CFC) with different reinforcement - KUP-VM (1D) and Desna 4 (4D) have been studied under 30 keV Ar⁺ high fluence (φt ∼ 10¹⁸-10²⁰ ion/cm²) irradiation in the temperature range from room temperature to 400 °C. Ion-induced erosion results in the changes of carbon fiber structure which depend on temperature and ion fluence. Monitoring of ion-induced structural changes using the temperature dependence of ion-induced electron emission yield has shown that for Desna 4 and KUP-VM at dynamic annealing temperature Т_а ≈ 170 °С the transition takes place from disordering at T < T_a to recrystallization at T > T_a. The annealing temperature Т_а is close to the one for polycrystalline graphites. Microscopy analysis has shown that at temperatures Т < T_a the etching of the fibers results in a formation of trough-like longitudinal cavities and hillocks. Irradiation at temperatures T > T_a leads to a crimped structure with the ribs perpendicular to fiber axis. After further sputtering of the crimps the fiber morphology is transformed to an isotropic globular structure. As a result the sputtering yield decreases for Desna 4 more than twice. This value is almost equal to that for KUP-VM, Desna 4, polycrystalline graphites and glassy carbons at room temperature.     

Phase relations in the U-Mo-Al ternary system

The phase relations in the U-Mo-Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo_2−xAl_20+x and U₆Mo_4+xAl_43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl₂ (cubic MgCu₂-type) giving a quite large extension (UAl_2−xMo_x, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl_2−xMo_x with the hexagonal MgZn₂-type. There is no detectable solubility of Mo in UAl₄, and it is of the order of 1 at.% in UAl₃. The interaction layers between the γU-Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.     

Influence of tempering temperature upon precipitate phases in a 11%Cr ferritic/martensitic steel

The effect of tempering temperature on the precipitate phases in a 11%Cr ferritic/martensitic steel normalized at 1050 °C for 1 h and tempered for 2 h at temperatures ranging from 600 to 780 °C has been investigated using transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that tempering temperature does not affect the existences of niobium-rich carbonitrides, (Nb_0.7V_0.2Cr_0.1)(C,N) and (Nb_0.55V_0.35Cr_0.1)(C,N), vanadium-niobium-rich carbonitride (V_0.45Nb_0.45Cr_0.1)(C,N), chromium-rich carbonitride (Cr_0.83V_0.12W_0.05)₂(C,N) and chromium-rich carbide (Cr_0.7Fe_0.25W_0.05)₂₃-C₆, whilst the precipitations of vanadium-rich carbonitrides, (V_0.65Nb_0.2Cr_0.15)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) are dependent on tempering temperature, which were detected only at the higher tempering temperatures of 750 and 780 °C. No coarsening was occurred during the temperings for the niobium-rich and spherical vanadium-rich carbonitrides. There was a low coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides with increasing the tempering temperature from 600 to 700 °C and 650 to 780 °C, respectively, and a high coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides at the tempering temperatures 750 through 780 °C and 650 °C, respectively. The compositions show an increase in vanadium and a decrease in niobium and chromium contents for the niobium-rich carbonitrides, and a decrease in niobium and an increase in vanadium and chromium contents for the vanadium-niobium-rich carbonitrides, and an increase in vanadium and a decrease in tungsten contents for the chromium-rich carbonitrides. The chromium-rich carbides show an increase and a decrease in their iron and chromium contents, respectively, with increasing the tempering temperature from 650 to 780 °C.     

TEM investigation of irradiation damage in single crystal CeO2

In order to understand the evolution of radiation damage in oxide nuclear fuel, 150-1000 keV Kr ions were implanted into single crystal CeO₂, as a simulation of fluorite ceramic UO₂, while in situ transmission electron microscopy (TEM) observations were carried out. Two characteristic defect structures were investigated: dislocation/dislocation loops and nano-size gas bubbles.The growth behavior of defect clusters induced by 1 MeV Kr ions up to doses of 5 × 10¹⁵ ions/cm² were followed at 600 °C and 800 °C. TEM micrographs clearly show the development of defect structures: nucleation of dislocation loops, transformation to extended dislocation lines, and the formation of tangled dislocation networks. The difference in dislocation growth rates at 600 °C and 800 °C revealed the important role which Ce-vacancies play in the loop formation process. Bubble formation, studied through 150 keV Kr implantations at room temperature and 600 °C, might be influenced by either the mobility of metal-vacancies correlated with at threshold temperature or the limitation of gas solubility as a function of temperature.     

Study of electron beam curing process using epoxy resin system

Polymeric matrix composite (PMC) has been used in engineering applications instead of metal in the last few years, due to its corrosion resistance and excellent relation between tensile strength/density and elastic modulus/density. However, PMC materials cured by thermal process require high temperature and are time-consuming. The electron beam (EB) curing technology allows its use at room temperature and reduced curing times, and this is one of the main advantages over thermal technology. The aim of this work is to investigate electron beam curable epoxy formulations to use in filament winding processes to produce composite material with similar or better properties than thermal curable composites. The study has been made with commercial epoxy resins and cationic initiators. The epoxy resin samples were irradiated for few minutes with total dose of 150 kGy. The glass transition temperatures (T_g) were determined by dynamic mechanical analyzer (DMA) and the result was 137 °C. The thermal process was carried out in a furnace following three steps: 4 h at 90 °C, increasing temperature from 90 °C to 130 °C during 4 h and 12 h at 130 °C. The total process time was 20 h. The T_g of this sample was 102 °C.     

Ion track formation in low temperature silicon dioxide

Low temperature silicon dioxide layers (LTO), deposited on crystalline silicon substrates, and thermally densified at 750 °C for 90 min or 900 °C for 30 min, jointly with thermally grown silicon dioxide layers, were irradiated with low fluence 11 MeV Ti ions. A selective chemical etch of the latent tracks generated by the passage of swift ions was performed by wet or vapour HF solution. The wet process produced conically shaped holes, while the vapour procedure generated almost cylindrical nanopores. In both cases thermal SiO₂ showed a lower track etching velocity V_t, but with increasing the densification temperature of the LTO samples, the V_t differences reduced. LTO proved to be suitable for wet and vapour ion track formation, and, as expected, for higher densification temperatures, its etching behaviour approached that of thermal silicon dioxide.     

An X-ray absorption spectroscopy study of Mo oxidation in Pb at elevated temperatures

The corrosion of fuel cladding and structural materials by lead and lead-bismuth eutectic in the liquid state at elevated temperatures is an issue that must be considered when designing advanced nuclear systems and high-power spallation neutron targets. In this work, lead corrosion studies of molybdenum were performed to investigate the interaction layer as a function of temperature by X-ray absorption spectroscopy. In situ X-ray absorption measurements on a Mo substrate with a 3-6 μm layer of Pb deposited by thermal evaporation were performed at temperatures up to 900 °C and at a 15° angle to the incident X-rays. The changes in the local atomic structure of the corrosion layer are visible in the difference extended X-ray absorption fine structure and the linear combination fitting of the X-ray absorption near-edge structure to as-deposited molybdenum sample and molybdenum oxide (MoO₂ and MoO₃) standards. The data are consistent with the appearance of MoO₃ in an intermediate temperature range (650-800 °C) and the more stable MoO₂ phase dominating at high and low temperatures.     

Post-annealing effects on the shallow-junction characteristics caused by high-fluence 77 keV BSi molecular ion implantations at room and liquid nitrogen temperatures

In this study, n-type <1 0 0> silicon specimens were liquid nitrogen temperature (LT) and room temperature (RT) implanted with 2 × 10¹⁵ cm⁻² 77 keV BSi molecular ions to produce shallow junctions. Post-annealing methods under investigation included furnace annealing (FA) at 550 °C for 0.5, 1, 2, 3 and 5 h and rapid thermal annealing (RTA) at 1050 °C for 25 s. Post-annealing effects on the shallow-junction characteristics were examined using one-step (FA) and two-step (FA + RTA) post-annealing treatments. Secondary ion mass spectrometry (SIMS), cross-sectional transmission electron microscopy (XTEM), a four-point probe and Raman scattering spectroscopy (RSS) were employed to analyze junction depths (x_j), damage microstructures, sheet resistance (R_s) and damage characteristics, respectively. The results revealed that the shallow-junction characteristics of the LT implant are better than those of the RT one when post-annealing time in FA exceeds 1 h. A post-annealing time of 3 h in FA is needed in order to obtain the optimal one- or two-step post-annealing effects on the shallow-junction characteristics in both the LT and RT implants.     

Vacuum plasma etching of 1 wt% La2O3 dispersed tungsten

Vacuum plasma etching of 1 wt% La₂O₃ doped tungsten alloy surfaces were carried out to refine the surface morphology for enhancing its bonding characteristics with copper for fusion reactor components. Three different gas compositions containing argon with zero, 14.3 and 25 vol% hydrogen were used to carry out the plasma etching from 30 to 120 s on the given samples. Mitutoyo surface roughness (R_a) measurement, FORM TALYSURF and scanning electron microscopy (SEM) were employed to measure the changes in the surface roughness. Plasma etching with 14.3 vol% hydrogen mixture was found to be the best in micro-roughening the alloy surface. The maximum increase of 44% in R_a value was obtained with this gas mixture, when etched for 90 s.     

Initial stage of etching through pores in PET films irradiated by Ar ions

Changes in the diameters and depths of pores were studied in the process of etching polyethyleneterephthalate (PET) films irradiated by Ar ions having an energy of 1 MeV/n. Information about the pore diameters and lengths was obtained with JSM-840 and TEM-125 electron microscopes. The solutions of NaOH (0.5 mol/dm³ and 2 mol/dm³) were used as etchants. Etching was performed at 55 °C and 70 °C. Two methods of sensitization were used: the first one by UV illumination and treatment in dimethylformamide (DMF), the second method just by UV illumination. It was found that the diameters and the depths of pores are larger in films treated according to the first sensitization method. Etching duration (breakthrough time), which leads to through-going pores of the minimal radius, was established. After sensitization according to the first method the track etch rate grows quicker than the transverse etch rate. This gives a possibility to obtain through pores with diameters ranging from 50 nm to several micrometers.     

Study of silver diffusion in silicon carbide

Diffusion of silver in 6H-SiC and polycrystalline CVD-SiC was investigated using α-particle channeling spectroscopy and electron microscopy. Fluences of 2 × 10¹⁶ cm⁻² of ¹⁰⁹Ag⁺ were implanted with an energy of 360 keV at room temperature, at 350 °C and 600 °C, producing an atomic density of approximately 2% at the projected range of about 110 nm. The broadening of the implantation profile and the loss of silver through the front surface during vacuum annealing at temperatures up to 1600 °C was determined. Fairly strong silver diffusion was observed after an initial 10 h annealing period at 1300 °C in both polycrystalline and single crystalline SiC, which is mainly due to implant induced radiation damage. After further annealing at this temperature no additional diffusion took place in the 6H-SiC samples, while it was considerably reduced in the CVD-SiC. The latter was obviously due to grain boundary diffusion and could be described by the Fick diffusion equation. Isochronal annealing of CVD-SiC up to 1400 °C exhibited an Arrhenius type temperature dependence, from which a frequency factor D_o ∼ 4 × 10⁻¹² m² s⁻¹ and an activation energy E_a ∼ 4 × 10⁻¹⁹ J could be extracted. Annealing of 6H-SiC above 1400 °C shifted the silver profile without any broadening towards the surface, where most of the silver was released at 1600 °C. Electron microscopy revealed that this process was accompanied by significant re-structuring of the surface region. An upper limit of D < 10⁻²¹ m² s⁻¹ was estimated for 6H-SiC at 1300 °C.