Deuterium retention in tungsten at fluences of up to 10 D/m using D ion beams

Deuterium retention in two types of polycrystalline tungsten (PCW) was studied as a function of incident ion fluence, ion energy, and specimen temperature. (i) D retention at 300 K, as a function of D⁺ fluence, demonstrated a trend to saturation in both the Rembar hot-rolled thin foil and Plansee tungsten plate. At 500 K, new D retention results for the Plansee PCW showed an increasing trend with increasing incident D⁺ fluence without any indication of saturation, in agreement with previous results for Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895]. Even when the incident D⁺ fluence was increased to 8 × 10²⁵ D⁺/m², which is in the fluence range of plasma devices, there was still no sign of saturation. (ii) The temperature dependence results for the Plansee PCW show a decreasing trend in D retention as the temperature is increased from 300 to 500 K. These results differ from previous studies of Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895], but are similar to those seen for single crystal tungsten [M. Poon, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, J. Nucl. Mater. 313-316 (2003) 199]; an explanation for the different behaviour is suggested. (iii) Varying the D⁺ energy from 100 to 500 eV/D⁺ plays a minor role in the amount of D retained, suggesting that D retention in W depends more on the W structure, incident ion fluence and specimen temperature, rather than on the incident ion energy when the energy is below the threshold for damage formation (∼960 eV for D on W).     

Deuterium retention in sintered boron carbide exposed to a deuterium plasma

Depth profiles of deuterium up to a depth of 10 μm have been measured using the D(³He,p)⁴He nuclear reaction in a resonance-like technique after exposure of sintered boron carbide, B₄C, at elevated temperatures to a low energy (≈200 eV/D) and high ion flux (≈10²¹ m⁻² s⁻¹) D plasma. The proton yield was measured as a function of incident ³He energy and the D depth profile was obtained by deconvolution of the measured proton yields using the program SIMNRA. D atoms diffuse into the bulk at temperatures above 553 K, and accumulate up to a maximum concentration of about 0.2 at.%. At high fluences (?10²⁴ D/m²), the accumulation in the bulk plays a major role in the D retention. With increasing exposure temperature, the amount of D retained in B₄C increases and exceeds a value of 2 × 10²¹ D/m² at 923 K. The deuterium diffusivity in the sintered boron carbide is estimated to be D = 2.6 × 10⁻⁶exp{−(107 ± 10) kJ mol⁻¹/RT} m² s⁻¹.     

Helium and deuterium implantation in tungsten at elevated temperatures

High temperature helium and deuterium implantation on tungsten has been studied using the University of Wisconsin inertial electrostatic confinement device. Helium or deuterium ions from a plasma source were driven into polished tungsten powder metallurgy samples. Deuterium implantation did not damage the surface of the specimens at elevated temperatures (∼1200 °C). Helium implantation resulted in a porous surface structure above 700 °C. A helium fluence scan, ion energy scan, and temperature scan were all completed. With 30 keV ions, the pore formation started just below 4 × 10¹⁶ He⁺/cm². The pore size increased and the pore density decreased with increasing fluence and temperature. The energy scan from 20 to 80 keV showed no consistent trend.     

Understanding of copper precipitation under electron or ion irradiations in FeCu0.1 wt% ferritic alloy by combination of experiments and modelling

This work is dedicated to the understanding of the basic processes involved in the formation of copper enriched clusters in low alloyed FeCu binary system (FeCu0.1 wt%) under irradiation at temperature close to 300 °C. Such an alloy was irradiated with electrons or with ions (Fe⁺ or He⁺) in order to deconvolute the effect of displacement cascades and the associated generation of point defect clusters (ion irradiations), and the super-saturation of mono-vacancies and self-interstitial atoms (electron irradiation). The microstructure of this alloy was characterised by tomographic atom probe. Experimental results were compared with results obtained with cluster dynamic model giving an estimation of the evolution of point defects (free or agglomerated) under irradiation on the one hand and describing homogeneous enhanced precipitation of copper on the other hand. The comparison between the results obtained on the different irradiation conditions and the model suggests that the point defect clusters (dislocation loops and/or nano-voids) created in displacement cascades play a major role in copper clustering in low copper alloy irradiated at 573 K.     

C and D reemission from mixed W-C-D layers during single-species and simultaneous irradiations of W with C and D

We present new experimental results on the reemission of C and D from tungsten during single-species and simultaneous irradiations with 6 keV C⁺ and 1 keV D⁺ ion beams. The relatively low C fraction in the combined total beam flux (∼4.5% C⁺/[C⁺ + D⁺]) was selected to prevent the formation of a carbon over-layer during C⁺ irradiation. The results show that the temperature dependence of D reemission from a mixed W-C-D surface is similar to that from pure W. In the case of a mixed W-C surface, the reemission of C was much lower than observed for pure carbon. Post-irradiation XPS analysis of the chemical bonding states of a W specimen irradiated at 973 K with 6 keV C⁺ shows that carbon in the mixed W-C surface is primarily in the form of WC.     

Modelling deuterium release during thermal desorption of D-irradiated tungsten

Thermal desorption profiles were modelled based on SIMS measurements of implantation profiles and using the multi-trap diffusion code TMAP7 [G.R. Longhurst, TMAP7: Tritium Migration Analysis Program, User Manual, Idaho National Laboratory, INEEL/EXT-04-02352 (2004)]. The thermal desorption profiles were the result of 500 eV/D⁺ irradiations on single crystal tungsten at 300 and 500 K to fluences of 10²²-10²⁴ D⁺/m². SIMS depth profiling was performed after irradiation to obtain the distribution of trapped D within the top 60 nm of the surface. Thermal desorption spectroscopy (TDS) was performed subsequently to obtain desorption profiles and to extract the total trapped D inventory. The SIMS profiles were calibrated to give D concentrations. To account for the total trapped D inventory measured by TDS, SIMS depth distributions were used in the near-surface (surface to 30 nm), NRA measurements [V.Kh. Alimov, J. Roth, M. Mayer, J. Nucl. Mater. 337-339 (2005) 619] were used in the range 1-7 μm, and a linear drop in the D distribution was assumed in the intermediate sub-surface region (∼30 nm to 1 μm). Traps were assumed to be saturated so that the D distribution also represented the trap distribution. Three trap energies, 1.07 ± 0.03, 1.34 ± 0.03 and 2.1 ± 0.05 eV were required to model the 520, 640 and 900 K desorption peaks, respectively. The 1.34 and 1.07 eV traps correspond to trapping of a first and second D atom at a vacancy, respectively, while the 2.1 eV trap corresponds to atomic D trapping at a void. A fourth trap energy of 0.65 eV was used to fit the 400 K desorption peak observed by Quastel et al. [A.D. Quastel, J.W. Davis, A.A. Haasz, R.G. Macaulay-Newcombe, J. Nucl. Mater. 359 (2006) 8].     

Temperature dependence of liquid Sn sputtering by low-energy He and D bombardment

Absolute sputtering yields of liquid tin from 240 to 420 °C due to irradiation by low-energy helium and deuterium have been measured. For ion energies ranging from 300 to 1000 eV, temperature enhancement of liquid tin sputtering was noted. These measurements were obtained by IIAX (the Ion-surface InterAction eXperiment) using a velocity-filtered ion beam at 45° incidence to sputter material from a liquid tin target onto deposition monitors. Sputtering yields from 500 eV ion bombardment at 45° incidence increase from 0.1 ± 0.03 and 0.019 ± 0.008 Sn particles/ion at room temperature, for He⁺ and D⁺ ions respectively, to 0.30 ± 0.12 and 0.125 ± 0.05 Sn particles/ion for 380 °C. Temperature enhanced sputtering has been seen in other liquid metals (namely lithium, tin-lithium, and gallium) using both ion beam and plasma irradiation.     

Isotope exchange reaction between tritiated water and hydrogen on SiC

SiC has been considered as a primary candidate material for a first wall component in future fusion reactor because it has been claimed that SiC has excellent high-temperature properties, good chemical stability and low activation. However, the behavior of tritium on SiC has not been discussed yet. In this study, tritium trapping capacity on the surface of SiC was experimentally obtained at the temperature range of 25-800 °C in consideration of tritium trapping to the experimental system. The capacity, which was independent of the water vapor pressure in the gas phase and the temperature, was determined as about 10⁶ Bq/cm². The isotope exchange reaction rate between tritiated water in a gas phase and hydrogen on the surface was quantified at the temperature of 25, 500 and 700 °C in consideration of the behavior of tritium trapping at change of experimental condition by the numerical curve fitting method applying the serial reactor model. The reaction rate was observed to be constant as 3.48 × 10⁻⁵ m/s. Additionally tritium release behavior from the surface of SiC in water vapor atmosphere was predicted and compared with that for graphite and stainless steel.     

Formation of helium induced nanostructure ‘fuzz’ on various tungsten grades

The response of a variety of W material grades to nanostructure ‘fuzz’ formation is explored. W targets are exposed to He or D₂-0.2He plasmas in PISCES-B at 900-1320 K to below sputter threshold He⁺ ions of energy 25-60 eV for up to 2.2 × 10⁴ s. SEM and XPS reveal nanoscopic reorganization of the W surface to a layer of ‘fuzz’ of porosity ∼90% as determined by a ‘fuzz’ removal/weight loss method. The variability of ‘fuzz’ growth is examined at 1120 K for 1 h durations: SR, SC and doped W grades - La₂O₃ (1% wt.), Re (5% and 10% wt.), and TiC (1.5% wt.) developed 2-3 μm thick ‘fuzz’ layers, while a VPS grade developed a layer 4 μm thick. An RC grade revealed additional ‘fuzz’ at deep (>100 μm) grain boundaries. However, heat treatment up to 1900 K produced reintegration of ‘fuzz’ with the bulk and He release at ∼1000 K and ∼1400-1800 K due to depopulation from vacancy complexes.     

Grazing-incidence electron microscopy of surface blisters in single- and polycrystalline tungsten formed by H, D and He irradiation

Blisters on single- and polycrystalline tungsten surfaces formed by hydrogen and helium ion irradiation were investigated by grazing-incidence electron microscopy (GIEM) with an ultra-high-voltage transmission electron microscope. It was found that the blister skin thickness formed by D⁺ irradiation of polycrystalline tungsten (PCW) was considerably larger than the calculated ion range of the implants; however, this skin thickness (or blister depth) is not related to the pre-existing grain boundaries in the PCW. Blister formation was also observed with GIEM for single crystal tungsten (SCW) irradiated with H⁺, D⁺, and He⁺. The critical ion fluence for blister formation in SCW is estimated to be ∼10²³ H⁺(D⁺)/m² for H(D) and ∼10²¹ He⁺/m² for He. The size of the blisters and their skin structure depends on the irradiating conditions. Typical skin thickness was about 50-150 nm. Based on the assumption that gas particles (H₂, D₂, and He) accumulate within the blisters during H⁺, D⁺, and He⁺ irradiation, the GIEM measurements provide a means to derive an estimate of the amount of gas so accumulated, by reproducing the observed blister shapes with finite element method (FEM) calculations. From the GIEM images and FEM calculations we have estimated the number of implanted ions being retained in the blisters, and compared these amounts with published retention measurements. A mechanism for the blister formation is proposed based on the present results.     

Microstructural analysis on helium retention of ion-irradiated and annealed tungsten foils

The helium retention characteristics and helium bubble distribution in tungsten were studied using ³He(d,p)⁴He nuclear reaction analysis (NRA) and transmission electron microscopy (TEM) on two forms of tungsten: single crystal and polycrystalline, implanted to 1 × 10^19 3He/m² at 850 °C and annealed at 2000 °C. The NRA results revealed that as-implanted single crystal and polycrystalline tungsten exhibited similar helium retention characteristics. Stepwise annealing reduced the helium retention in both single crystal and polycrystalline tungsten when the number of implantation steps and annealing time were increased. The TEM results indicated that microstructure played a large role in helium trapping; the existence of grain boundaries led to significant cavity formation and greater cavity growth. Single crystal tungsten had less trapping sites for helium, allowing long range He diffusion during annealing. The decrease of He retention in polycrystalline tungsten during stepwise annealing was probably due to significant recrystallization, resulting in decrease of grain boundary density.     

Helium desorption in He implanted tungsten at low fluence and low energy

The behaviour of helium in polycrystalline ³He implanted tungsten at low energy (60 keV) and low fluence (2 × 10¹³ cm⁻²) has been studied as a function of post-implantation annealing temperature until 1873 K by means of Nuclear Reaction Analysis. Helium desorption has been observed only from ∼1500 K, suggesting a helium trapping at mono-vacancies. Only ∼75% of the implanted helium has been released after the annealing during 1 h at high temperature (1873 K); besides, the desorption rate decreased from 1673 K. The presence of a second type of helium trapping site is likely to explain this strong helium retention.     

Permeability of hydrogen and deuterium of Hastelloy XR

Permeation of hydrogen isotope through a high-temperature alloy used as heat exchanger and steam reformer pipes is an important problem in the hydrogen production system connected to be a high-temperature engineering test reactor (HTTR). An experiment of hydrogen (H₂) and deuterium (D₂) permeation was performed to obtain permeability of H₂ and D₂ of Hastelloy XR, which is adopted as heat transfer pipe of an intermediate heat exchanger of the HTTR. Permeability of H₂ and D₂ of Hastelloy XR were obtained as follows. The activation energy E₀ and pre-exponential factor F₀ of the permeability of H₂ were E₀=67.2±1.2 kJ mol⁻¹ and F₀=(1.0±0.2)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5, respectively, in the pipe temperature ranging from 843 K (570 °C) to 1093 K (820 °C). E₀ and F₀ of the permeability of D₂ were respectively E₀=76.6±0.5 kJ mol⁻¹ and F₀=(2.5±0.3)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5 in the pipe temperature ranging from 943 K (670 °C) to 1093 K (820 °C).     

Surface morphology and deuterium retention in tungsten oxide layers exposed to low-energy, high flux D plasma

Surface morphology and deuterium retention in tungsten oxide layers (WO_3−z, z ? 0.25) grown on polycrystalline and recrystallized W substrates have been examined after exposure to a low-energy (38 eV/D), high flux (10²² D/m² s) D plasma to an ion fluence of 10²⁶ D/m² at various temperatures (up to ∼700 K). Characterization methods used were scanning electron microscopy, X-ray diffraction, Rutherford backscattering spectroscopy, and the D(³He,p)⁴He nuclear reaction analysis. During exposure to the D plasma at temperatures of 340-615 K, a partial reduction of the tungsten oxide takes place in the near-surface layer up to 0.3 μm in depth. Even at around room temperature, deuterium atoms diffuse several micrometers into the tungsten oxide. The high D concentration of about 0.1 D/W observed in the first micrometers below the surface at temperatures below 500 K can be related mainly to D atoms chemically bonded to O atoms. As the exposure temperature increases, the D concentration decreases, reaching about 2 × 10⁻⁴ D/W at 615 K. At plasma exposure temperatures of about 700 K, the oxide layer shrinks and loses a large fraction of oxygen.     

Refinement in the ultrasonic velocity data and estimation of the critical parameters for molten uranium dioxide

Based on the previously reported internal pressure approach and by making use of recent and more accurate experimental measurements on the density, and heat capacity of liquid UO₂ up to ∼8000 K, reliable data on the sound propagation velocity in molten uranium dioxide have been obtained. An equation of state for liquid urania has also been developed which predicts a critical temperature (T_c) ≈ 10 500 K, critical pressure (P_c) ≈ 219 MPa and critical density (ρ_c) ≈ 2510 kg m⁻³.     

Deuterium retention in plasma spray tungsten coatings exposed to low-energy, high flux D plasma

Two types of porous plasma spray tungsten coatings deposited onto stainless steel and graphite substrates were exposed to low-energy (76 eV ), high-flux (10²² D/m² s) D plasma to ion fluences of (3-4) × 10²⁶ D/m² at various temperatures. Deuterium retention in the W coatings was examined by thermal desorption spectroscopy and the D(³He,p)⁴He nuclear reaction, allowing determination of the D concentration at depths up to 7 μm. The relatively high D concentration (above 0.1 at.%) at depths of several micrometers observed after D plasma exposure at 340-560 K can be related to accumulation of D₂ molecules in pores, while at temperatures above 600 K deuterium is accumulated mainly in the form of D atoms chemisorbed on the inner pore surfaces. At exposure temperatures above 500 K, the D retention in the plasma spray W coating on graphite substrate increases significantly due to trapping of diffusing D atoms at carbon dangling bonds located at the edge of a graphite crystallite.     

Electrochemical behaviour of gadolinium ion in molten LiCl-KCl eutectic

This work presents the electrochemical study of GdCl₃ in the molten LiCl-KCl eutectic in the temperature range 723-823 K. Transient electrochemical techniques such as cyclic voltammetry and chronopotentiometry, on an inert metallic tungsten working electrode, have been used in order to investigate the reduction mechanism and transport parameters. This study shows that Gd³⁺ ions are reduced to Gd metal by a single step mechanism with exchange of three electrons. Diffusion coefficient of GdCl₃ ions was determined at various temperatures, at 723 K the value is D = 0.88 10⁻⁵ cm² s⁻¹. Apparent standard reduction potential of the redox couple Gd³⁺/Gd has been determined by the open-circuit chronopotentiometry technique at several temperatures. Also the Gibbs free energy of GdCl₃ formation was determined and compared with thermodynamic data for pure compounds in the supercooled state in order to estimate the activity coefficient of Gd³⁺ in the molten LiCl-KCl eutectic.     

High energy primary knock-on process in metal-deuterium systems initiated by bombardment with noble gas ions

An experimental study confirms the possibility of nuclear fusion reactions initiating in metal-deuterium targets by bombarding them with ions that are not the reagents of the fusion reaction, in particular, with noble gas ions. The yields of (d,d) and (d,t) reactions were measured as functions of energy (0.4-3.2 MeV) and mass of incident ions (He⁺, Ne⁺, Ar⁺, Kr⁺ and Xe⁺). Irradiation by heavy ions produced a number of energetic deuterium atoms in the deuteride and deuterium + tritium metal targets. At ion energies of ∼0.1-1 MeV the d-d reaction yields are relatively high. A model of nuclear fusion reaction cross-sections in atomic collision cascades initiated by noble gas ion beam in metal-deuterium target is developed. The method for calculation tritium or deuterium recoil fluxes and the yield of d-d fusion reaction in subsequent collisions was proposed. It was shown that D(d,p)t and D(t,n)⁴He reactions mainly occur in energy region of the recoiled D-atom from 10 keV to 250 keV. The calculated probabilities of d-d and d-t fusion reactions were found to be in a good agreement with the experimental data.     

Oxygen glow discharge experiment to remove deposited layers and to release trapped hydrogen isotopes in HT-7 superconducting tokamak

An experiment to remove re-deposited layers and to release hydrogen using a glow discharge in oxygen (O-GDC) has been performed in the HT-7 superconducting tokamak. In the absence of magnetic fields, the O-GDC wall conditioning had produced rapid, controlled co-deposit removal. Average removal rates, 5.2 × 10²² H-atoms/h, 5.65 × 10²¹ D-atoms/h and 5.53 × 10²² C-atoms/h, respectively, were obtained during 145 min O-GDC experiment in the pressure range 0.5-1.5 Pa. The corresponding removal rate of co-deposited films was ∼1.19 μm/day (26.5 g/day for carbon) based on an area of 12 m². Compared to thermo-oxidation and O-ICR experiment, high pressure O-GDC wall conditioning promoted the oxidation and improved the C and D atoms removal. In the O-GDC experiment, the removal rates of H-atoms and D-atoms as H₂O, HDO and D₂O were higher than that of H₂ and D₂ by factors of about 20 and 50, respectively. During the 145 min O-GDC experiment, about 14.5% O-atoms were converted into carbon oxides and hydroxides, and about 5.37 × 10²² O-atoms were adsorbed on the walls corresponding to a coverage of 4.5 × 10²¹ O/m² on an wall area of 12 m². In a 100 min helium glow discharge (He-GDC) following the O-GDC experiment, 1.53 × 10²² O-atoms, about 28.5% oxygen retained on the walls, were removed. The removal rate of H-atoms in He-GDC cleaning after O-GDC experiment was lower than that in He-GDC cleaning before O-GDC experiment, which indicates that the O-GDC wall conditioning had effectively reduced hydrogen retention on the walls.     

Laser annealing in combination with mass spectroscopy, a technique to study deuterium on tokamak carbon samples, a tool for detritiation

In this study, a method is presented based on mass spectroscopy to measure the areal density of deuterium on a graphite surface exposed to tokamak discharges. The studied sample was cut from a bumper limiter exposed in the TEXTOR tokamak and annealed by a 1 J Excimer laser (KrF). The energy used was 400 mJ cm⁻², which is below the threshold for ablation, 1 J cm⁻². The release of HD and D₂ was measured by a mass spectroscopy set-up and no other species released from the sample were detected in this experiment. The amount of D released from the sample after 20 laser pulses was measured to 7 × 10¹⁶ D atoms per cm⁻² (for this particular sample) and most of the hydrogen at the surface was released in the first pulse, as checked by nuclear reaction analysis (NRA) techniques, which gave changes of the amount of deuterium before and after laser annealing. The sensitivity in this experiment was 5 × 10¹⁴ atoms per cm⁻² for HD and 5 × 10¹³ atoms per cm⁻² for D₂.