Behavior Of Hydrogen Isotopes Research Articles

Speciation and Diffusivity of Hydrogen in Molten 2LiF-BeF2 for Nuclear Fission and Fusion Applications

The behavior of hydrogen isotopes in molten 2LiF-BeF2 (FLiBe) is of interest to both advanced fission and fusion reactor designs. Tritium (hydrogen-3) is both a fission product and neutron activation product of lithium-6 fluoride in fission reactors which use FLiBe as a heat transfer fluid. In this case, tritium is an undesirable side product. In fusion reactors, FLiBe may be used as a blanket around the plasma which, upon neutron irradiation, generates tritium by the same neutron–Li-6 reaction. Here, tritium is a necessary and desirable product; the tritium is used to continuously fuel the deuterium-tritium fusion reaction. A thorough understanding of tritium’s behavior and residence time is relevant to managing tritium inventories in fission and fusion systems, which is essential for both the safety cases and functional operation of these reactors. Using hydrogen as a non-radioactive surrogate for tritium, we study the speciation and diffusivity of hydrogen in molten FLiBe at various temperatures relevant to reactor operating conditions.Here we present results of electrochemical investigations of hydrogen in FLiBe using cyclic and square wave voltammetry. We introduce hydrogen to FLiBe through 1) LiH additions and as 2) H2 gas, electrochemically desorbed from graphite. The expected solubility of H2 in FLiBe at 1 atm partial pressure is as low as 1 appm. Previous studies saw a higher apparent solubility of H2 at atmospheric pressure (up to 8000 appm). The possible presence of the postulated quasi-stable intermediate BeH2 is explored to explain the high apparent solubility of H2. The two methods of introducing H (as LiH reacting with BeF2 to form BeH2, which decomposes to H2, and the surface reduction of H2 on graphite) are compared to investigate the different species in which hydrogen may exist in molten FLiBe. Additionally, we attempt to quantify the diffusivity of these hydrogen species in FLiBe at 500ºC and 800ºC and compare findings with predicted diffusivities from computational models of Be and H additions to FLiBe. Overall, these studies seek to elucidate the speciation of hydrogen in FLiBe to aid in the management and operation of fission and fusion systems.

Microelectrode-Based Diffusivity Measurements of Hydrogen in Molten 2LiF-BeF2

Detailed knowledge of the mass transport behavior of hydrogen isotopes in molten 2LiF-BeF2 salt (FLiBe) is essential to the design of new nuclear fusion technologies that make use of this molten salt to breed tritium to fuel the fusion reaction. However, typical studies of hydrogen transport using macroelectrodes are limited to solely measuring the permeability of an analyte—a property defined as the product of concentration squared and diffusivity. This challenge necessitates the development of microelectrodes capable of dynamically measuring diffusion coefficients when concentrations are variable. Unlike macroelectrodes, microelectrodes can measure diffusivity and concentration independently due to a steady-state current proportional to the product of diffusivity and concentration captured in addition to the Cottrellian current response.This steady-state current is established due to a continuously expanding diffusion layer from the electrode that cannot expand indefinitely within finite experimental setups. Therefore, COMSOL Multiphysics simulations mirroring experimental conditions were performed to validate the analytical mass transport model based on the solution of Fick’s Second Law in radial coordinates. These simulations employed a 2D axisymmetric model to examine the conditions under which the infinite bulk solution approximation is valid, confirming that our electrode design would yield true steady-state current responses indicative of radial diffusion.Platinum microelectrodes developed in this study served a dual purpose: they not only addressed previous limitations in high-temperature molten salt experimentation but also enabled dynamic measurements of both the diffusion coefficient and concentration of hydrogen in molten FLiBe introduced by lithium hydride. Square wave voltammetry and chronoamperometry were performed on a three-electrode cell using a platinum working microelectrode, a tungsten counter electrode, and a Ni/Ni2+ thermodynamic reference electrode. The successful measurement of hydrogen diffusivity in these experiments not only offers essential insights into molten FLiBe behavior but also sets the stage for future investigations into the effects of redox conditions, impurities, and equilibration time on tritium within nuclear fusion reactors.

Modeling the Co-permeation of hydrogen isotopes through metals in all rate-limited regimes

Tritium safety plays a crucial role in D-T fusion engineering. During normal operations, tritium(T) is typically dissolved with protium(H) or deuterium(D) in structural materials. However, there are still controversial issues related to the synergistic isotopic effects when considering T permeation through metals with H/D. In this work, a numerical model is built to describe the co-permeation behavior of multi-component hydrogen isotopes through metals in all rate-limited regimes. This model involves isotope exchange reactions in the gas phase, molecular dissolution, and atomic recombination at the gas-solid interface, as well as the diffusion process in the bulk. By simulating the mixed hydrogen gas permeation through various metals, different results are observed. In surface limited regime, the effective permeation flux of D is slightly enhanced by the presence of H due to isotopic effect. In diffusion-limited regime, the recombination of heteronuclear hydrogen molecules (HD) will deviate the equilibrium concentration of D on the upstream side from Sievert's Law, thereafter suppressing the D permeation flux on the downstream side.

Effects of the diffusion path on the effective diffusion coefficient of hydrogen isotope in tungsten with helium bubbles

In irradiated tungsten (W), the formation of numerous helium (He) bubbles has a significant impact on the diffusion behaviors of hydrogen (H) isotopes. To investigate the influence of the microstructure of He bubbles on the diffusion behaviors of deuterium (D), we utilize the reconstructed experimental data, the phase-field method, and the steady-state diffusion equation to calculate the effective diffusion coefficients of D based on four possible diffusion paths, i.e., D atoms diffuse in the bulk W, inside the He bubbles, and along the outer and inner surface of He bubbles. Simulation results based on the reconstructed depth-dependent distribution of He bubbles reveal that the diffusion paths remarkably affect the effective diffusion coefficient of D. By varying the radius and number density of He bubbles, the effective diffusion coefficient of D undergoes significant changes. Subsequently, we fit an empirical formula of the effective diffusion coefficient of D as a function of the radius and number density of He bubbles for different diffusion paths of D based on simulation data. Besides, the growth behaviors of He bubbles are simulated by the phase-field model, and the effective diffusion coefficient of D as a function of the evolutionary time for different diffusion paths is also discussed. At last, we investigate the influence of the D concentration and temperature on the effective diffusion coefficient of D along different diffusion paths. These results indicate that the effective diffusion coefficient of D increases with decreasing the D concentration and increasing the temperature for four diffusion paths. The current study provides a reference for investigating the diffusion behaviors of D in W in the presence of He bubbles, and may benefit the efforts of developing low D retention W-based materials.

Measurement of Hydrogen Permeation Fluxes Through Tungsten Deposition Layer Growing by Hydrogen Plasma Sputtering and Observation of Microstructure

Hydrogen isotope behavior, especially permeation and retention, at the first wall is important for the safety and fuel sufficiency of fusion reactors. This study focuses on the deposition layer formed on the first wall by sputtered particles. Hydrogen permeation flux was measured under the co-deposition environment of hydrogen and tungsten, and the microstructure of the deposition layer was observed by a transmission electron microscope. Then the relationship between the observed hydrogen permeation behavior and the formation of the deposition layer was evaluated. The results showed that the deposited layers had three different microstructures and that the permeation flux decreased with its formation. However, it was concluded that the permeation behavior could be evaluated simply by the increase in the thickness of the deposited layer and that there was no clear effect of the different structures on the permeation behavior.

Water speciation and hydrogen isotopes in hydrous stishovite: implications for the deep Earth water cycle

Stishovite is a key mineral for understanding the deep Earth water cycle because of its potential as a main carrier for water into the transition zone and lower mantle. During subduction-related metamorphism of basaltic oceanic crust, stishovite stabilizes at 8–9 GPa and comprises 10–25 vol% of the bulk mineralogy, with some experimental studies indicating that stishovite can accommodate 3.5 wt% H2O or more in the transition zone and upper lower mantle. This large water solubility has been explained by a hydrogarnet substitution mechanism (1Si4+ ↔ 4H+) and/or the incorporation of interstitial molecular water. To investigate water speciation and hydrogen isotope behavior, we synthesized partially deuterated hydrous stishovite at 9 GPa and 450 °C in a multi-anvil press (MA). The hydrous stishovite contains on average 1.69 ± 0.05 wt% water, which is consistent with earlier MA studies but is significantly lower than the 3.5 wt% reported from in situ diamond anvil cell (DAC) studies made at higher pressures and temperatures. 1H MAS NMR spinning sideband characteristics suggest a high abundance of interstitial molecular water in hydrous stishovite, while the presence of a hydrogarnet defect cannot be ruled out. Unit-cell volumes and deuterium enrichment in the quenched hydrous stishovite indicate that ~ 45% of the water is lost from the stishovite upon quenching and decompression of the experiment, consistent with a higher solubility. This implies that the pristine water contents of a P–T–fO2 equilibrated hydrous stishovite cannot be quenched to 1 atm and room temperature from classical MA experiments. We further present a capillary-based recovery method for fluid from experimental capsules, allowing direct determination of the D/H ratio of the experimental fluid and indirect determination of the hydrous stishovite. Using Rayleigh modeling to account for the quench-related water loss, we find that, at 450 °C and 9 GPa, deuterium is 3.5–4.5 times enriched in hydrous stishovite relative to coexisting aqueous fluid. This is opposite of what is commonly observed for mineral–fluid pairs above 300 °C, rendering hydrous stishovite a potential sink for deuterium and decreasing the D/H ratio of coexisting aqueous fluids. Partial decomposition (30–60%) of hydrous stishovite during mantle upwelling and production of primary basaltic melts could be accompanied by high-temperature D/H fractionation, decreasing the hydrogen isotope composition of such melts towards “mantle-like” δD values between −75 and −220‰.

The promotion effect of uniaxial strain on hydrogen aggregation in tungsten

The aggregation behavior of hydrogen (H) isotopes after low-energy plasma irradiation is particularly important for tungsten (W) in future fusion devices. Here, using the first-principles calculation, we demonstrated an interesting strain effect that may drive the planar aggregation of interstitial H atoms in W. Although there are attractive interactions between H atoms, the spontaneous nucleation of these platelet-like H clusters in strain-free W appears impossible even at high H concentrations (up to 0.1 at.%) because of the extremely low configurational entropy. However, applied uniaxial strain significantly increased the binding energy of H platelets and enabled planar H clusters to form. These results suggest that uniaxial strain enhances the nucleation and growth of H platelets in W, regardless of whether it is a compressive or tensile strain. Moreover, the binding energy of one-layer H clusters was lower than that of parallel H platelets, implying that the formation of multi-layer H clusters in W and their stability is also promoted by uniaxial strain. Meanwhile, the presence of planar H clusters dramatically reduced the vacancy formation energy in W, which in turn provided an extra trapping site to accommodate excessive H atoms. These results provide an important reference for understanding the H evolution in W-PFMs.

Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

The diffusion of molecular water (H2Om) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δDH2O) to the glass (δDgl) offset by a glass-H2O fractionation factor (ΔDgl-H2O = δDgl – δDH2O) which is approximately −33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H2O during eruption, transport, and emplacement. To aid in the interpretation of δDgl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 °C for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 °C and below 250 °C. Lower temperature experiments yield raw ΔDgl-H2O values in the range of −33 ± 11‰. Experiments at 225 °C using both positive and negative initial ΔDgl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H2O vapor (103lnαgl-H2O) below 250 °C. Variation in ΔDgl-H2O (−33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 °C yield unexpectedly low δDgl values with ΔDgl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H2O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H2Om and 1.0 wt% hydroxl (OH−) in the highest P-T experiment at 375 °C and 21.1 MPa. Therefore, we employ a 1D isotope diffusion–reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 103lnαgl-H2O values of −33‰ ± 5‰ and −25‰ ± 5‰ at 175 °C and 225 °C, respectively. At higher temperatures, ΔDgl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δDgl value is highly responsive to changes in the δDgl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δDgl fractionation with progressive glass hydration. These results indicate that the observed ΔDgl-H2O of ∼−33 ± 11‰ can be cautiously applied as an equilibrium 103lnαgl-H2O value to natural silicic glasses hydrated below 250 °C to identify hydration sources. This approximate ΔDgl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H2O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface.

Effect of initial exposure temperature on the deuterium retention and surface blistering in tungsten

• Exposure temperature in the initial stage strongly affects the subsequent deuterium behavior in tungsten. • Initial exposure at 420 K significantly inhibits the formation of intergranular blisters and leads to a reduction in the size of intragranular blisters. • Initial exposure at a relatively high temperature of 600 K leads to a reduction in the size of intergranular blisters and reduces deuterium retention by ∼ 34 %. • The gradient distribution of deuterium over a larger depth leads to the intergranular cracks more prone to propagate along sloping grain boundaries. Temperature is one of the critical factors that strongly affect the behavior of hydrogen isotopes in tungsten (W). This work investigates the effect of initial exposure temperature on the deuterium (D) retention and surface blistering in recrystallized W. D plasma exposure is started at 420 K and 600 K, respectively, and then changed to the same temperature of 520 K. For each temperature, the irradiation fluence is ∼ 1 × 10 26 D m −2 , and therefore the total fluence is ∼ 2 × 10 26 D m −2 for each sample. A reference W sample was also exposed at 520 K with the same total fluence. Compared with the reference sample, it is found that the initial temperature of 420 K significantly inhibits the formation of intergranular blisters and leads to a reduction of intragranular blisters in size. Although the initial temperature of 600 K facilitates the formation of intergranular blisters, the intergranular blister size is reduced. This is possibly related to the relatively higher concentration of trapped D over a large depth range as revealed by the TMAP simulation results, leading to the fact that intergranular cracks are more prone to propagate along sloping grain boundaries. Thermal desorption spectra results show that the initial exposure temperature of 420 K and 600 K reduce D retention by a factor of ∼ 56 % and ∼ 34 %, respectively. It is considered to be connected with the blistering-induced defects and D transport in W, which are strongly affected by the initial exposure temperature. This work demonstrates that the initial exposure temperature plays a crucial role in D behavior in W, improving the understanding of the temperature effects of hydrogen isotopes behavior in W.

Deuterium transport and retention properties of representative fusion blanket structural materials

Reduced activation ferritic-martensitic (RAFM) steels have been developed for decades for use as fusion blanket structural materials, and have advantages in both mechanical properties and irradiation resistance following careful engineering of the microstructure. However, the hydrogen isotope behavior in these proposed fusion structural materials is not well understood, but is important to assess since it impacts the fusion reactor safety and self-sufficient tritium fuel cycle. In this work, we investigated deuterium transport and retention in representative advanced RAFM steels, including castable nanostructured alloys (CNAs), and oxide-dispersion-strengthened (ODS) steels. A gas-driven permeation (GDP) system was used to measure the permeability, diffusivity and solubility of the studied materials, covering the temperature range from 623 K to 873 K, and the loading pressures from 1.8×104 to 1.0×105 Pa. The results indicated that the deuterium permeability has little material dependence. In contrast, the deuterium diffusivity of the studied materials showed significant variation. The deuterium diffusivity in ODS steels is one order of magnitude lower than that in RAFM steels and CNAs, and correspondingly, have an effective solubility that is 2–10 times larger than RAFM steels and CNAs. In addition, thermal desorption spectroscopy (TDS) measurements were performed to assess the deuterium retention and desorption of these materials following a static thermal deuterium charging at 723 Kfor 1 hour under the deuterium pressure of 1.0×105Pa. It was found that ODS steels exhibit the highest deuterium retention and have broader desorption peaks. Microstructural features contributing to deuterium retention and impacting deuterium transport are discussed to rationalize the observed deuterium behavior in the studied RAFM steels.

Hydrogen isotope exchange behavior of protonated lithium metal compounds

The exchange behaviors of hydrogen isotopes between protonated lithium metal compounds and deuterated water or tritiated water were investigated. The various protonated lithium metal compounds were prepared by acid treatment of lithium metal compounds with different crystal structures and metal compositions. The protonated lithium metal compounds could more effectively reduce the deuterium concentration in water compared with the corresponding pristine lithium metal compounds. The H+ in the protonated lithium metal compounds was speculated to be more readily exchangeable with hydrons in the aqueous solution compared with Li+ in the pristine lithium metal compounds, and the exchanged heavier isotopes were speculated to be more stably retained in the crystal structure compared with the light protons. When the tritiated water (157.7 kBq/kg) was reacted with the protonated lithium metal compounds, the protonated lithium manganese nickel cobalt oxide was found to adsorb and retain twice as much tritium (163.9 Bq/g) as the protonated lithium manganese oxide (69.9 Bq/g) and the protonated lithium cobalt oxide (75.1 Bq/g) in the equilibrium state.

A high temperature dual-mode quartz crystal microbalance technique for erosion and thermal desorption spectroscopy measurements.

An improved quartz crystal microbalance measurement method is described, which allows us to determine erosion, implantation, and release rates of thin films, during changing temperatures and up to 700 K. A quasi-simultaneous excitation of two eigenmodes of the quartz resonator is able to compensate for frequency drifts due to temperature changes. The necessary electronics, the controlling behavior, and the dual-mode temperature compensation are described. With this improved technique, quantitative in situ temperature-programmed desorption measurements are possible and the quartz crystal microbalance can be used for quantification of thermal desorption spectroscopy measurements with a quadrupole mass spectrometer. This is demonstrated by a study of the retention and release behavior of hydrogen isotopes in fusion-relevant materials. We find that more than 90% of the deuterium implanted into a thin film of beryllium is released during a subsequent temperature ramp up to 500 K.

Deuterium transport in ITER-grade tungsten

The transport behavior of hydrogen isotopes in ITER grade tungsten (IG-W) and commercial rolled polycrystalline tungsten (CR-W) was investigated using deuterium gas. Analyses using field-emission scanning electron microscopy revealed that both IG-W and CR-W had a polycrystalline structure, but their surface grain structures were apparently different. The permeability and diffusivity of deuterium in IG-W and CR-W were obtained using a time-dependent gas-phase technique in the temperature range of 650–850 °C, being observed to be affected by surface morphology and grain boundaries. Our results in this study were also compared with previous results for polycrystalline W reported by other authors.

Dissolution and Release Behavior of Hydrogen Isotopes from Barium-Zirconates

Dissolution and Release Behavior of Hydrogen Isotopes from Barium-Zirconates

Chemisorption, diffusion and permeation of hydrogen isotopes in bcc bulk cr and cr(100) surface: First-principles dft simulations

Chromium, a transition metal, is one of the important constituents, used widely in steel. It is of paramount importance to study the behavior of hydrogen isotopes with Cr to design a suitable barrier materials for arresting the permeation of H isotopes. Here, the interaction and dynamical behaviours of hydrogen isotopes in bcc Cr lattice have been studied employing plane wave based density functional theory. Nudge elastic band methods have been used to determine the activation energy barrier whereas phonon calculations are performed to calculate the zero point energy in order to investigate the isotope effects. The dissociative surface adsorption of hydrogen is predicted to be exothermic, whereas surface to sub-surface and bulk absorption is found to be endothermic but the vacancy induced absorption energy for H atom in bulk Cr has been found to be exothermic. The diffusion of H atom from one tetrahedral hole to the adjacent tetrahedral hole has been established to be the most favoured pathway. The lighter H atom was shown to have higher diffusion coefficients, permeability coefficients, solubility and rate constants than heavier D and T atoms. The computed values of diffusion, permeability and solubility of H in Cr follow the experimental prediction. The computed results presented here might be of assistance in the future design of suitable materials to arrest the permeation of tritium.

An electrochemical study of hydrogen in molten 2LiF-BeF2 (FLiBe) with addition of LiH

The chemical and transport behavior of hydrogen isotopes in molten 2LiF-BeF2 (FLiBe) is of interest for the design of tritium management systems in nuclear fission and fusion reactors that use FLiBe. The chemical reaction of LiH with FLiBe is used to introduce hydrogen in the molten salt and electrochemical methods are used for in situ studies of hydrogen in FLiBe. LiH reacts with molten FLiBe to generate an electroactive species whose voltammetry peak is proportional to the added quantity of LiH. The cyclic voltammetry reaction potential of 2.009±0.050VvsBe/Be2+ and the electron exchange of n=0.8±0.5 are consistent with the one-electron oxidation of dissolved H in the zero valence state, Ho to H+. The concentration of Ho is estimated by linear sweep voltammetry at 60–80% of the hydrogen introduced by the reaction of 0.42 mol% LiH with FLiBe and after eleven hours it remains above 40%. It is postulated that covalent BeH2 is formed in FLiBe upon LiH addition, as a FLiBe-soluble quasi-stable intermediate product. The results provide an evaluation of LiH as a means of introducing dissolved hydrogen in FLiBe, enabling electrochemical methods as tools to advance the understanding of the chemistry of hydrogen isotopes in FLiBe.

Deuterium behavior in tungsten exposed to deuterium plasma with rising or declining temperature

Exposure temperature is a key factor affecting the behavior of hydrogen isotopes (H) in tungsten (W). This work focuses on the effect of temperature variations in plasma exposure on deuterium (D) behavior in recrystallized W. Low-energy and high-flux D plasma exposure was conducted in the linear plasma generator STEP. Exposure temperature was set as three patterns, keeping constant at 520 K, rising from 420 K to 590 K, and declining from 590 K to 420 K. Surface observations show that as compared to the constant type the declining type leads to a much more severe surface blistering, whilst the rising type gives rise to a mitigation of intragranular blisters. Thermal desorption spectra indicate that D retention is remarkably enhanced in the declining case while a slight enhancement is found in the rising type. This study demonstrates that the trend of temperature variation during plasma exposure has a great influence on the behavior of D in W, providing a new insight into the temperature effect of H behavior in W.

Experimental Study on the Release Behavior of Deuterium from Advanced Tritium Breeder Core-Shell Li2TiO3-Li4SiO4

Core-shell Li2TiO3-Li4SiO4 has been suggested as an advanced tritium breeder for outstanding mechanical strength and rich lithium. However, no study has reported on the release behavior of hydrogen isotopes from core-shell Li2TiO3-Li4SiO4. This paper focuses on a deuterium exposure experiment to investigate deuterium release behavior. X-ray diffraction, Raman, and electron spin resonance measurements were adopted to analyze the phase change, crystal structure modification, and defects before and after deuterium exposure. Thermal desorption spectroscopy was used to study the deuterium release behavior of core-shell Li2TiO3-Li4SiO4. Deuterium release from single Li4SiO4 and Li2TiO3 was also examined for comparison. The similarity of the deuterium release profiles between core-shell Li2TiO3-Li4SiO4 and Li4SiO4 was observed, which was considered to be caused by the incomplete coverage of shell Li2TiO3 on the core-shell samples.

Transport parameters and permeation behavior of hydrogen isotopes in the first wall materials of future fusion reactors

Abstract The first wall of a future magnetic fusion device is essentially defined as the plasma-facing surface of the breeding blankets, which is supposed to be subjected to bi-directional hydrogen isotopes permeation: in one direction by edge plasma-driven permeation (PDP) of deuterium as well as tritium into blankets, and in the other direction by breed tritium gas-driven permeation (GDP) into the edge plasma. Deuterium and tritium PDP will complicate the recovery of tritium from the blanket, while tritium GDP will lead to an unwanted increase of particle recycling in the first wall region, which could even affect core confinement performance. Reduced activation ferritic/martensitic (RAFM) steels are widely proposed as candidate structural materials for the blanket of a DEMO reactor, the surface coatings made of tungsten are necessary to protect the plasma-facing wall from sputtering under high-energy particle bombardment. Therefore, the characterization of hydrogen isotopes transport through a multi-layer W + RAFM wall is of crucial importance to evaluate major reactor design issues including tritium retention, particle recycling and breeding feasibility, etc. This paper is intended to provide a review over the transport parameters of hydrogen isotopes in RAFM steels, including permeability, diffusivity, solubility and surface recombination coefficient. In addition, the present research status of hydrogen isotopes permeation and retention behavior of tungsten coated RAFM steels is briefly introduced.

Deuterium Gas-Driven Permeation and Retention Through Tungsten-Coated CLAM Steel

Hydrogen isotope behavior in tungsten coated on reduced activation ferritic/martensitic (RAFM) steels such as China low activation martensitic (CLAM) steel has attracted more attention in the fusion engineering research community. This paper is mainly devoted to the investigation of the effect of tungsten coating on deuterium permeation and retention behavior in RAFM steels. The permeability and diffusion coefficients of CLAM, W-CLAM, and W were determined by gas-driven permeation (GDP) tests followed by thermal desorption spectroscopy to measure deuterium retention. It was found that the observed deuterium permeability and diffusivity of the composite W-CLAM specimen was reduced to about ~60% of the pure CLAM steel, whereas deuterium retention increased, evidently owing to the W coating on the surface that caused the slower release of D into the environment and increased of the effective surface area. In addition, a key finding was that the lath martensite–coarsened and more precipitate phase was found, which may be due to the migration of lath interface during the GDP test.