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Group 5 metals (V, Nb and Ta) are potential candidates of tube material in vacuum permeator for tritium recovery from Pb-17Li liquid blanket system. From this viewpoint, the influence of oxygen on the surface reaction rates of hydrogen on V and Ta were examined in an ultra-high vacuum apparatuses at elevated temperatures, and the results were compared with the data on Nb acquired in a previous study. The surface reaction rates of hydrogen on V and Ta, and consequently permeation rates, decreased with increasing oxygen concentration in the bulk as previously observed for Nb. These observations were ascribed to the increase in surface oxygen coverage with increasing bulk oxygen concentration. The weakest influence of oxygen on hydrogen permeation rate was observed for V. The expected permeation rate through V under typical blanket conditions, however, was not necessarily high due to high oxygen solubility in V. The evaluation indicated that the highest permeation rate should be obtained with Nb under typical blanket conditions. (authors)

The superpermeability phenomenon was suggested to use in fusion machines to separate the fuel mixture from helium ashes, to arrange in-vessel fuel recirculation contours and effective evacuation of...

The capability of a niobium panel to absorb atomic hydrogen was investigated for particle control application in divertors. The absorption rate (1.3 · 10 20 H/m 2 s) stayed almost constant up to a dose of 4 · 10 23 H/m 2 with no hydride formation occurring. The absorption panel can be easily regenerated. The absorbed hydrogen can be retained in the panel even after 48 h at room temperature. The absorption rate was found to decrease with panel temperature for T < 200 � C, remarkably dropping in the vicinity of room temperature. This is connected most probably with the formation of a multilayer impurity coverage. The multilayer carbon coverage deposited during the panel exposure in the divertor region of LHD decreased the absorption rate, but the panel properties could be completely restored by heating at 850 � C in spite of panel carbidization. � 2004 Elsevier B.V. All rights reserved.

In view of benefits expected from the employment of membranes for particle control in fusion devices and for separation of hydrogen from its mixtures with hydrocarbons, the behavior of a Pd sample is investigated in a plasma-membrane device with a graphite target. The permeation of hydrogen through a 0.2 mm-thick Pd membrane with clean surfaces was found to be limited by the bulk diffusion. An incident flux of hydrocarbon radicals (approx. 2×10 12 cm -2 s -1 ) in hydrogen plasma forms no carbon layer on the Pd surface. Applying of a negative bias to the target gives rise to target sputtering, and to the deposition of carbon onto the membrane surface. The formation of carbon layer results in a decrease of the absorption probabilities of both H 2 molecules and H atoms. The effect of the deposition of carbon is found to depend non-monotonically on membrane temperature.

Niobium membrane was immersed in hydrogen plasma in order to study its interaction with hydrogen ions in the energy range from a few eV to 200 eV, controlled by changing the membrane bias. Degradation of plasma driven permeation (PDP) and enhancement of permeation driven by thermal molecules of cold gas (GDP) were observed when bias voltage was applied to the membrane exposed at plasma because of’ sputtering of non-metal impurity layer at upstream surface. Both dissolution of oxygen and increase of membrane temperature lead to decreasing of membrane bias effect due to rise of surface layer recovering rate.

A new divertor pumping system with superpermeable membranes has been installed in the JFT-2M tokamak to investigate the applicability of membrane pumping to fusion devices. The membrane pumping is based on suprathermal (>1 eV) hydrogen permeation through a niobium membrane. Deuterium pumping is observed with divertor plasmas in tokamak environment. The pumping flux through the membrane increases by NBI heating and additional gas puffing and it is found that the pumping flux is directly proportional to the pressure in the divertor chamber where the pumping system is placed. The maximum permeation flux density is 7.3×10 19 D/m 2 s for a tokamak discharge with a dense and cold divertor plasma.

Membranes of Nb and V were examined with the use of a few plasma devices simulating different fusion machine conditions. A reproducible long-time plasma driven superpermeation was observed at all the flux densities up to 2 x 10 17 H cm -2 s -1 at membrane temperatures, T m , 500-1000°C. The deposition of stainless steel components on the membrane upstream surface resulted in a permeation drop at T m < 750°C, though the membrane annealing at T m ≥ 750°C restored superpermeation. A superpermeable membrane in combination with He plasma was shown to be a suitable means to isolate hydrogen impurities from He.

To develop the membrane pumping technology by means of superpermeable membranes at RFNC-VNIIEF in the “Prometheus” setup, the experiments on superpermeation of hydrogen isotopes through metal membr...

Metal membranes (e.g. of Nb, Pd, Fe, etc.) may be, under certain conditions, superpermeable to hydrogen fast particles and thermal atoms, i.e. their permeability may approach that of an opening of the same area. A proposal is put forward that superpermeable membranes be installed in the ITER pumping ducts in order to isolate the major part of D/T mixture and thus to avoid its accumulation in the cryopumps that pump helium. Nonthermalized particles coming from the divertor region will be immediately pumped by the membrane, while thermalized H 2 molecules will be atomized at incandescent surfaces specially introduced. The latter mechanism is experimentally demonstrated on a Nb membrane, with hydrogen atomizers made of Nb and Ta. The specific pump speed reached amounts up to ≅1.5 1s −1 per 1 cm 2 of the membrane area. Pumping of hydrogen is accompanied by its automatic compression beyond the membrane by a factor of ≅10 4 .

A non-metallic coating thicker than the implantation depth may protect a metal against tritium retention and permeation. However, a thinner film has quite the opposite effect: a dramatic increase of permeation and retention, and a corresponding suppression of re-emission. In view of the benefits expected from particle control with a superpermeable membrane placed right inside the divertor, the behavior of a Nb sample was investigated in a plasma-membrane device having a graphite target. Even polyatomic carbide coating was found not to hinder hydrogen absorption and permeation. Polyatomic non-carbide C films effectively inhibits it, but the formation of such films depends on H and C fluxes, H energy and metal temperature. A durable isolation of suprathermal hydrogen with the superpermeable membrane was observed at a high enough ratio between H and C fluxes, and the effects of carbon were found to have a non-monotonic temperature dependence.

In order to understand the capability of vanadium panels and membranes for fuel particle pumping at relatively low temperatures, absorption of neutral hydrogen atoms by vanadium sheet was examined at/below 350 °C under wide variety of experimental conditions. A niobium sheet kept at high temperature (420 °C) was used as a reference specimen. Sufficiently high absorption rates were obtained even at around room temperature in the range of incident fluxes from 101 7 to 1021 m -2 s -1 . No noticeable reduction in absorption rates was observed up to the H retention level of 0.1 at%. The influence of CO and water vapor was negligibly small up to an exposure of 1023 m -2 . Significant reduction in the absorption rate was observed only when an oxide film was formed on the surface by exposure to O 2 to 10 20 m -2 and to H 2 O over 1023 m -2 at room temperature.

Superpermeable membranes whose permeability to energetic hydrogen approaches the permeability of an opening of the same area can be employed to separate DT and He in fusion machine exhausts, to control the edge plasma and divertor conditions (by pumping and/or arranging of gas circulation through SOL or divertor) and to pump and recuperate DT in auxiliary systems e.g. in pellet or neutral beam injection. One of the key points is the operation at permeation flux densities of up to 1016–1019 cm−2 s−1. Theory predicts that the highest flux densities may be reached with superpermeable membranes based on the V group metals: the limit conditioned by a maximum permissible hydrogen concentration in bulk metal is expected to be as high as ∼ 1019 cm−2 s−1. The experimental membrane system comprised a cylindrical Nb membrane and an incandescent TaNb atomizer placed inside. The hydrogen pumping speed by this system amounts to ∼ 103 1/s, with a specific pumping speed of ∼ 1 1/s per cm2 membrane area and ∼ 8 1/s per cm2 atomizer area. Superpermeability was observed at record parameters referring both to the flux density of 3 × 1017 H/cm2/s (by one order of magnitude larger than ever before) and to the operational pressure of 3 × 10−2 Torr. A long-term reliable operation of this system proved being possible even in a vacuum far inferior to UHV conditions.

The experimental setup is put in operation designed to study the phenomenon of the superpermeation of hydrogen isotopes, including tritium, through metals and to demonstrate the possibility of membrane pumping. The permeation of atomic hydrogen through the niobium membrane was shown to occur in the superpermeation regime. For the first time superpermeation of tritium through a metallic membrane was experimentally observed. The possibility of effective pumping, compression and recuperation of hydrogen isotopes by means of superpermeable membrane was demonstrated.

Niobium is a potential candidate of tube material in vacuum permeator for tritium recovery from Pb-17Li liquid blanket system. From this viewpoint, the permeation of hydrogen through a Nb membrane ...

One of key points for the applications of superpermeable membranes in fusion is the question of the upper limit of the permeation flux density. Theoretical estimates give the maximum fluxes of ∼ 10 19 cm −2 s −1 for the V group metal membranes. The permeation fluxes of up to 3 × 10 16 and 4 × 10 17 cm −2 s −1 were achieved using Nb and thermal atomic hydrogen in the direct membrane experiments and in simulation absorption experiments respectively. Achievement of higher fluxes has been limited by the experimental equipment opportunities. Another important point is the reliability of membrane operation in the presence of chemically active gases. No negative effects of CH 4 , CO and O 2 specially introduced on a superpermeable Nb membrane were detected during 3000 h of operation. Such membranes coupled with special hydrogen atomizers can be easily incorporated into the up-to-date ITER design concept. The membranes installed in every of the 24 ducts might isolate > 90% of D/T with the flux densities already reached.