Vanadium Superpermeability to Hydrogen Isotopes
Superpermeability to Hydrogen Isotopes of the Group V Metals and Development of Vanadium Membranes for Applications in Nuclear Fusion
Tech Area / Field
- MAT-ALL/High Performance Metals and Alloys/Materials
- PHY-SSP/Solid State Physics/Physics
8 Project completed
Senior Project Manager
Tyurin I A
VNIIEF, Russia, N. Novgorod reg., Sarov
- St Petersburg State University of Telecommunications, Russia, St Petersburg
- Toyama University / Hydrogen Isotope Research Center, Japan, Toyama\nCompagnie Europeenne des Technologies de l'Hydrogene/Ecole Polytechnique, France, Palaiseau\nGraduate School of Engineering, University of Tokyo, Japan, Tokyo\nNational Institute for Fusion Science, Japan, Gifu-ken\nCNRS / Ecole Polytechnique, France, Palaiseau\nForschungszentrum Karlsruhe Technik und Umwelt / Institute For Technical Physics, Germany, Karlsruhe
Project summaryReactors based on nuclear fusion are to resolve not only the problem of providing the humankind with an energy source upon the exhaustion of fossil organic fuels, but also a number of global ecology problems, such as the global warming due to CO2 emission, acid rains, etc. Since fusion reactors are going to use the synthesis of D and T, a safe and accident-free operation with tritium counts among the most salient tasks here.
One of the fusion machine features is a rigid limitation on the content of the reaction product, He, in the fuel mixture: no more than 5% of He is permissible. Hence He must be continuously evacuated out of the reactor by cryogenic pumps which will simultaneously freeze D/T mixture. Frequent cryopanel regeneration cycles are required to extract D/T. Hence tritium will be accumulated at the cryopanels according to the current concept, and the tritium outburst hazard emerges in the case of an “off-normal” warm-up. Besides, each tritium recuperation cycle involves the operation of auxiliary systems resulting in an extra accumulation of tritium in such systems and in an ensuing reduction of the overall reactor operation reliability. Finally, tritium accumulation in the cryopumps means an enhancement of the full amount of tritium in the fuel cycle, which is disadvantageous in economic terms.
The Project participants are suggesting that a major part of D/T (e.g., ~ 99%) be extracted from the exhaust gas and returned into the fuel cycle before the gas reaches the cryopumps, the latter serving to evacuate virtually only helium and other impurities. This is to be attained by means of superpermeable membrane (SPM) installed between the operating plasma and the cryopumps; such membranes are capable of letting through and compressing hydrogen (D/T) selectively, continuously, and with a high pumping speed.
SPM also can be employed for an active control of D/T mixture fluxes aimed at a fusion machine operation enhancement. That refers in the first place to the control of pertor gas regimes by way of effective D/T evacuation.
The problem addressed by this Project is securing a safer and environmentally cleaner generation of energy in fusion reactors due to the development of a new technology based on the superpermeability of metals to hydrogen; the purposes are: to reduce the amount of tritium in the reactor, to secure its safer recycling, and to decrease the amount of tritium-containing waste.
The objective of the Project is the development of a new type of SPM based on the material most suitable for this purpose, vanadium, distinguished by a low induced radioactivity, as well as the development of a concept of technical employment of SPM in nuclear fusion. Deeper investigations into superpermeability of the Group V metals (where V belongs) are also to serve to Project objective.
The current state of art in the relevant field and ensuing tasks.
Superpermeability. “Superpermeability” means that a metallic membrane becomes virtually as “transparent” to suprathermal (і 1 eV) hydrogen particles as an opening of the same area, the permeability depending neither on membrane temperature nor on its thickness. As such membranes are virtually impermeable both to thermal molecular hydrogen and to any other gases, they are capable of compressing the permeating hydrogen by orders of magnitude and purifying it of any impurities, including He. Superpermeability is conditioned by a potential barrier for the reemission of absorbed hydrogen given rise to by a monatomic nonmetal film at the metal surface. Superpermeability has been obtained on membranes of Fe, Ni, Pd and Nb with respect to protium and deuterium particles of an energy below the threshold of sputtering of nonmetallic films.
One of the Project tasks consists in defining of superpermeability maintenance mechanisms under the action of ion sputtering of the surface and in making an attempt of superpermeability advancement into a higher ion energy range or even of finding means to maintain superpermeability at any incident energy. Another task is pioneer investigations of superpermeability to tritium and, in particular, of the effects of radiogenic helium generated by tritium decay.
Advantages of vanadium. The Group V metals, V, Nb and Ta, are most suitable material to achieve superpermeability. Vanadium appears to be the best one among them in this regard, and, at the same time, V is the cheapest one, and it exhibits the best manufacturability. Still the main V advantage is its low induced radioactivity. This feature is particularly valuable in the context of the radiation and ecological safety enhancement. However, more or less thorough investigations of superpermeability have been mainly conducted on Nb so far. Publications about the superpermeability of vanadium are virtually nonexistent. One of the main Project tasks is obtaining and studying the superpermeability of vanadium-based materials and developing of a membrane module based on vanadium.
Controlling D/T flows in fusion machines. Typical of SPM are an immense hydrogen particle permeation rate, an automatic compression of the permeating hydrogen behind the membrane, a very high permeation selectivity, the continuity of operation, low hydrogen inventory, and the absence of moving mechanical parts. All that permits the development of effective SPM-based means to control D/T fluxes in fusion machines in their models. In the first place, a short-way separation of D/T mixture from He could be effected: D/T mixture can be isolated from He before the cryopumps serving to pump He, and not after the pumps as it is currently accepted, due to a very large hydrogen pumping rate by SPM. That would reduce the influx of tritium into the cryopumps by an order of magnitude or more. Thus fusion machine safety would be effectively improved; besides, the fusion machine would become much more compatible with the environmental requirements due to a reduction of the amount of tritium in the fuel cycle and a reliability enhancement of the whole fusion machine exhaust system (due to a reduction of cryopump regeneration cycle frequency).
Another opportunity is the D/T mixture recycle control at interactions with plasma facing components. For example, one can obtain the low recycling pertor regime by an effective evacuation with SPM of D/T particles escaping from the plasma as charge-exchange neutrals, which can enhance the core plasma performance in the fusion machine.
Finally, SPM-based technologies could be employed in fusion machine periphery systems, such as neutral beam injectors or pellet injectors, particularly, at operation with tritium.
The Project tasks here are as follows: (1) building up of a concept of employment of vanadium-based SPM in both the existing (e.g., JET and LHD) and future (e.g., ITER) fusion machines; (2) designing, manufacturing and testing (including tests with tritium) of a new type of SPM, based on vanadium, including the manufacture and tests of a membrane module that might serve a component of membrane systems of large surface area and complicated geometrical configuration.
Expected results and their applications
1) Experimental data on superpermeability of the Group V metals, including vanadium, and on the specific effects of tritium and of the ions of an energy exceeding the sputtering threshold.
2) Physical model based on experimental data of the superpermeability of the Group V metals, including vanadium, including tritium, and comprising the case of ion sputtering.
3) Effecting and maintaining superpermeability of the Group V metals, including vanadium, at incident hydrogen energies above the sputtering threshold of light impurities, including the case of operation with tritium.
4) Membrane module as a basic element of membrane pumping systems of any dimension and shape, based on vanadium and tested for operation with tritium, designed for fusion machine applications.
5) Concepts of practical application of SPM in fusion machines and their models, including both the existing and the future ones.
6) A laboratory setup for plasma driven tritium permeation experiments.
Competence of the project team in the project area
RFNC–VNIIEF (Sarov), the research group headed by Dr. A.A. Yukhimchuk. The team that has earlier been working on weapon technologies has got a lot of experience in studies of material interactions with hydrogen isotopes, in particular, with tritium. The data of these investigations are broadly published, including papers in international scientific journals. In the past two years, a new unique experimental device has been designed, constructed and put into operation to stage experiments on tritium superpermeability, and pioneer experiments of this kind has begun.
Bonch-Bruyevich University (St. Petersburg), research group headed by Prof. A.I. Livshits (Scientific Leader of the Project). It is this team that has discovered the superpermeability phenomenon and made pioneer studies of it. These results are broadly published in international scientific journals. The research group continues actively working in this field, including the work at laboratories abroad as invited scientists. There is a unique equipment specially designed to study superpermeability, including an ion-beam apparatus as well as a plasma apparatus constructed during the past two years.
Addressing the ISTC objectives
The Project is aimed at securing of a safer and environmentally friendlier generation of nuclear energy in fusion machines. The goal is supposed to be reached by a new technology to be developed during the Project implementation, based on the superpermeability phenomenon first discovered by the Project participants. The expertise and research potential of the leading Russian nuclear weapons facility, RFNC–VNIIEF, is to be taken advantage of for the Project purposes; RFNC–VNIIEF will be incorporated into the international collaboration on controlled fusion. In the case of success, RFNC–VNIIEF may become a developer, manufacturer and commercial supplier of membrane pumping systems for the fusion machines.
Scope of activities
1) Developing of experimental methods and adapting the experimental equipment to the project purposes. Duration 18 months.
2) Studies of superpermeability of the Group V metals. Duration 33 months.
3) Developing of methods to maintain superpermeability of the Group V metals at incident hydrogen particle energy exceeding the sputtering threshold of light impurities. Duration 33 months.
4) Obtaining and studying superpermeability of vanadium to suprathermal hydrogen, and developing of a vanadium-based membrane pumping module. Duration 36 months.
5) Developing of a concept of the application of SPM in fusion machines. Duration 12 months.
Role of foreign collaborators: Stipulated by the Project program are: joint experiments and tests, including those in the collaborators’ (M. Bacal, N. Ohyabu, P. Mioduszewski, K. Watanabe, M. Yamawaki and R. Causey) laboratories, exchange of information and collaborative work on the concept of superpermeable membrane applications (P. Mioduszewski, N. Ohyabu, Ch. Day and M. Bacal), expert consultations and discussion of the results on vanadium-based materials (R. Causey and A. Hassanein).
Technical approach and methodology: Experimental studies are planned of the permeation of hydrogen particles of varied incident energy (~ 1–104 eV) and isotopic composition (H, D and T) through membranes of V and Nb with a controlled chemical composition of their surfaces within a wide metal temperature range (200–1500 °C). Specially designed ultra-high vacuum plasma-membrane devices (including the operation with tritium) and atomic and ion beam techniques will be employed. A layer-by-layer Auger electron spectroscopy (AES) analysis, secondary ion mass-spectroscopy (SIMS), electronography, Rutherford backscattering spectrometry (RBS), and electron microscopy, will be used as surface analysis methods.