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Hydrogen and Helium in Metals

#2276


Hydrogen and Helium in Metals

Tech Area / Field

  • FIR-ISO/Isotopes/Fission Reactors
  • FIR-MAT/Materials/Fission Reactors
  • PHY-SSP/Solid State Physics/Physics

Status
8 Project completed

Registration date
14.08.2001

Completion date
04.07.2007

Senior Project Manager
Osipov E A

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • St Petersburg State University / Institute of Physics, Russia, St Petersburg

Collaborators

  • Forschungszentrum Karlsruhe Technik und Umwelt, Germany, Karlsruhe\nArgonne National Laboratory (ANL), USA, IL, Argonne\nSandia National Laboratories, USA, CA, Livermore

Project summary

The radioactive decay of absorbed tritium atoms gives rise to an accumulation of radiogenic helium in metals operating in tritium-containing media. This brings about changes in the physical and mechanical properties of the metal (mainly, degradation), i.e. metal aging occurs, which markedly affects its performance. Investigations into the interactions of hydrogen isotopes with aged materials are hoped to provide a deeper insight into the in-service performance of structural materials (SM) after their long-term operation in a tritium-containing environment, and such studies are of great scientific and practical interest.

The basic concept of the Project consists in studying the interactions of hydrogen isotopes with metals that have been subjected to accelerated aging and that have performance characteristics basically similar to those of metals after long-term service in tritium-containing media.

The so-called "tritium trick" technique will be used for the accelerated inventory of radiogenic helium in materials of interest to concentrations of up to 103- 104appm. The basic underlying idea of the method consists of the fact that metal samples are exposed at high pressures and high temperatures of tritium to provide for a solution with a considerable amount of tritium, its radioactive decay resulting in a helium inventory. To reach these ends the unique tritium handling facilities at VNIIEF will be employed, which permit tritiation of metals at 50MPa of pressure and at temperatures of up to 900K. Under such conditions, the amount of radiogenic helium produced in metal over a few months will be of the same order of magnitude as that possibly accumulated during 10 years or more in the operation of a fusion reactor or any other tritium-containing installation.

The Project objective is:

development of a technique to predict the behavior and in-service performance of metals operating in tritium and tritium-containing media and to elucidate the degradation mechanism of these materials.

The following activities will be undertaken for this purpose:


development of a method of accelerated aging through the production of 3He and of related defect structures in metals and SM operating in tritium-containing media;
investigation of hydrogen isotope transport, trapping and retention in metals containing radiogenic helium;
investigation of the effects of radiogenic helium on the interaction of metals with non-equilibrium hydrogen isotopes, including tritium;
study of the synergistic effects of hydrogen isotopes and radiogenic helium upon mechanical properties of metals and SM.
To date, the processes of hydrogen isotope interactions with metals containing radiogenic helium have not been adequately investigated, and it is this that determines the novelty of the proposed Project. In most of the previous studies, helium was introduced into metals by either ion bombardment or by irradiation with high-energy neutrons. So far, there is much less knowledge as regards the behavior of helium produced by radioactive decay of tritium, although such experiments are certainly attractive, for a number of reasons. Firstly, it will be possible, due to the high mobility of tritium, to produce a high level of helium concentration throughout the metal bulk, and not just in a near-surface implantation zone. Secondly, the proposed helium inventory technique will produce no radiation damage and, thus, the bearing of the original material structure on helium-induced embrittlement can be clarified. Thirdly, this method enables more reliable results on the effects of helium on the inventory and diffusion mobility of hydrogen in metals.
The amount of helium to be produced in the blanket region of the ITER international thermonuclear reactor may vary, by different estimates, in the range of 170 to 270appm. With such a helium content, the fracture toughness of SM, which defines the reliability of materials, drastically decreases to ~0.2- 0.4 of the initial value. On the other hand, changes in mechanical properties are clearly observed, according to some reported data, at helium concentrations as low as 30appm.
The scientists involved in Project implementation have unique experience, both in the saturation of metals with radiogenic helium and in investigations of the physical and mechanical properties of metals. For instance, the effects of radiogenic helium on the structure and properties of B1 palladium alloy have been studied. The maximum achieved 3He concentration in the alloy samples amounted to ~3000appm. To obtain a concentration of 3He of an order of 100appm in austenitic stainless steels, according to estimates, samples must be maintained in tritium at a pressure level of 50MPa and a temperature level of 900K for about 1000 hours, which appears feasible in the framework of the proposed Project.
Comprehensive studies of the interaction of hydrogen isotopes with metals containing radiogenic helium and defects at a level characteristic of materials that have endured long-term operation in tritium-containing media, to be fulfilled during Project implementation, are hoped to provide the basis for the development of a new method to predict the behavior of metals at their long-term operation in tritium or tritium-containing media and to elucidate the mechanisms of degradation of these materials.

Fundamental importance of the Project consists in the following


A new approach is suggested to study synergistic effects of dissolved radiogenic helium and hydrogen isotopes on the physical and mechanical properties of metals and structural materials.
Existing theoretical models for hydrogen inventory and diffusion in solids and for hydrogen interactions with defect structures are to be verified and refined.
Mechanisms of material degradation in tritium-containing media are to be studied.

Practical applications

Practical applications of the results, to be obtained in the framework of the Project, are mainly associated with the rapid method to be developed for prediction of the behavior of metals and structural materials over long operation in contact with tritium-containing mixtures. Such a method may find application in nuclear fission and nuclear fusion power engineering, including: fusion reactor blanket region, superpermeable membranes for fuel pumping, purification, and recycling; tritium production systems; facilities for basic research in nuclear physics utilizing various tritium-containing materials, etc.

The following scientific problems are to be resolved through Project implementation:


1. A new technique and equipment will be developed for the accelerated saturation of metals and structural materials with radiogenic helium.
2. The kinetics of hydrogen isotope absorption in metals and SM with different amounts of accumulated radiogenic helium will be studied.
3. The mechanisms and kinetics of the release of radiogenic helium from metals will be investigated.
4. The effects of radiogenic helium on hydrogen diffusion and inventory in metals will be studied.
5. The effects of radiogenic helium on the superpermeability of Group V metals will be studied.
6. The influence of radiogenic helium on the mechanical properties and structure of SM and the joint effects of radiogenic helium and hydrogen isotopes will be studied. 7. Results of the investigations will be summarized to make detailed recommendations on how materials operating in a tritium-containing environment should be handled.

Main expected results:


A method of accelerated aging of metals and SM (through the production of 3He and of related defects) designed for operation in tritium-containing media.
New experimental data on the kinetics of generation, accumulation and localization of radiogenic helium in metals under various external conditions (temperature, tritium pressure, initial material structure, etc.).
Data on the effects of radiogenic helium on the kinetics of hydrogen isotope diffusion, solution, trapping, permeation and inventory in metals and SM.
Data on the synergistic helium / hydrogen effects upon mechanical properties of SM.
Data on the effects of radiogenic helium on the interaction of metals with nonequilibrium hydrogen and tritium.
Mathematical models of hydrogen transport in metals in the presence of radiogenic helium and of helium-induced defects.
A database on the parameters of hydrogen diffusion and inventory in metals and SM in the presence of radiogenic helium.
Recommendations on the employment of materials in a tritium-containing environment.


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