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First Wall and Divertor Effects

#1419


Investigation of the Effect of Exposure to Tritium and Reactor Irradiation on the Properties of the First Wall and Divertor Materials for Magnetic Fusion Experiment

Tech Area / Field

  • FUS-MCS/Magnetic Confinement Systems/Fusion

Status
3 Approved without Funding

Registration date
16.12.1998

Leading Institute
VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Supporting institutes

  • All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow\nUral Branch of RAS / Institute of Metal Physics, Russia, Sverdlovsk reg., Ekaterinburg\nMIFI, Russia, Moscow

Project summary

The high radiation hazard presented by tritium makes the task of organizing experiments involving tritium very difficult, and thus the majority of investigations in this area are carried out on stable hydrogen isotopes, with subsequent extrapolation of these experimental results to those for tritium, based on the assumption of chemical identity and observing the isotope ratios for diffusion coefficients.

It has been shown [1], however, that extrapolation cannot be regarded as sufficiently reliable, since gas absorption may vary noticeably for some metal-deuterium and metal-tritium systems, and the isotope ratios for diffusion coefficients are often not satisfied. Besides, experiments with stable hydrogen isotopes completely exclude the unique property of the metal-tritium system—the generation and subsequent exposure of the system to radiogenic helium, the product of the radioactive decay of tritium.

The objective of the proposed Project is to gather experimental data on the effect of tritium, radiogenic helium and reactor irradiation on the structure and properties of the materials and bimetallic elements of the first wall and pertor of a fusion reactor employing magnetic plasma confinement, as well as to study the behavior of the first wall materials under conditions of long-term static stresses (delayed failure strength).

Implementation of the proposed Project will give vital new knowledge on the following topics:


- Penetration parameters and levels of hydrogen accumulation in the investigated materials from the gaseous phase, glow discharge plasma, or ion beams.
- Effect of radiogenic helium on hydrogen isotope mass transfer and on the structure and properties of the transition zones between layers of bimetallic elements used for the first wall and the pertor.
- Localization and concentration of residual hydrogen atoms in the samples after vacuum degassing.
- Mechanical properties of hydrogen isotope-saturated samples under conditions of long-term exposure to tensile stresses.
- Erosion and changes in the structural and phase states of surfaces under conditions simulating plasma disruptions.
- Interaction of tritium atoms with initial and radiation-induced defects in the investigated materials.
- Influence of the surface state on tritium interaction with materials.

The expected results will be of great scientific and practical value, contributing both to basic understanding and to the provision of a basis for making a carefully verified choice of materials and process-engineering solutions in fusion installations that will maximize component lifetimes and minimize tritium escape to the environment.

The proposed Project will carry out the world's first experiments to study hydrogen penetration through helium-saturated materials and the influence of tritium, radiogenic helium, and neutron irradiation on the properties of contact zones of bimetallic-layered structures.

Such data are essential to accurately predict tritium losses and operating reliability of the first wall and pertor elements for two reasons. First, the presence of helium in the material lattices may bring about noticeable changes in hydrogen interaction with the materials, due to the formation of point defect complexes, which speed up the transport and/or raise hydrogen solubility in the material. Second, hydrogen embrittlement and the formation of gas pockets in the transition zone between layers in bimetallic structural elements may substantially change the mechanical properties and heat transfer rate from the first wall elements.

The levels of tritium accumulation and the kinetics of tritium release will be investigated through vacuum annealing of tritium-saturated samples. The kinetic degassing curves will be plotted, and the residual tritium will be measured. These experiments will be conducted both before and after reactor irradiation.

The distribution of hydrogen isotopes in the structure of the metals being investigated will be determined by microautoradiography. Information about the interaction of hydrogen isotopes and radiogenic helium with defects of different origins (dislocations, grain and subgrain boundaries, phase boundaries, etc.) will be derived from the experimental data, showing the changes in mechanical properties, electrical resistance, and residual tritium concentration. In addition, X-ray structural analysis, positron annihilation, electron microscopy, and nuclear microanalysis will be used.

Reactor irradiation and post-reactor investigations will be carried out at the IVV-2M test reactor in Zarechny, which provides fast neutron (E>0.1 MeV) fluences up to 1020 n/cm2, even at the temperature of liquid nitrogen.

To investigate the transition state between bimetallic layers, special samples made from bimetallic slabs will be used. They will be tested before and after saturation with tritium and radiogenic helium. Mechanical tests, combined with fractographic and metallographic analyses, will give unique information about the state transformations in the transition zone between two layers of different metals.

Previous investigation [2] showed that it is important to account for the influence of tritium, radiogenic helium, and neutron irradiation on the degradation of mechanical properties of structural materials by long-term tensile tests. Short-term testing fails to reveal the actual role of hydrogen in the transition to a brittle state under long-term loads (which is primarily the case in actual operation), while long-term tests allow the sensitivity of plastic properties of structural materials to be determined, even with the smallest quantities of gas admixtures.

The opportunity to use the equipment and techniques acquired while implementing Project 019-94 and involve international cooperation with the scientists of the USA, European Union and Japan will be of paramount importance in implementing the proposed Project. For example, the present Project Proposal is based on the ideas formulated at joint meetings with H. Roehrig, A. Perujo, D. Ruzic, R. Causey, the text was edited by C. Elliott.

The proposed Project implementation fully agrees with the ISTC goals, since it offers a peaceful application alternative to the efforts of nuclear weapons designers and allows their knowledge and practical experience to be switched over to solving the peaceful problems of thermonuclear fusion energy production, environment protection and fusion reactors safety, and promoting further integration of the Russian scientists in the world scientific community.

Potential role of foreign collaborators

In 1997 and 1998 there were several discussions of this proposal with foreign specialists in Russia and abroad [3]. Preliminary agreement concerning future collaboration with A. Perujo (JRC, Ispra, Italy, e-mail: adolfo.perujo@jrc.it) and R. Causey (SNL, Livermore, California, USA, e-mail: causey@california.sandia.gov) was achieved in spheres of scientific consultations, analysis of results, organization of joint investigations and workshops

Till this moment Prof. D. Ruzic and C. Elliott (they are employees of the University of Illinois) expressed their wish to become official collaborators of this project. They conduct the following activities: ensuring communication with foreign specialists, supply of information concerning the project topics, delivering samples for investigations, discussion of task formulation, analysis of experimental results, editing of reports, papers and articles.

Discussions with specialists have shown that objectives of this investigation are of great interest for science. That's why it was decided together with foreign specialists to start some activities before receiving financial support. To realize this idea C. Elliott in February, 1999 have brought to Snezhinsk (Russia) bimetal slab, provided by Prof. J. Stubbins (University of Illinois) for samples manufacturing. To discuss progress of these activities Prof. D. Ruzic will come to Snezhinsk with working visit in September, 1999.

References


1. R.Lasser, Tritium and Helium-3 in Metals, Springier series in materials science, v. 9, 1989.
2. V.V.Zabilsky, V.V.Velichko, S.G.Ilyina. Zamedlennoye razrusheniye martensitnistareyushchikh stalei. 1. Rol sredy ispytanii. FMM, 1995, v. 80, num. 6, p. 108-118.
3. Final Report on the ISTC Project #019-94, ISTC, 1998.


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