Effect of modification of the surface on the hydrogen permeability of reactor steels and their welded joints
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EFFECT OF MODIFICATION OF THE SURFACE ON THE HYDROGEN PERMEABILITY OF REACTOR STEELS AND THEIR WELDED JOINTS V. V. Fedorov 1, 2 and T. M. Zasadnyi 1
UDC 669.788
We study the characteristics of hydrogen permeability of 10Kh9VFA and 12Kh18N10T reactor steels in the intact state and in the presence of welds and analyze the influence of oxidation and irradiation with protons on these characteristics. It is shown that the proton irradiation increases the thermal stability and the degree of protection against the penetration of hydrogen into the oxidized specimens. To determine the contribution of the welds to the solubility of hydrogen, we apply, for the first time, a combined approach, namely, the diffusion coefficient of hydrogen in the weld is found according to the changes in the electric resistance in the process of hydrogenation and the hydrogen permeability of the welded joint is determined from the ratio of the areas of the base metal and the weld. It is shown that the oxide films with structures of chromium spinel formed on the surface of steels decrease their hydrogen permeability by 1.5–2 orders of magnitude. Keywords: hydrogen, tritium, hydrogen permeability, solvability, diffusion coefficient, oxides, proton irradiation, weld.
Parts of the reactors and pipes of heat exchangers in the power-generating units are most often produced from stainless austenitic and ferritic-martensitic steels whose main drawbacks include the susceptibility to radiation swelling, activation under irradiation, and high gas permeability, which leads to the loss of hydrogen and its isotopes into the environment. Moreover, the main part of losses of tritium occurs in the welded joints of reactor steels. It can be found solely by measuring the hydrogen permeability of steels [1]. As an advantage of this method, we can mention the fact that it is not necessary to directly determine the rate of penetration of radioactive tritium because all structural materials obey the following empirically established relation: PT ⎛ m ⎞ 1/ 2 = ⎜ H⎟ , ⎝ mT ⎠ PH where P and m are, respectively, the permeability and mass of the corresponding isotope [1]. According to this relation and in view of the condition that the level of radiation safety in the course of operation of a thermonuclear power-generating unit should not exceed 1 Ci / day, the upper ecologically safe value of the coefficient of penetration of ordinary hydrogen (protium) is specified as follows: Pcr = 2.4 ⋅ 10– 12 mole / (m ⋅ sec ⋅ Pa1/2 ) (the dashed lines in Figs. 1 and 2) [2]. The comparison of this value with the hydrogen permeability of reactor steels within the range of working temperatures of the reactor 673 – 973°K [1] shows that the flow rate of hydrogen through these steels should be made 6–200 times lower. Thus, for the reactor materials and their welded joints, this parameter must be additionally lowered as a result of alloying, thermochemical treatment, or the application 1 Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. 2 Corres
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