The operation experience of sodium-water steam generators has demonstrated that the main source of hydrogen ingress into the secondary sodium is the process of hydrogen depolarization (equation 2) of the cathodic electrochemical corrosion process (equation 1) on the steam-generating steel surfaces from the water side:
3Fe + 4H2O = Fe3O4 + 4H2 (1)
2H2O+2e- = 2Hkt + 2OH- (2)
Hydrogen generation and its concentration in water and sodium increases in the course of chemical washing and when a steam generator is started up, as with them the metal corrosion rate goes significantly up. Determination of the predominant direction of hydrogen transfer is required, for example, for estimation of steam generator tubes’ 10X2M steel hydrogenation.
In this work, the specific rates of hydrogen supply to sodium were determined with analyzing the data of systematic monitoring of the hydrogen content in the third (steam-water) and secondary (sodium) circuits. The mechanism of diffusion of hydrogen formed in the cathode sections of the circuit through steel was considered in terms of two processes, i.e. recombination of H atoms adsorbed into H2 molecules with their subsequent release into the solution, and dissolution of atomic hydrogen in the metal itself with diffusion into sodium of the secondary circuit, where hydrogen is bound, thus forming sodium hydride. Based on the assumptions made and the calculations performed, as well as with consideration of the BN-600 reactor operational data, the following results were obtained:
- The content of hydrogen dissolved in the third circuit water does not influence the flow.
- Hydrogen diffusing through a steel wall of steam generating tubes is of electrolytic (corrosion) origin.
- At a low corrosion rate typical of the nominal operating mode of the sodium-water steam generator, most of the corrosion hydrogen (about 90%) diffuses into the secondary sodium, and at a significant corrosion rate (typical of chemical washings) it goes into water. Therefore, it is not correct to control the metal corrosion of the steam-generating surfaces in the nominal operating mode of the sodium-water steam generator only by the difference in the hydrogen concentration in the superheated steam and in the feed water.
- It was found that at all the real rates of metal corrosion in heat-exchange tubes in a steam-water environment, hydrogenation of 10X2M steel is insignificant. Thus, hydrogen embrittlement of steel is practically ruled out in the course of steam generator operation and chemical washing of its pearlitic surfaces.
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