THE PHYSICAL MODEL OF MASS TRANSFER CORROSIVE HYDROGEN IN FAST REACTOR STEAM GENERATORS OF THE SODIUM-WATER TYPE

Abstract

For timely and prompt detection of the inter-circuit depressurization of the steam generator of the “sodium-water” type in the 2nd circuit, continuous monitoring of the content of hydrogen, as a product of the interaction of sodium with water, is carried out in all operating modes. However, in reality, the main source of hydrogen in the sodium of the 2nd circuit is the process of electrochemical corrosion of 10X2M steam generator steel in a steam-water medium, the speed of which is never zero and is controlled at the minimum achievable level by means of the water-chemical regime of the 3rd circuit. At the same time, during the operation of the steam generator, deposits of corrosion products accumulate on the surface of the structural steel from the side of the 3rd circuit. The removal of deposits is carried out with the help of operational chemical flushes (ECPs). However, during the ECP, the corrosion rate of steel inevitably increases significantly (by about three orders of magnitude), which leads to an increase in the concentration of hydrogen in water and in sodium. This phenomenon has previously caused concern to the operational services of nuclear power plants due to the likely through-corrosion during regular ECP (every year, one of the three steam generators is washed). However, it is unacceptable to operate a steam generator without ECP, and to reduce the corrosion rate of steel of steam generator pipes in a steam-water environment, after ECP, a chemical passivation stage is carried out with different formulations, the effectiveness of which is different. This article presents a physical model of the mass transfer of corrosive hydrogen in a steam generator during chemical washing, passivation and start-up of the steam generator. An operational criterion for evaluating the effectiveness of passivation by the rate of increase in the concentration of hydrogen in the sodium of the second circuit during the subsequent start-up of BN-600 steam generators is proposed: if the rate of increase in the concentration of hydrogen in the 2nd circuit is less than 0.05 ppm/h, then passivation is effective, if more than the passivation formula requires revision. For BN-800 steam generators, the technology of passivation after ECP has not yet been determined, the article proposes a calculated criterion for its efficiency of 0.015 ppm/h.