HELIO code prediction of hydrogen accumulation in non-vented steam pipelines

Abstract

The thermal-hydraulic and physico-chemical code HELIO is developed for numerical simulations and analyses of radiolytic gases (hydrogen and oxygen) accumulation in non-vented steam pipelines of nuclear power plant systems. The HELIO code simulates all relevant process, such as the steam-radiolytic gases mixture convection, the condensate drainage, the buoyancy forces induced by concentration and related gas mixture density differences, the gas mixture and condensate film interfacial shear, the steam condensation in the presence of non-condensables, the noncondensables diffusion, and the radiolytic gases absorption and degassing on the liquid film surface. This paper presents HELIO code models, numerical solving procedure and three cases of results of three-dimensional radiolytic gases accumulation: (a) in a non-vented steam pipeline of the Residual Heat Removal System in the Hamaoka Nuclear Power Plant, (b) in a non-vented horizontal pipe connected with an elbow and a vertical pipe, and (c) in a slightly inclined, nearly horizontal pipe. The radiolytic gases accumulation in the mixture with steam is a very slow and long duration transient that is induced by the steam condensation due to heat losses to the atmosphere. The condensate is drained, while the concentration of remaining non-condensable radiolytic gases increases; the process that could lead to the formation of a critical hydrogen and oxygen concentration and subsequent explosion, as in case of the Hamaoka plant incident. In case of the Hamaoka steam pipeline geometry, presented results show an accumulation of the radiolytic gases from the closed top end of the pipeline, a formation of the concentration front and its propagation towards the open end. In the second analyzed case, the gas mixture circulates in the elbow and horizontal pipe due to buoyancy forces induced by concentration and related density differences. The circulation flow prevents the formation of radiolytic gases concentration front and an accumulated radiolytic gases concentration is low. The third case shows that under lower heat loss rates or higher pipe diameters the hydrogen-oxygen accumulation does not occur due to the buoyancy driven circulation of gas mixture; the circulation expels the gas mixture stream with a higher radiolytic gases concentration out of the pipe. Obtained results clarify mechanisms of the radiolytic gases accumulation and they are a support to the prescription of safety measures that should be applied in order to prevent an accumulation and eventual explosion.