Helios code simulation of condensation induced hydrogen accumulation in slightly inclined non-vented pipe

Abstract

The radiolytic gases (hydrogen and oxygen) accumulation in a gas mixture with steam, due to the steam condensation in a nearly horizontal pipe, closed at one end and open at the other, is simulated and analyzed with the HELIOS code. The thermal-hydraulic and physico-chemical code HELIOS is based on three-dimensional two-fluid model of steam, hydrogen and oxygen gas mixture and water condensate flow, as well as on the radiolytic gases mass conservation equations. Transport processes at the gas mixture-water condensate interface are modelled with appropriate closure laws. Obtained results show that the occurrence of the radiolytic gases accumulation in the non-vented horizontal pipe depends on the complex influence of several parameters such as pipe diameter, thermo-physical properties of gas mixture and condensate and heat loss rate from the gas mixture to the ambient atmosphere. Performed parametric analyses show that under lower heat loss rates or higher pipe diameters, the hydrogen-oxygen accumulation does not occur due to the buoyancy driven circulation of the gas mixture, which expels the gas mixture stream with the higher radiolytic gases concentration out of the pipe. This behaviour is opposite to the hydrogen and oxygen accumulation in the vertical pipe closed at the top and open at the bottom, where the front of the accumulated radiolytic gases always propagates from the pipe top.