Condensation induced non-condensables accumulation in a non-vented vertical pipe

At the end of the year 2001 the radiolytic gases hydrogen and oxygen accumulated and exploded in non-vented steam pipelines at two nuclear power plants. The HELIO code has been developed for the numerical simulation and analyses of these incidents and as a support to the prescription of safety measures. This paper presents modelling and numerical approach, verification and results of the two-dimensional module of the HELIO code for the conditions of hydrogen-oxygen accumulation in a vertical pipe closed at the top. The results show a dynamics of hydrogen-oxygen accumulation, formation and propagation of the concentration front, gas mixture natural convection due to concentration and temperature imposed buoyancy forces and gas mixture-liquid film interface shear. The verification against plant data and for the common problem of steam condensation in the presence of air is presented.