Hydrogen risk in the Containment Filtered Venting System

It was observed that the thermal hydraulic conditions in the CFVS (Containment Filtered Venting System) vessel sharply changed markedly, especially the steam condensation in the early CFVS operation can affect the hydrogen behavior such as the combustion. This paper summarizes the calculated results on hydrogen concentration in the CFVS vessel, which was presented at The Ninth Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS9) last year. The MELCOR computer code calculated the thermal-hydraulic conditions in the containment building of OPR 1000, and in the cylindrical CFVS vessel with 3 m in diameter and 6.5 m in height under an SBO. After the operation of the CFVS, the pressure and temperature in the containment building decreased, and those in the CFVS vessel jumped from the initial conditions of atmosphere pressure and room temperature. These big differences of thermal-hydraulic conditions can make the volumetric concentrations of steam and gas mixtures in the CFVS vessel to be changed in comparison with those in the containment building. The volumetric concentration of hydrogen increased from 6% in the containment to 14% in the CFVS vessel after the operation of the CFVS, while the concentration of steam decreased from 58% in the containment to 3% in the CFVS vessel. The increased volumetric concentration of hydrogen (14%) with the other concentrations of steam (3%) and air (60%) in the CFVS vessel exists within the region of the burn limit in the Shapiro diagram. This possibility of the hydrogen combustion can threaten the integrity of the CFVS