## 3 Results
### Application to PWR Containment
#### 3.2.2 Steam and Hydrogen Behaviors in the Containment  
In the early stage of the SBLOCA accident, the water vapor release rate is high due to the high reactor pressure, and the release rate decreases as the accident progresses (Figure 13a). Figure 14 shows the distribution of water vapor in the containment building over time. At 5000 s after the start of the accident, the water vapor concentration in the containment increases up to 70%. Afterward, as the water vapor release rate decreases, its concentration in the containment decreases due to the reduced mass flow and the vapor condensation. In the SBLOCA, the flow of the released gas is dependent on the orientation and location of a break. In this study, it was assumed that a small break occurred in the upper part of the cold leg, which resulted in a strong upward flow from the SG compartment to the containment dome, as shown in Figure 15. It is also found in the figure at 9992 s that a flow is developed by the PARs installed below the containment dome. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Accident Scenario** & **Water Vapor Mass** & **Hydrogen Mass** \\ \hline SBLOCA & 163 ton & 950 kg \\ \hline SBO & 67 ton & 975 kg \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Total masses of water vapor and hydrogen. Figure 14: Change in water vapor distribution during the SBLOCA in the APR1400 containment. Figure 15: Change in flow field during the SBLOCA in the APR1400 containment. Figure 16 shows the evolution of hydrogen distribution in the containment over time, with hydrogen being well mixed by the strong upward flow of the released water vapor. In the figure at 9992 s, the hydrogen concentration is higher in the lower part of the containment, which is strongly related to the water vapor distribution. Figure 17 shows the hydrogen mass inventory in the containment and pressure change over time during the SBLOCA. From 4500 s after the start of the accident, hydrogen is released into the containment, increasing the amount of hydrogen in the containment. Compared to the case where PARs are not installed, the start of hydrogen recombination by the PARs can be estimated from 6000 s. This time lag of 1500 s is called the PAR startup delay. The delay in the PAR operation includes the time it takes for the released hydrogen to arrive by convection at the point where PAR is installed and the time it takes for the PAR's catalyst to be heated by catalytic reaction. The main reason for the longer delay of the PAR startup during the SBLOCA of APR1400 compared to the PAR experiments shown in Figure 8 is the height of the containment, which is about 10 times larger than the vessel used in the experiment. As shown in Figure 17b, when water vapor begins to release, the containment pressure rises rapidly. In the case without PARs, the pressure gradually decreases after 6000 s, whereas in the case with PARs, the pressure does not decrease due to the heat generated by hydrogen recombination. As shown in Table 2, the amount of water vapor released into the containment atmosphere was significantly reduced during the SBO accident. Figure 18 shows that there  
Figure 16: Change in hydrogen distribution during the SBLOCA in the APR1400 containment. Figure 17: Simulation results for the SBLOCA accident: (**a**) change in hydrogen mass inventory; (**b**) change in containment pressure. is no significant change in the water vapor distribution over time, and the water vapor concentration is very low compared to the SBLOCA. Additionally, it can be seen that the water vapor distribution in the containment is strongly stratified. In the SBO accident, water vapor and hydrogen are released through a nozzle horizontally near the top of the steam generator compartment.