## 5 Conclusions  
In this study, the three-dimensional analysis code for hydrogen distribution in containment was applied to the APR1400 NPP. Two severe accident scenarios such as SBLOCA and SBO were simulated using the 3D code.  
During the SBLOCA, a huge amount of water vapor is released into the containment, and it was confirmed that its thermal hydraulic behaviors directly affect the distribution of the hydrogen released with the steam. A noticeable finding from the 3D analysis is a phenomenon of the reverse stratification of hydrogen in the containment because of smaller steam concentration in the lowest annular region than in the upper region of the containment. Unlike SBLOCA, in the event of the SBO accident, water vapor is initially sparged into the IRWST water through a safety depressurization value and condenses. Afterward, as the core damage progresses, hydrogen and water vapor are released into the SG compartment. Due to the small amount of water vapor released, the pressure of the containment is maintained about 1 bar lower than that of SBLOCA. The 3D analysis showed that hydrogen could not move to the containment dome due to the thermal stratification formed by the operation of the PARs, and a hydrogen cloud developed in the area below the dome.  
Methodology, J.K.; software, J.K.; validation, J.K. and K.L.; writing--original draft preparation, J.K.; writing--review and editing, K.L.; project administration, K.L.; funding acquisition, K.L. All authors have read and agreed to the published version of the manuscript.  
Figure 20: Simulation results for the SBO accident: (**a**) change in hydrogen mass inventory; (**b**) change in containment pressure.  
**Funding:** This research was funded by the Korea Foundation of Nuclear Safety (KOFONS) (No. 2106007).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflicts of interest.  
**Nomenclature**  
\(\alpha\) Ratio of fog mass-to-gas mass  
\(\delta\) Normal distance from wall to cell center  
\(\rho\) Density  
\(C_{p}\) Specific heat  
\(C_{v}\) Coefficient for transient friction force  
\(C_{blk}\) Bulk condensation coefficient  
\(D_{w}\) Water vapor diffusivity at wall  
\(D\) Darcy coefficient vector  
\(F\) Forchheimer coefficient tensor  
\(g\) Gravity vector  
\(\Gamma_{t}\) Turbulent fog diffusivity  
\(h\) Heat transfer coefficient  
\(h_{m}\) Mass diffusion coefficient  
\(h_{s}\) Sensible enthalpy  
\(J\) Mass diffusion flux vector  
\(k\) Thermal conductivity  
\(K\) Kinetic energy (\(1/2\rho U^{2}\))  
\(m\) Mass  
\(p\) Pressure  
Pr Prandtl number  
\(\phi_{par}\) Ratio of thermal energy transferred to gas  
\(q_{w}\) Wall heat flux  
\(q\) Heat flux vector  
\(R\) Recombination rate  
\(R\) Reynolds stress tensor  
\(S\) Sink or source  
\(C\) Schmidt number  
\(T\) Temperature  
\(T^{+}\) Dimensionless temperature  
\(t\) time  
\(U^{+}\) Dimensionless velocity  
\(u_{\tau}\) Shear velocity  
\(U\) Velocity vector  
\(Y^{+}\) Dimensionless distance  
\(Y_{l}\) Mass fraction of gas species i