## 1 Introduction  
As seen in the Fukushima nuclear power plant (NPP) accident [1; 2], hydrogen mitigation and the prevention of overpressure caused by water vapor is very important to maintain the integrity of the reactor containment in the event of an accident. During a loss of coolant accident (LOCA) or a station blackout (SBO) accident in a pressurized water (PWR) reactor, hot water and steam are released into the containment building. The water vapor released into the containment mixes with the atmosphere and partially condenses into droplets or condenses on the surface of the structure in the containment building, raising the temperature of the structure. However, a large amount of water vapor is not condensed and remains in the containment building. When hydrogen generated by reactor core oxidation [3] is released into the containment, it spreads as it mixes with the water vapor and air exiting in the containment. As the water vapor condenses in any space in the containment, the volume fraction of the hydrogen increases relatively, and the potential for explosion or flame acceleration will be raised. Hydrogen concentrations below 4 vol% are known to be non-flammable, meaning that combustion does not occur. Also, as the concentration of water vapor increases and reaches 65 vol% or more, combustion becomes unlikely even if the concentration of hydrogen is high [4]. Thus, the behavior of water vapor in a containment building directly affects the distribution of hydrogen. As the supersaturated water vapor changes phase in the containment and becomes liquid water, it no longer contributes to the concentration of hydrogen but can indirectly affect the behavior of hydrogen through thermal and flow resistance. Therefore, hydrogen mitigation needs to be carried out simultaneously to preventing the overpressure of water vapor. Many NPPs operating around the world are adopting PAR-based hydrogen mitigation systems (HMSs) to control hydrogen concentration in the event of a design-based and design-extended severe accident. Due to the passive nature of PAR, it is affected by thermal-hydraulic conditions such as water vapor and temperature distributions within the containment. Various analysis codes have been developed or are being developed to evaluate the hydrogen safety in an NPP containment and to prove the effectiveness of the HMS installed in containment under severe accident conditions. Each code simulates the behavior of hydrogen over time based on computational fluid dynamics (CFDs) which solves the mechanistic conservation equations of species masses, momentum, and energy of fluid. Generally, it is divided into lumped parameter (LP) and three-dimensional (3D) code depending on the method of discretizing the volume of the containment [5]. Because the LP code relies on many correlation-based physical models in the volume-averaged form to simulate severe accident phenomena, it is dominantly used to assess the hydrogen safety in an NPP containment. Meanwhile, compared to LP codes, the key feature of 3D analysis codes is that they can resolve 3D turbulence phenomena. Conservative evaluation is very important in hydrogen safety evaluation using the LP codes because the parameters of the correlation-based models are problem-dependent. Sometimes, it can be difficult to decide the conservativity of the analysis results of the LP codes due to the diversity in hydrogen behaviors in the containment. It is still difficult to evaluate hydrogen behaviors in containment buildings under severe accident conditions using 3D codes. Difficulties in the development of 3D physical models for thermal hydraulic phenomena within NPP containments and the development of numerical schemes for fast calculation speed are limiting the use of 3D codes. However, 3D codes can be of great help in assessing the conservativity of LP code analysis results or selecting conservative values for LP code model parameters [6].