A deflagration to detonation transition (DDT) occurrence is one of the most important issues concerning safety during severe accidents in nuclear power plants because it can damage the integrity of the containment. It is possible to arrest the acceleration of a flame which can cause DDT by installing quenching meshes between the compartments. To evaluate the applicability of a quenching mesh to nuclear power plants, it requires a means to evaluate a flame arrest of a quenching mesh under a given combustion condition. The flame-quenching models developed by previous researchers were derived to fit the experimental geometry and to consider various thermal boundary conditions from a flame to the mesh wall. Flame-quenching tests were carried out at the 10% hydrogen concentration in a dry air by changing atmospheric pressure to 2.2 bar as the initial pressure. The quenching criterion of a quenching mesh with a 0.3 mm gap distance for hydrogen–air mixtures is established by using the experimental data. The flame-quenching models are also evaluated by using the experimental data. A flame-quenching model that can be used to evaluate a flame arrest for various hydrogen–air mixtures in nuclear power plants is proposed.

Korea Atomic Energy Research Institute (KAERI) established a multi-dimensional hydrogen analysis system to evaluate hydrogen release, distribution, and combustion in the containment of a Nuclear Power Plant (NPP), using MAAP, GASFLOW, and COM3D. In particular, KAERI developed an analysis methodology for a hydrogen flame acceleration, on the basis of the COM3D validation results against measured data of the hydrogen combustion tests in the ENACCEF and THAI facilities. The proposed analysis methodology accurately predicted the peak overpressure with an error range of approximately ±10%, using the Kawanabe model used for a turbulent flame speed in the COM3D. KAERI performed a hydrogen flame acceleration analysis using the multi-dimensional hydrogen analysis system for a severe accident initiated by a station blackout (SBO), under the assumption of 100% metal–water reaction in the Reactor Pressure Vessel (RPV), to evaluate an overpressure buildup in the containment of the Advanced Power Reactor 1400 MWe (APR1400). The magnitude of the overpressure buildup in the APR1400 containment might be used as a criterion to judge whether the containment integrity is maintained or not, when the hydrogen combustion occurs during a severe accident. The COM3D calculation results using the established analysis methodology showed that the calculated peak pressure in the containment was lower than the fracture pressure of the APR1400 containment. This calculation result might have resulted from a large air volume of the containment, a reduced hydrogen concentration owing to passive auto-catalytic recombiners installed in the containment during the hydrogen release from the RPV, and a lot of stem presence during the hydrogen combustion period in the containment. Therefore, we found that the current design of the APR1400 containment maintained its integrity when the flame acceleration occurred during the severe accident initiated by the SBO accident.

Korea Atomic Energy Research Institute (KAERI) established a multi-dimensional hydrogen analysis system to evaluate a hydrogen release, distribution, and combustion in the containment of a nuclear power plant using MAAP, GASFLOW, and COM3D. KAERI developed the COM3D analysis methodology on the basis of the COM3D validation results against the experiments of ENACCEF and THAI. The proposed analysis methodology accurately predicts the peak overpressure with an error range of approximately ±10% using the Kawanabe turbulent flame speed model. KAERI performed a hydrogen flame acceleration analysis using the multi-dimensional hydrogen analysis system for a severe accident initiated by a station blackout (SBO) under the assumption of 100% metal-water reaction in the reactor pressure vessel for evaluating an overpressure buildup in the Advanced Power Reactor 1400 MWe (APR1400). The COM3D calculation results using the established analysis methodology showed that the calculated peak pressure in the containment was much lower than the fracture pressure of the APR1400 containment. This calculation result may have resulted from a large air volume of the containment, a reduced hydrogen concentration owing to passive auto-catalytic recombiners installed in the containment, and a lot of stem presence during the hydrogen flame acceleration in the containment. Therefore, we can know that the current design of the APR1400 containment maintains its integrity when the flame acceleration occurs during the severe accident initiated by the SBO accident.

In the postulated severe accidents of nuclear power plants, the interaction mode of the molten corium with water happens differently depending on the height of the water level in the reactor cavity. The interaction of the molten corium with the partially filled water in the reactor cavity has been extensively studied. The molten corium in this case was released into the water after free falling to some distance. Meanwhile, some advanced reactors have adapted the in-vessel corium retention concept by cooling the reactor vessel’s outside wall. If a reactor vessel failure happens in this case, the molten corium in the reactor vessel is injected directly into the water without any free fall. Triggered steam explosion experiments were carried out to compare the explosion behavior conditions of the partially flooded cavity and ex-vessel cooling. It was found that the jet breakup process before the explosion appeared differently between the two experiments. These behaviors contributed to the differences in the maximum dynamic pressure and load that express the explosion’s strength. The explosion’s strength under the partially flooded cavity condition was about two times stronger than that under ex-vessel cooling. Accordingly, it is believed that the steam explosions under conditions of ex-vessel cooling are of less concern than the partially flooded cavity condition.

We performed a hydrogen combustion analysis in the Advanced Power Reactor 1400 MWe (APR1400) containment during a severe accident initiated by a small break loss of coolant accident (SBLOCA) which occurred at a lower part of the cold leg using a multi-dimensional hydrogen analysis system (MHAS) to confirm the integrity of the APR1400 containment. The MHAS was developed by combining MAAP, GASFLOW, and COM3D to simulate hydrogen release, distribution and combustion in the containment of a nuclear power plant during the severe accidents in the containment of a nuclear power reactor. The calculated peak pressure due to the flame acceleration by the COM3D, using the GASFLOW results as an initial condition of the hydrogen distribution, was approximately 555 kPa, which is lower than the fracture pressure 1223 kPa of the APR1400 containment. To induce a higher peak pressure resulted from a strong flame acceleration in the containment, we intentionally assumed several things in developing an accident scenario of the SBLOCA. Therefore, we may judge that the integrity of the APR1400 containment can be maintained even though the hydrogen combustion occurs during the severe accident initiated by the SBLOCA.

The suppression of a vapor explosion is reviewed from a void fraction point of view from previous research results and the results of an experiment and analysis for TROI using a prototypic reactor material. In a tin-water system, a high fraction of air which played the role of a steam reduced the peak pressure of a steam explosion. According to the sensitivity analysis that was carried out with an increase in the vapor volume fraction, an energetic vapor explosion hardly took place in the mixture with a high void fraction. In higher vapor fraction conditions (αv> 0.3), the vapor explosion was very weak. A prototypic corium shows a relatively high void fraction compared to the ZrO2 which is known as an explosive material because the corium system generated many smaller particles compared to the ZrO2 system. The corium system shows a relatively low explosivity compared to the ZrO2 system because the high void fraction of the corium system plays the role of preventing a contact between the water and the hot melt drops in the triggering stage. When considering the experimental results for the role of air instead of steam, an air supply system to provide a high volume fraction during a premixing process can radically prevent and/or mitigate a steam explosion.