[en] OECD/NEA THAI (Thermal hydraulics, Hydrogen, Aerosol and Iodine) Project was conducted from 2007 for three years to address open questions concerning the behavior of hydrogen, iodine and aerosols in the containment of water cooled reactors during severe accidents. In the project nine OECD member countries joined including Korea (KAERI and KINS). Recently the final report of the project was published to provide information on research results relevant to the open hydrogen and fission product issues. In this paper, major experimental results of HR (hydrogen recombiner) test series for passive autocatalytic recombiners (PARs) that can be referred to or considered in domestic R and D projects, are introduced. We quote the OECD report to obviously transfer the facts found in test series. Since new or refurbished nuclear power plants (NPPs) in Korea are considering the installation of PARs, the results obtained from HR test series may also give useful information to the licensees as well as the regulators. Therefore this paper aims for sharing the fruitful PAR related findings with Korean Nuclear Society members

[en] Following TMI-2 and Chernobyl accidents, concerns on the safety problems of nuclear power plants (NPPs) have been increased, especially for severe accident phenomena including hydrogen risk. To reduce the potential of hydrogen combustion, much of international efforts have been made for decades and it resulted in development of an innovative device, i.e. passive autocatalytic recombiner (PAR), for hydrogen removal in containment during an accident. On the designing and implementing a PAR, Bachellerie et al. provided a good instruction. Actually not only most new NPP designs but also NPPs to be refurbished are considering the installation of PARs for hydrogen control following a design basis accident (DBA) and severe accident (SA). For some Korean NPPs, the utility, KHNP, also has decided to equip PARs in containment and KINS has performed relevant regulatory reviews and preoperational inspections. In this paper, the status of PAR installation in Korean NPPs and KINS' experiences in regulatory activities including onsite findings as well as some recommendations, are described

[en] Concerns about the local hydrogen behavior in a nuclear power plant (NPP) containment during severe accidents have increased with the 10CFR50.34(f) regulation after TMI accident. Consequently, investigations on the local hydrogen behaviors under severe accident conditions were required. An analytical model named HYCA3D was developed at Seoul National University (SNU) to predict the thermodynamics and the three dimensional behavior of a hydrogen/steam mixture, within a subdivided containment volume following hydrogen generation during a severe accident in NPPs. In this study, the HYCA3D code was improved with a steam condensation and spray model, and verified with hydrogen mixing experiments executed in a SNU rectangular mixing facility. Helium was used to simulate hydrogen in both the calculations and the experiments. The calculation results show good agreement with the experimental data

[en] IVR-ERVC is selected as a Severe Accident Management Strategy for APR1400. Design Requirements: • To provide flexibility in severe accident management; • To provide an additional function for in-vessel retention by using the existing systems; • To flood the external surface of reactor vessel lower plenum before the relocation of molten corium. Implementation: • Initial flooding of the reactor vessel using a shutdowm colling pump (SCP); • Reactor insulation design for effective water intrusion and flow; • Supplementary water injection by boric acid makeup pump (BAMP) to compensate boiling-out.

[en] OECD-NEA performed THAI (Thermal hydraulics, Hydrogen, Aerosol and Iodine) Project from 2007 for three years to address open questions concerning the behavior of hydrogen, iodine and aerosols in the containment of water cooled reactors during severe accidents. In the project nine OECD member countries joined including seven European countries, Canada and Korea. In Korea, KAERI and KINS participated in the project. There was a benchmark study concerning hydrogen mixing phenomena occurred in HM-2 test and KINS took part in the benchmark program to assess MELCOR code characteristics and capabilities for the relevant phenomena. This paper summarizes major results obtained from the benchmark program. In HM-2 test, hydrogen was injected into the THAI test vessel for 4,200sec (phase 1) and an atmospheric stratification occurred. After 2 minutes, Steam was released at the nozzle located at the central region of the lower plenum from 4,320sec to 6,820sec (phase 2). In phase 2, a stagnation of steam plume in the inner cylinder occurred and the stratified hydrogen region was eroded by the plume and finally a global natural circulation flow was formed to mix overall atmosphere in the vessel

[en] The combustible gas control in containment is one of the important items for plant safety, but there are still large uncertainties in analyzing the hydrogen behavior in the containment for the regulatory review of new plants. Moreover since the Shin Kori 1 and 2 and Shin Wolsong 1 and 2 plants named Optimized Power Reactor 1000 (OPR1000) are under licensing process now, so it is important for KINS (Korea Institute of Nuclear Safety) to have a reliable analysis methodology for the assessment of hydrogen risk. An integral severe accident code MELCOR which has been widely used was intensively verified against experiments, but we should always be careful about the capability of a code. As part of our efforts to establish a confident analysis environment we need to simulate experimental data using MELCOR code. This paper describes the first step of our efforts for the code verification. The HM2 test of OECD/THAI (Thermal hydraulics, Aerosol and Iodine) project was selected as a benchmark problem, which focused on the hydrogen mixing phenomenon in containment especially for the erosion process of stratified atmosphere by steam plume. THAI-HM experiment is part of the OECD/THAI Project run in Germany. Brief results of the first step approach are described

[en] Following the Fukushima accident, the Korean government set up a response led by a Prime Ministerial Task Force to precisely assess the safety of operating nuclear power plants (NPPs) against such an event in Japan in March 11^th, 2011. KINS organized a targeted Special Safety Inspection (SSI), and a team composed of 37 KINS staffs and 36 external experts stemming from various different organizations was brought together to undertake the mission. The assumed scenario which was investigated mirrored the Fukushima accident. Accordingly, areas for improvement were promptly identified. This paper describes the findings of the SSI and their technical backgrounds, especially on the severe accident countermeasures of operating NPPs

[en] The purpose of this study is to explore input development and the audit calculation using TEXAS-V code for ex-vessel steam explosion for a flooded reactor cavity of APR1400. TEXAS computational models are one of the simplified tools for simulations of fuel-coolant interaction during mixing, triggering and explosion phase. The models of TEXAS code were validated by performing the fundamental experimental investigation in the KROTOS facility at JRC, Ispra. The experiments such as KROTOS and FARO experiment are aimed at providing benchmark data to examine the effect of fuel-coolant initial conditions and mixing on explosion energetics with alumina and prototypical core material. TEXAS-V code used in this study was to analyze and predict the ex-vessel steam explosion for a reactor scale. The input deck to simulate the flooded reactor cavity of APR1400 is developed and base case calculation is performed. This study will provide a base for further study. The code will be of use for the evaluation and sensitivity study of ex-vessel steam explosion for ERVC strategy in the future studies. Analysis result of this study is similar to the result of other study. Through this study, it is found that TEXAS-V could be the used as a tool for predicting the steam explosion load on a reactor scale, as fast running computer code. In addition, TEXAS-V code could be to evaluate the impact of various uncertainties, which are not clearly understood yet, to provide a conservative envelope for the steam explosion

[en] When a high-pressure accident occurs in APR1400, large amount of steam-hydrogen mixture may be released to the in-containment refueling water storage tank (IRWST) through the pilot-operated safety relief valves (POSRVs). If the heat removal is not successful, core melt may proceed and massive hydrogen will be generated by zirconium-water reaction. The hydrogen concentration in the atmosphere of IRWST is supposed to be very high and a detonable condition can be formed. This paper introduces the results of sensitivity study executed by KINS on the IRWST hydrogen issue. The station blackout (SBO) accident scenario was selected as the result of a comprehensive consideration based on the probabilistic core damage frequency (CDF) and the deterministic engineering judgment

[en] In this study, MELCOR simulation was carried out for THAI HM-2 experiment of OECD. As a results, stratification of hydrogen cloud was reasonably captured in MELCOR simulation. Furthermore, the pressure from simulation results in cases where mass transfer coefficient of MELCOR condensation model was modified was good agreement with the experimental results. Containment Filtered Ventilation System (CFVS) has been introduced as facility to prevent containment failure during severe accident. However, possibility of hydrogen risk has been issued due to inflow of hydrogen, condensation and removal of steam and complicated inner structure in CFVS. Preferentially benchmark work for THAI HM-2 experiment of OECD was decided to validate the methodology before detailed assessment of hydrogen risk in CFVS. The objectives of THAI HM-2 experiment were evaluation of hydrogen behavior, verification of numerical analysis tools and so on. In this paper, therefore, MELCOR simulation was carried out in comparison with the experiment results. Additionally, steam condensation effect was considered for detailed simulation. Hydrogen concentration from MELCOR results was underestimated in comparison to the experimental results