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Core catchers are being installed in advanced light water reactors after TMI and Fukushima. The core catchers are designed to stabilise and cool molten corium within the reactor containment for a long time, reducing the risk of core meltdown. Some core catchers use sacrificial material to lower the specific volumetric heat release of the molten corium. The low density oxidic component of molten pool migrate to the top layer of the core catcher after melt-sacrificial material interaction. The high density metallic component migrate to the bottom layer. In the event that the core catcher is top flooded, this prevents metallic components from reacting with water and generate hydrogen. Post experimental analysis of the cooled stratified melt pool was performed. The experimental scaled down core catcher used reactor the prototypic sacrificial material while maintaining the actual reactor flooding conditions. About 550 kg of simulant corium was melted at about 2500 °C using heat of thermite reaction. The core catcher vessel was partially filled with sacrificial bricks. After the test, solidified debris samples from different locations of the core catcher model were analysed for density and SEM-EDS for quantitative and qualitative characterisation. Post-test analysis showed the melt inversion characteristics. [ABSTRACT FROM AUTHOR]

This article analyzes methods and technologies for assessing the way of formation, composition, characteristics, and features of corium, i.e., a mixture of nuclear and structural materials of a nuclear reactor core, which formed as a result of an accident that is accompanied by partial or complete melting of the core. The analysis is based on data from studying the corium formed as a result of the accident at the Fukushima Daiichi nuclear power plant, which are in the public domain and are the result of the work of many scientific organizations around the world. The study of corium is a main problem in improving nuclear safety in the future and an objective in developing the successful procedure for eliminating the consequences of the accident at the Fukushima Daiichi nuclear power plant. Without a detailed analysis of the neutron physics, materials science, gravimetric, and other characteristics of the corium and without creation of a corium data set that combines these characteristics, it is impossible to create a full fledged model of the corium in the future and to organize an efficient and safe process for removing nuclear materials from the damaged units of the Fukushima Daiichi nuclear power plant. This study is aimed at combining the existing research results into a data set, which can later be used to simulate the corium using neutron physics codes. The set being created includes data, such as the size, density, and morphology of corium samples and their approximate nuclide composition. The created data set will allow one not only to perform tasks related to increasing the level of safety when accomplishing the procedure for eliminating the consequences of the accident at the Fukushima Daiichi nuclear power plant through simulation but also will serve as a basis for creating an international benchmark for simulating a mixture that contains nuclear materials (NMs). [ABSTRACT FROM AUTHOR]

The core melt composition resulting from the Fukushima Daiichi Unit1 Nuclear Power Plant (FD-U1) accident is essential for the corium characterization phase before decommissioning or handling of radioactive waste. Several models were applied by different research groups for the estimation of corium composition. In this paper, the investigation of the isotopic composition, and radioactivity of the radio-nuclides in the corium10 and 50 years post-accident were calculated using Monte Carlo code, MCNPX 2.7. The results showed that the estimated core materials inventory at reactor scram before core melt was about 123.97 ton, and after the formation of the corium melt was about 140.702 ton, which agrees with the predictions calculated using other models. Also, the total corium activity was about 6.046E+17 Bq and 1.89E+17 Bq 10 and 50 years post- accident, respectively. The radionuclide compositions in the corium are necessary for decommissioning plan of F-D-U1. Furthermore, RELAP/SCDAPSIM MOD3.4 code was used for the analysis of thermal performance of the FD-U1 reactor core starting from the time of the accident up to corium formation and slumping to the lower head of the reactor pressure vessel (RPV). Our analysis indicates that the hydrogen generation started on March11th, around 18:39. The results were compared with MELCOR code and OECD/NEA BSAF Phase I results, which were found in good agreement. • The corium formation and characterization in case of severe accident were reviewed. • Fukushima Daiichi Unit-1 accident was modeled using MCNPX and RELAP/SCDAPISM codes. • The corium inventory and radioactivity were calculated 10 and 50 years post- accident. • Additional heat in the core due to the exothermic reaction between zirconium and steam. [ABSTRACT FROM AUTHOR]