Already as a student of Nuclear Faculty at Czech Technical University in Prague I had opportunity to joint SA research in the frame of SARNET mobility programme. After graduating the university I started my PhD study at IRSN. My doctoral thesis is focused on the modelling of reflood of a severely damaged reactor core. This topic is one of the key issues of SARNET research within the WP 5 (“Corium and debris coolability”). As a post-graduate student at this institution I am working on development of a reflooding model which in term will be integrated in the ASTEC integral code for severe
analysis. The TMI-2 accident and recently the Fukushima accident demonstrated that the nuclear safety philosophy has to cover accident sequences involving massive core melt in order to develop reliable mitigation strategies for both existing and advanced reactors. Although severe accidents have a very low likelihood and might be caused only by multiple failures, accident management is implemented for controlling their course and mitigating their consequences. In case of severe accident, the fuel rods may be severely damaged and oxidized. Finally, they collapse and form a debris bed on core support plate. Removal of decay heat from a damaged core is a challenging issue because of the difficulty for water to penetrate inside a porous medium. The reflooding (injection of water into core) may be applied only if the availability of safety injection is recovered during accident. If the injection becomes available only in the late phase of accident, water will enter a core configuration that will differ from original rod-bundle geometry and will resemble to the severe damaged core observed in TMI-2. The higher temperatures and smaller hydraulic diameters in a porous medium make the coolability more difficult than for intact fuel rods under typical LOCA conditions. The modelling of such thermal-hydraulic and heat transfer phenomena is one of key objectives in development of the future coupling between the ASTEC and CATHARE codes. The objective of my work is to develop a 3D reflood model that would be able to treat a degraded configuration of the core in a case of severe accident. The modelling of reflood of degraded cores is currently based on assumption of thermal non-equilibrium between the solid, liquid and gas phases. It includes two momentum balance equations (one for each phase). It takes into account the multidimensional characteristics of flow during a reflood process. In parallel to this modelling work, IRSN started an experimental program (tests PRELUDE and PEARL) to enhance a database of 2D debris bed reflooding data. The objective of the program is to provide data for validation of numerical tools for prediction of reflooding of severely damaged reactor core, where a large part of the fuel rods has collapsed and formed a debris bed. A series of PRELUDE experiments performed in 2010-2011 has provided a large amount of new data that were analysed. On the basis of those experimental results, the thermal-hydraulic features of the quench front have been analysed and the efficiency of heat transfer regimes was estimated. Based on the results of this experimental program, the reflooding model was improved. Significant improvements were done especially on the field of heat transfer modelling for different flow boiling zones. The criteria characterizing the transition between different flow regimes were completed. The quantitative validation of experimental results showed that the model provides satisfactory results. The calculation results are in a good agreement with experimental results of the whole range of injectoin velocities and particle diameters.
These results are promising because the previous models largely underestimated the progression of quench front and the steam production. The future studies will focus on validation of model with new experimental data and particular attetion on analysis of 2D effects is ongoing.
Contact: Andrea Bachrata
andrea.bachrata@irsn.fr