Analysis of Debris Bed Formation, Spreading, Coolability, and Steam Explosion in Nordic BWRs
Pavel Kudinov, Alexander Konovalenko, Dmitry Grishchenko, Sergey Yakush, Simone Basso, Nazar Lubchenko, Aram Karbojian,
Severe accident management strategy adopted in Nordic type BWRs employs core melt fragmentation and quenching in a deep water pool below the reactor vessel. However, there is a risk that formed debris bed will not be coolable or energetic steam explosion will threaten containemnt integrity. The goal of the project is to reduce uncertainties in assessment of (i) debris bed properties and coolability, (ii) steam explosion impact.
In this work the DECOSIM code developed for analysis of porous debris coolability was further validated against new COOLOCE data for different configurations: (i) cylindrical debris bed with open side walls, (ii) conical bed on a cylindrical base. An analytical model is proposed based on the analysis of DECOSIM calculations for prediction of the maximum temperature of the debris. The model for prediction of particulate debris spreading was implemented in the DECOSIM code for ananlysis of possible feedbacks between dryout and spreading effectiveness. DECOSIM code was extended to in-vessel problems by implementing models for complex geometries, as well as taking into account the effect of congesting structures available in the lower plenum (CRGTs and IGTs).
Scaling approach and universal semi-empirical closure have been developed for prediction of particulate debris spreading using PDS-C tests. The apporach has been validated against experimental data with different particle misxtures.
An approach for analysis of steam explosion sensitivity to the uncertain modeling and scenario parameters has been further developed. First results onbtained with using TEXAS-V code indicate that the most influential parameters are water level and water temperature. Obtained database of impulse and pressure is used for development of the computationally efficent surrogate model which can be used in extensive uncertainty analysis.
Nordic BWR, severe accident, debris bed formation, coolability, steam explosion