Analysis of composition and temperature of debris bed based on settling characteristics of relocated core debris in pool type SFR

The hypothetical core meltdown accident in sodium cooled fast reactors (SFR) is a severe accident situation during which the energetic dispersion of core materials and their relocation may pose a threat to the integrity of reactor containment. During core meltdown accident, part of the molten core materials relocated downwards would be fragmented in the lower plenum sodium pool and settle over the in-vessel core catcher plate and forms a particulate debris bed. The recriticality potential and coolability of the debris bed are strongly influenced by the composition of fragmented particles of different core materials and their size distribution. In this paper, simplified models have been developed to evaluate the incipient conditions i.e., composition profile and initial temperature of the particulate debris bed formed over an in-vessel core catcher plate following the core meltdown accident in pool type SFR. The particle mixture settling process subsequent to the fragmentation of molten core materials in sodium pool has been analyzed to evaluate the composition profile of the debris bed. A 1-D model for particle motion through the stagnant fluid under the influence of gravity has been considered to evaluate the settling characteristics of core materials debris (mainly UO2 and stainless steel) in liquid sodium for a wide range of possible sizes. By extending this model, the differential settling behaviour of mixture of UO2 and stainless steel particles in liquid sodium pool has been investigated. The composition profile of the debris bed along its thickness has been evaluated for the material relocation scenario considered with various mass ratios of stainless steel and UO2 mixture using the published experimental results based particle size distributions. Further, a 1-D transient conduction model is implemented to estimate the temperature of debris particles when they settle on the core catcher plate. Simulations revealed that the particles will be quenched to near the temperature of the liquid sodium in lower pool while reaching the core catcher. The results are useful in assessing the coolability characteristics of debris bed formed on the core catcher and its recriticality potential in the analysis of the Post Accident Heat Removal.