Analysis of fuel vapour bubble dilation dynamics and in-vessel actinide transport under severe accident scenario in SFR

In pool type Sodium cooled Fast spectrum Reactors (SFR), the core is kept permanently submerged in the liquid sodium pool. The fuel (UO₂ + PuO₂ mixture) in SFR core is kept inside the hermitically sealed stainless steel tubes. A sustained Power Coolant Mismatch (PCM) due to Anticipated Transient Without Scram (ATWS) could result in coolant boiling, core material (fuel and stainless steel) melting and relocation. In case of prompt neutronic recriticality under the new core configuration, rapid dispersion of core materials (~ few hundred milliseconds) takes place and the phenomenon is termed as energetic Core Disruptive accident (CDA). Several inherent shutdown mechanisms make it a highly unlikely event and therefore energetic CDA is generally considered as the bounding phenomenon for the safety analysis of ATWS in SFR. In this regard, a parametric analysis is carried out to understand the expansion behavior of fuel vapour bubble and the associated actinide release and transport from the disrupted core to liquid sodium pool under the energetic CDA conditions. Results show that fuel particles with diameter ranging from few tens of microns to few hundreds of microns are effectively removed from the fuel vapour bubble by virtue of particle inertia and retained by the sodium pool and not expected to contribute to the radiological source term