Abstract

Sodium cooled Fast Reactors (SFRs) are built with several engineered safety features to prevent and mitigate core melt accidents. Core catcher is a passive mitigation device, which retains core debris resulting from hypothetical core melt accident in a subcritical and coolable geometry. The core catcher of Prototype Fast Breeder Reactor (PFBR) uses Austenitic Stainless Steel (ASS) as structural material. However, in the view of enhanced safety, core catchers for future reactors need to be designed for large thermal loads resulting from whole core melting. Provision of suitable refractory material as protection layer for ASS made core catcher plate can be one of the promising options to enhance the melt retention capability. Thermal stability of protective layer during impact of molten corium or fragmented core debris at high temperature in presence of liquid sodium is very essential for its satisfactory performance. Based on preliminary studies, sintered magnesia has been chosen as candidate material and experiments were carried out to investigate its stability in liquid sodium. Accelerated sodium compatibility tests were carried out by exposing magnesia specimens in sodium for 240 h at 850 °C. Tests were also carried out to assess its stability in liquid sodium under condition envisaged during whole core relocation over core catcher. Tests were carried out by subjecting the specimens under dead weight up to the boiling point of sodium in lower plenum of a typical SFR. Sodium exposed magnesia specimens were subjected to XRD and SEM analysis to assess material degradation. The specimens were also tested for change in compressive strength due to high temperature sodium exposure. The experimental results are discussed in the paper.

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