Evaluation of plasma sprayed sacrificial thermal barrier coatings for core catcher of future Sodium cooled Fast Reactors

Abstract

Core catcher is a passive safety device in nuclear reactors for collection and long term retention of corium (mixture of molten fuel and structural material) in subcritical and coolable geometry during hypothetical core melt accidents. Core catchers of earlier Sodium cooled Fast Reactors (SFR) were designed for partial core melt accidents. However in view of enhanced safety, the core catcher design for future SFR considers whole core melt scenario, which results in higher thermal and structural loads. Provision of a sacrificial layer is one of the methods to reduce the thermal load on core catcher and enhance its corium retention capability.An experimental study has been taken up to assess the compatibility of few candidate materials for sacrificial layer, with liquid sodium at high temperature conditions. Based on literature, alumina and Yttria Stabilized Zirconia (YSZ) are chosen due to their favorable properties like high melting point and lower thermal conductivity. Test specimen with Stainless Steel (AISI 316 LN, UNS No. S31653) substrate and candidate ceramic layer coated by plasma spray process, were prepared and exposed to sodium at 400°C for 500h.Post exposure investigations indicated that YSZ coating was leached out completely, whereas alumina coating was intact with the substrate. At low pressure, alumina had shown weak interaction with sodium i.e., formation of sodium aluminate within a shallow depth. However, at slightly higher pressure, delamination of coating was observed. The results indicated that plasma sprayed alumina and YSZ coatings are not stable in present test configuration. Alternate methods for sacrificial lining as well as materials need to be explored.