[en] Highlights: • Oxidation resistant SiC-ZrB₂ interlayer is successfully deposited on graphite. • Adherent Y₂O₃ coating over SiC-ZrB₂ surface shows good interfacial bonding. • Three-fold enhancement in the thermal cycle life of Y₂O₃ coating is demonstrated. • SiC-ZrB₂ show active to passive oxidation behavior by forming glassy SiO₂, ZrO₂ and ZrSiO₄. • Tensile residual mismatch stresses are relieved by the formation of cracks in topcoat. The SiC-ZrB₂ composite interlayer coating is developed by pack cementation over high density graphite (HDG) for the deposition of plasma sprayed yttria (Y₂O₃) topcoat for U-Zr (Uranium-Zirconium) melting application. The performance of Y₂O₃ coating with interlayer for enhancing its durability are investigated by performing thermal cycling experiments at 1450 and 1550 °C. The interlayer morphology with infiltrated and diffused coating of SiC with entrapped ZrB₂ particles reduces the differential thermal mismatch stresses and offers passive oxidation protection for HDG. The role of composite SiC-ZrB₂ interlayer enhancing threefold increase in the life of Y₂O₃ coating and its mechanisms at high temperature are provided.

[en] Nuclear grade graphite and pyrolytic graphite (PyG) are proposed construction materials for high-temperature molten LiCl-KCl for the pyrochemical reprocessing of metallic fuel and also as structural/ core materials for molten salt reactors (GEN IV reactor). Pyrolytic graphite (PyG) is synthetic polycrystalline graphite produced by the thermal pyrolysis of hydrocarbons by chemical vapour deposition (CVD) at very high-temperatures. Low temperature PyG pyrolysis < 1600 °C results in dark laminar texture with isotropic properties suitable for sealing nuclear graphite to improve chemical resistance for Gen IV reactors and bio-medical implants. While for applications like a rocket nozzle, thermal managements, semiconductors, magnetic levitation etc., anisotropic rough texture produced at pyrolysis >1800 °C is most suitable

[en] Yttria (Y₂O₃) coated High density Graphite (HDG) crucibles are extensively used as the process crucibles in pyrochemical reprocessing application. The durability of HDG crucible can be further enhanced with carbide interlayers by avoiding interaction between yttria and graphite beyond 1400 °C. Thus, the present study focus on examine the durability and performance of yttria coated HDG with and without Silicon Carbide interlayer during uranium melting cycles. Novel pack cementation method was used to develop SiC interlayer followed by top yttria coating by Atmospheric Plasma Spray (APS) method. The performance of coating improved by introducing SiC interlayer and the same was analyzed by XRD, SEM, and thermal cycle studies. The results demonstrated that yttria coated HDG crucibles with SiC interlayer withstood 5 uranium melting cycles without any cracking or delamination of coating. (author)

[en] Fe and Ag, as well as Fe and C are immiscible species. They are likely to form either individual atomic nanoclusters or core shell nanoclusters with Fe at the core. This is due to implantation sequence with Fe implanted first, followed by either Ag or C which have higher diffusion constant in quartz at the same depth and concentration. Raman spectroscopy and LFRS measurements confirm the presence of nanoclusters. The size of nanoclusters in the case of co-implantation with two species into quartz is smaller compared to size of nanoparticles formed when a single species is implanted. In case of Fe nanoclusters alone, there is no PL emission. PL emission is there in samples with Ag and highly enhanced in the case (Fe + Ag) indicating smaller size of Ag nanoparticles. It is likely that Ag forms the shell around the Fe core. The observed absorption data are similar to the reported data for core-shell nanoclusters. (author)

[en] Hot section components of thermal power plant boilers, gas turbine and chemical and food processing industries are exposed to severe erosive and corrosive conditions leading to premature failure of conventional ferrous based alloy systems. One of the widely accepted methods to impart protection against corrosion, hot oxidation, wear and erosion with cost effectiveness is by cladding or overlay coating of Ni based super alloys. The conventional ways for such overlays are: thermal spray, plasma transferred arc (PTA), arc welding process (GMAW, GTAW), laser and electron beam cladding/welding. Among the arc welding methods, Cold Metal Transfer (CMT) is a newcomer in the industry, well known for its 'pulsed wire feeding' technology. This brings low dilution, defect free claddings, narrow HAZ, highly controlled heat input, spatter free weld beads, strong metallurgical bond between substrate and cladding, energy efficiency and economy. In the present study, Inconel 617 weld overlay coatings were deposited on SS 316L (3 mm thickness) at different current inputs such as 100 A, 90 A and 80 A, using a Fronius VR 7000 CMT machine aided by a CNC automation for overlaying. Micro Vickers hardness profile of the cross section (from substrate end to cladding end) will be discussed in the paper. Detailed SEM analysis of phase changes as well as microstructure evolution is in progress. Micro chemical analysis using EDS, qualitative and quantitative evaluation of weld dilution etc. will be conferred in the advancement of the study. (author)

[en] Currently, stainless steel grade 316LN is the material of construction widely used for core catcher of sodium-cooled fast reactors. Design philosophy for core catcher demands its capability to withstand corium loading from whole core melt accidents. Towards this, two ceramic coatings were investigated for its application as a layer of sacrificial material on the top of core catcher to enhance its capability. Plasma-sprayed thermal barrier layer of alumina and partially stabilised zirconia (PSZ) with an intermediate bond coat of NiCrAlY are selected as candidate material and deposited over 316LN SS substrates and were tested for their suitability as thermal barrier layer for core catcher. Coated specimens were exposed to high-temperature thermite melt to simulate impingement of molten corium. Sodium compatibility of alumina and PSZ coatings were also investigated by exposing samples to molten sodium at 400 °C for 500 h. The surface morphology of high-temperature thermite melt-exposed samples and sodium-exposed samples was examined using scanning electron microscope. Phase identification of the exposed samples was carried out by x-ray diffraction technique. Observation from sodium exposure tests indicated that alumina coating offers better protection compared to PSZ coating. However, PSZ coating provided better protection against high-temperature melt exposure, as confirmed during thermite melt exposure test.

[en] Highlights: • Two proposed candidate ceramic sacrificial materials are tested under sodium exposure for compatibility test. • Substrate coating was carried out using air plasma spar coating. • The results for both plasma sprayed alumina and YSZ coatings are discussed for the present test configuration. - 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 500 h. 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.

[en] Highlights: • Laser-remelted YSZ TBCs showed chemical inertness in molten sodium. • LSR produced a dense, columnar and smooth surface architecture in YSZ TBCs. • Vertical segmented cracks allowed molten sodium infiltration and disintegration at non-remelted interfaces. • The corrosion mechanism of LSR YSZ in molten sodium is primarily of physical decohesion at weak boundaries. Liquid-metal corrosion in molten sodium is a significant issue to Thermal Barrier Coatings (TBCs) proposed for in-core fast breeder reactor applications. In this study, the viability of attenuating molten sodium corrosion of plasma-sprayed yttria-stabilized zirconia TBCs is investigated by microstructure re-engineering using Laser Surface Remelting (LSR). LSR produced a smooth and dense surface with columnar microstructure and segmented vertical cracks. At the onset of molten sodium corrosion, the remelted layer retained structural integrity but exhibited localized corrosive attack along the surface-network cracks. However, on further exposure, the vertical cracks lead to molten sodium infiltration down the non-remelted layers, causing corrosive damage and spallation of coating.