Innovative Coating of Nanostructured Vanadium Carbide on the F/M Cladding Tube Inner Surface for Mitigating the Fuel Cladding Chemical Interactions
The project is to conduct applied and fundamental research towards the development of diffusion barrier coatings on the inner surface of ferritic/martensitic fuel cladding tubes. Advanced cladding steels such as T91, HT-9 and NF616 have been developed and extensively studied as advanced cladding materials due to their excellent irradiation and corrosion resistance. However, the fuel-cladding chemical interaction (FCCI), accelerated by the elevated temperature and high neutron exposure anticipated in fast reactors, can have severe detrimental effects on the cladding steels through the diffusion of Fe and Ni into fuel and lanthanides towards into the claddings. For this proposed study, the diffusion couple experiments will be focused on T91, NF616 and HT9 for clad, and both surrogate and uranium bearing fuel.
The work is aimed at developing a low temperature coating process using organometallic precursor, vanadocene Cp2V (Cp=C5H5). An innovative coating setup will be built to deposit a uniform nanostructured vanadium carbide layer (less than 10μm in thickness and nano-sized grain structure) on the inner surface of long F/M cladding tube. Compared with mechanical lining and conventional CVD technologies, the proposed low temperature process can offer several advantages: 1) no significant residual stress or axial texture from cold-drawing, 2) no detrimental effects on the martensitic substrate, and 3) a strongly bonded thin and uniform vanadium carbide coating can be readily obtained.
The functionality of the coating as FCCI barrier will be examined experimentally using diffusion couple tests (cerium and uranium). The integrity of the vanadium carbide coating will be evaluated using ion irradiation for radiation stability and quench tests for cracking resistance. The diffusion coefficients of U and Ce in V, V2C and F/M steels will be systematically mapped over a range of temperatures and annealing schedules. These results will provide benchmark against to which compare atomistic modeling. Even though vanadium carbide has been identified as one of most effective FCCI barrier element, the fundamental underlying mechanisms have not been fully understood. The proposed atomistic modeling will help understand these fundamentals processed
Understand the phase transformation of thermally aged and neutron irradiated duplex stainless steels used in LWRs
The lifetime of reactor components made of duplex stainless steels can be limited by the embrittlement from thermal aging, neutron irradiation or a synergistic effect between thermal aging and neutron irradiation. Previous studies showed that the spinodal decomposition in the delta ferrite phase is a primary embrittlement mechanism of the duplex structure stainless steels, while the G-phase precipitates were also identified. Most of the past studies focused on characterizations of fine-scale precipitates and phase decomposition using transmission electron microscopy and atom probe tomography. The fundamental mechanism and kinetics of elemental segregations occurring in the ferrite has not been fully understood. The exact concurrent evolution mechanism of the solute clustering and spinodal decomposition is not clear. This knowledge gap has hindered the development of thermodynamics and kinetic modeling of phase evolution in duplex stainless steels. It was speculated that the cracks initiate in hardened ferrites and then propagate along the phase boundaries between ferrite and austenite. However, the fundamental mechanism of how the microstructural changes decrease materials’ fracture toughness has yet to be determined. And it must be determined in order to construct a physical model that can be used to predict materials’ mechanical response in the extended reactor lifetime.
The project uses the capability of the high energy X-ray MRCAT facility, including X-ray diffraction (XRD), Extended X-ray Absorption Fine structure Spectroscopy (EXAFS) and in-situ tensile testing with wide angle X-ray scattering (WAXS) to further probe the elemental segregations, phase precipitations and lattice strain status under tensile load of different phases in selected cast austenitic stainless steels (CASS) and welds. The results from X-ray diffraction/ EXAFS/WAXS will provide an unprecedented multi-scale understanding of the structural evolutions in duplex stainless steel under different thermal aging and/or neutron irradiation conditions, and when complemented by the TEM/APT studies and in-situ micro tensile tests of phase boundary strength, the microstructure interpretation can be more accurately correlated with degradations in mechanical properties.
Synergistic effect of Thermal Aging and Low Dose Irradiation on the Cast Stainless Steels and Stainless Steel Welds
Reactor internal components made of duplex stainless steel (cast and weld) can be potentially affected by a synergy between thermal aging and irradiation embrittlement, and there have not been sufficient studies to approve or disprove this argument. We propose to perform a systematic study on a set of cast stainless steels and welds at aged and irradiated conditions to understand the combined effects of thermal aging and neutron irradiation. This study is focused on microstructural examinations using advanced characterization tools and mechanical properties tests using novel micro/nano indentation tests benchmarked by conventional uniaxial tensile tests. This study will give the first direct microstructural evidence of whether or not duplex stainless steels are truly suitable for long-term operation, to time frames of 60-80 years. The deliverables from this proposal will provide a strong technical basis for regulating the materials aging for LWRs life extension.