Search

Oxygen is the most promising non-metallic inhibitor in lead-bismuth cooled fast reactors (LFRs). The addition of oxygen to the coolant can format a protective oxide layer on the surface of structural materials, which will effectively alleviate the corrosion of structural materials by the liquid lead-bismuth eutectic (LBE). LFR is a complex environment characterized by the interaction of multiple physical fields. For instance, the growth and removal behaviors of the oxide layer are influenced by various factors such as temperature, oxygen concentration, coolant velocity, and time. Moreover, the formation of the oxide layer changes the thermal-hydraulic characteristics and neutronics parameters of the reactor core. Therefore, studying the coupled effects of oxidation corrosion, thermal-hydraulics, and neutronics is of paramount importance for the development, design, and safety assessment of LFRs. A multi-physics framework that couples neutron physics, thermal-hydraulics, and material corrosion was proposed to investigate the multi-physics coupling characteristics of the fuel assembly in LFRs. Within the coupling framework, neutronics calculations were performed using the open-source neutron diffusion equation solver Moltres, thermal-hydraulic calculations were conducted using the Navier-Stokes module included in the multi-physics object-oriented simulation environment (MOOSE) platform, and corrosion calculations were carried out using the Seal module developed based on the MOOSE. The coupling framework involves two types of coupling parameter transfer relationships: 1） The oxidation corrosion field obtains coolant temperature and flow velocity from the thermal-hydraulic field to compute the oxide layer thickness and transfers the oxide layer thickness to the thermal-hydraulic field to calculate the convective heat transfer coefficient; 2） The neutron physics field receives temperature distribution from the thermal-hydraulic field to compute keff, neutron flux distribution and power distribution, and transfers the power distribution to the thermal-hydraulic field for thermal-hydraulic calculations. In terms of numerical system solving, the coupling framework employs the concept of directly coupled equations of the three physical fields and solves them using the Newton-Krylov iteration method. A neutronics-thermal-hydraulics-material coupling problem of a 19-rod bundle fuel assembly in an LFR was computed using the coupling framework, and the effects of oxygen concentration and coolant inlet temperature on the temporal variations of key coupling parameters and the distribution of oxide layers were investigated. The results indicate that under the benchmark condition, after 10 000 hours of oxidation corrosion, the average thickness of the oxide layer on the fuel assembly cladding surface is approximately 9.86 μm. The maximum temperature rises of the fuel and cladding are 13.36 K and 5.63 K, respectively, with a decrease in keff of 7 pcm. Increasing oxygen concentration is beneficial for inhibiting magnetite dissolution and enhancing the self-repair ability of the oxide layer, but the promotion effect of increasing oxygen concentration on the growth of Fe-Cr spinel is limited after reaching a certain concentration. Although raising the coolant inlet temperature leads to an increase removal rate of magnetite on the inner surface of the cladding at the center of the assembly, it significantly promotes the growth of Fe-Cr spinel.

The effect of Zn addition on general corrosion and metal release behavior of alloy 690 were investigated in simulated pressurized water reactor (PWR) primary water with 0, 10 and 40 ppb Zn additions. The corrosion rate and the metal release rate were calculated by weight loss method after3000 h exposure. The surface and cross-sectional morphology, thickness and element distribution along the oxide film were analyzed by scanning electron microscope (SEM) and transmission electron microscope-energy disperse spectroscopy (TEM-EDS). The crystallographic structure of the outer and inner oxides was identified using atomic-resolution TEM imaging with fast Fourier transformation (FFT). The results indicate that the corrosion rate, metal release rate and oxide film thickness of alloy 690 all decreased with the increase in Zn concentrations. The addition of 10 ppb Zn results in an approximately 18% reduction in the average corrosion rate and 14% reduction in the metal release rate, which further decreases by about 35% with an increase in Zn concentration to 40 ppb. Meanwhile, the average thickness of oxide film of alloy 690 significantly decreases from 110 nm to 25 nm when the concentration of Zn is increased from 0 to 40 ppb. The microstructural analysis of the oxide film indicates that the oxide film of alloy 690 exhibits a three-layer structure in 40 ppb Zn addition condition. The outermost oxide film has less Zn and is primarily composed of (Zn, Ni)Fe2O4. The intermediate oxide film contains much more Zn with a structure of Zn(Cr, Fe)2O4. The innermost oxide film at the oxide/metal (O/M) interface is mainly composed of Cr2O3. In PWR primary water, the free energy required for the formation of spinel ZnCr2O4 is lower than that of FeCr2O4 and NiCr2O4, and ZnCr2O4 exhibits a much lower solubility compared to other spinel, which results in its greater stability within the oxidation film. Meanwhile the Zn2+ promotes the denser Zn(Cr,Fe)2O4 outer and intermediate oxide film formation by filling the cation vacancies and replacing the Ni2+ and Fe2+ in spinel. In summary, the Zn addition can significantly enhance the stability and compactness of the oxide film on alloy 690, thereby retarding the outward diffusion of metal ions and inward diffusion of oxygen ions. Additionally, it reduces the oxygen partial pressure at the O/M interface, promoting the formation of an inner layer composed of Cr2O3. Consequently, this effectively suppresses corrosion and minimizes metal release from alloy 690 in PWR environments.

Supercritical carbon dioxide is one of the most promising candidate working fluids for nuclear power plants. Structural materials using in this kind of system face severe challenges due to its high temperature and high pressure. Corrosion, especially oxidation and carburization, will occur, which leads to the failure of material. Traditional metallic materials like austenitic stainless steel, which is regarded as the candidate cladding materials, will break away because of their limited properties. To further improve the corrosion resistance of austenitic stainless steel as the candidate cladding material for supercritical carbon dioxide nuclear reactor, the general corrosion behavior of three kinds of alumina-forming austenitic stainless steel with different Al contents and their Al-free steel was investigated in supercritical carbon dioxide at 650 ℃/20 MPa. The purity of carbon dioxide was 99.99%. All samples were firstly ground, and then polished to eliminate the surface damage regions. Scanning electron microscopy equipped with backscattered electron, electron backscatter diffraction detector and energy dispersive spectroscopy was used for microanalysis. Focused ion beam system was used to prepare the cross-sectional transmission electron microscopy foils. Selected area electron diffraction was also carried out on the transmission electron microscopy foils for revealing the detailed microstructural characterization. Glow discharge optical emission spectrum was used for carburization analysis. The results show that the weight gain of materials decreases with the increase of Al content, and the weight gain of 3.5wt% Al steel is only about 0.022 mg/cm2. The weight gain against exposure time of different materials follows the parabolic growth law approximately, which indicates that the corrosion behavior is mainly controlled by diffusion. When the content of Al is less than 1.5wt%, a dual-layer Fe-rich oxide film is formed on the surface with poor protection. Some pores were observed within it, which can cause oxidation and carburization. Carburization occurs on Al-free steel and the depth of the carburization region can be up to about 12 μm. When the content of Al is higher than 2.5wt%, protective oxide films are formed on the surface. Its outer layer is mainly rich in Cr and the inner layer is rich in Al. The Al layer on 2.5wt% and 3.5wt% Al containing steel is more continuous. These oxides own better corrosion resistance than that formed on 1.5wt% steel. Although more protective oxides are formed, carburization still exists in the oxide film and matrix of this kind of materials, while the thickness of the carburization region decreases to about 6 μm. Higher Al content, above 2.5wt%, can effectively facilitate the formation of protective Al-rich oxide film, which hinders the outward diffusion of Fe and the inward diffusion of C. It further improves the oxidation and carburizing resistance of materials, which will be useful in the supercritical carbon dioxide nuclear power plant.

Understanding the deformation mechanism in polycrystalline metals is critical to use them in high-value high-risk applications. Here, we report an automated framework based on lattice rotation analysis for accurately identifying slip system and assessing the multiple slip activities in large data set of polycrystalline Zr, aims to statistically provide deep insight on deformation mechanism of Zr. Results show that multiple slip is the dominant slip system rather than single slip system. This method can be applied as a complementary method to the intragranular misorientation axis (IGMA) method and can act as bridges between macro-mechanical response and microstructural deformation mechanisms. [ABSTRACT FROM AUTHOR]