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In a light water reactor, nuclear fuel element melting is critical. This paper will introduce the experimental studies on the melting and relocation behaviors of the single fuel rod and 3 × 3 fuel bundle imitators based on aluminum and zinc as alternative materials. The eutectic reaction can occur between Al and Zn. Moreover, the lower melting points of Al and Zn allow us to observe the experimental phenomena directly. In this paper, three single-rod experiments and a rod bundle experiment are discussed in detail. The results of the experiments are analyzed at both macro and micro levels. In the experiments of slowly increasing temperature, the melting phenomena of fuel rod imitators mainly include rod deformation, eutectic reaction, melting, erosion, and molten materials relocation. Among them, the existence of a eutectic reaction between materials will make the materials melt under their melting point, which significantly influences the melting time and behavior of the rod. Through this study, the nuclear fuel rods melting and relocation behaviors under the influence of the eutectic reaction can be observed, and the experimental results can provide data support for the development of related models. [ABSTRACT FROM AUTHOR]

The isothermal steam oxidation behavior of Cr-coated Zr-1Nb alloy at 1350 ℃ and 1400 ℃ was investigated. The temperatures are above the Cr-Zr eutectic temperature. The reliability of the experimental setup and methodology was confirmed by conducting isothermal steam oxidation experiments on bare Zr-1Nb specimens at 1100∼1400 ℃, and the corresponding oxidation kinetics correlation was obtained. After the eutectic-oxidation reaction, the surface of the coated specimens exhibited a "crocodile skin" morphology composed of "ridges" and "hollows", which was attributed to the non-uniform eutectic reaction. Due to the much higher O affinity of Zr compared to Cr, and the higher solubility of Cr in β-Zr compared to α-Zr(O) and ZrO 2 , the ZrO 2 and α-Zr(O) layers initially grew on the Cr-Zr eutectic during oxidation. Cr continuously diffused inward and eventually precipitated as Cr-rich phases within the metallic matrix. Three types of Cr-rich phases were observed, with varying oxygen content and reflecting the oxidation process of the Cr-rich phases. The Cr-rich phases were ultimately oxidized into a mixture of Cr 2 O 3 and ZrO 2. The parabolic weight gain rates for the oxidation of Cr-Zr eutectic are compared with the oxidation kinetics correlation derived from the bare Zr-1Nb specimens, and the model applicability of the oxidation kinetics correlations of Zr alloys in the simulations of Cr-Zr eutectic-oxidation reaction is discussed. [ABSTRACT FROM AUTHOR]

The oxidation and degradation mechanisms of Cr coating on Zr alloy cladding under high temperature steam atmosphere are summarized. Six basic phenomena are considered for model development, including: oxidation of Cr, growth of ZrCr 2 , formation of Zr(Cr) solid solution, outward diffusion of Zr, reduction of Cr 2 O 3 , and volatilization of Cr 2 O 3. The mathematical analysis model is established and validated, demonstrating its good capacity to predict the increase and subsequent decrease in the thickness of Cr 2 O 3 scale, as well as the thickness changes in other individual layers. The developed model is applied in the analysis of the structure evolution of Cr coating under steady state, transient heating conditions and postulated accident scenarios. [ABSTRACT FROM AUTHOR]

The threshold displacement energy of Fe and Fe-Cr-Al alloys and the influence factors were investigated using molecular dynamic simulations. The mechanism of defect recombination was discussed, as was the evaluation of the recombination radius for Fe and Fe-Cr-Al. The simulation results show that Fe and Fe-Cr-Al have big differences in threshold displacement energies and directional dependencies at low temperatures, but the differences become less pronounced at high temperatures. The cause might be that the replacement collision sequence can effectively separate an interstitial from a vacancy in Fe-Cr-Al and greatly reduce threshold displacement energy at low temperatures. In addition, the energy transfer efficiency of primary knock-on Al atoms is low due to the difference in atomic mass, and the recombination effect for Cr is hindered by the formation of Fe-Cr 〈110〉 dumbbell. However, the effect of solute atoms on threshold displacement energy is insignificant at high temperatures, due to the increase in the recombination radius and a reduction in the effect of the aforementioned factors. [ABSTRACT FROM AUTHOR]

A reactivity-initiated accident is a typical nuclear reactor accident during which the core fission rate and power increase unexpectedly, making the temperature of the fuel element soar, which causes a decreasing strength, large deformation, and even the failure of the element. To simulate the performance of the plate-type fuel during RIA, a neutronics–thermal hydraulics–mechanics coupling method was implanted in a fuel performance code BEEs-Plates. The coupling scheme was firstly verified and validated against the MTR benchmark. Then the RIA results with the consideration of the deformation effect were compared with the benchmark case, showing that the mechanics as well as the burnup effect have little impact on the neutronic–thermal hydraulics results when using the constant thermal properties. Lastly, the thermal-mechanical analysis was conducted for the fuel plate at different burnups (0% FIMA, 10% FIMA, 20% FIMA) under the RIA. The results show that the peak stress of the cladding increases with the burnup increases while it has an opposite trend for the fuel meat. Some special features such as over-shrinkage of the fuel meat have also been noticeable and analyzed. [ABSTRACT FROM AUTHOR]

The spherical fuel element (SFE) dispersed with many tri-isotropic fuel (TRISO) particles has been primarily employed in the high-temperature gas-cooled reactor. Owing to the complex structure of SFE, behaviors will be evolved for both TRISO particles and the fuel matrix with the increasing burnup. To evaluate the fuel performance of SFE, the homogenization method has been conventionally adopted for SFE scale modeling, and dispersed TRISO particles are analyzed subsequently with fixed temperature boundary conditions. However, owing to the varying properties of TRISO particles under reactor operation, the homogenization models dedicated to certain circumstances can hardly reflect the properties evolution of TRISO with high fidelity, which may lead to uncertainties in analysis results. To this end, a hybrid model, namely the conventional homogenization model informed by the lower scale model online, has been developed in this paper, which may increase modeling fidelity for both SFE and particles. Specifically, property models for the TRISO particle and graphite matrix have been implemented in COMSOL. Subsequently, based on the linking of COMSOL and MATLAB, the hybrid model for SFE analysis was established. Through the hybrid model, the temperature field and stress distribution of TRISO, the evolution of effective properties, and the thermal and mechanical state of the SFE were obtained and analyzed. The results show that within the scope of this paper, the central temperature of TRISO in the innermost region rises with the increasing burnup for the decreasing thermal conductivity of materials, and the peak value is about 1324 K, which is about 60 K higher than that in the outermost region. The difference in the maximum plenum pressure is more than 5 MPa among TRISO particles, which is mainly caused by the variations in CO production. Consequently, the hoop stress of SiC in the outermost region is slightly elevated. The distribution of effective thermal conductivity (ETC) for TRISO is nearly 4.3 W/(m·K) at the beginning of life (BOL), while it was reduced to 3.6 W/(m·K) at the end of life (EOL). There are marginal variations along the radial direction for the ETC of TRISO. Meanwhile, the ETC of SFE decreases from about 21 W/(m·K) for fresh fuel to 13 W/(m·K) for spent fuel. The predicted effective elastic modulus and coefficient of thermal expansion of SFE increase gradually versus time. The former is caused by the densification of pyrolytic carbon, and the latter is mainly determined by the high volume fraction of matrix and the changes of CTE for pyrolytic carbon. In addition, the comparison in the temperature field between the proposed model and the Differential Effective Material Theory (D-EMT) model may further demonstrate the feasibility and effectiveness of the hybrid model. [ABSTRACT FROM AUTHOR]

In order to explore the core corrosion characteristics of horizontal lead-bismuth reactor in oxygen-controlled environment, a liquid lead-bismuth eutectic (LBE) corrosion model including the growth and removal process of oxide layer was established in this paper. The coupling method of double-layered oxidation model and computational fluid dynamics (CFD) was used to simulate the long-term oxidation corrosion of horizontal LBE reactor core numerically. The results show that the average total oxide layer thickness of fuel rod surface is 1.59 μm, and the thickest oxide layer is 3.48 μm on the fuel rod surface at the core outlet after 20,000 h. The fuel rods maintain a double-layer oxide layer structure. The growth rate and removal rate of the oxide layer on fuel rod surface increase with the increase of inlet temperature and the decrease of inlet flow rate. The growth rate of the oxide layer on fuel rod surface increases with the increase of inlet oxygen concentration, while the removal rate decreases. At higher temperature above 420℃, lower flow velocity below 0.25 m/s and lower oxygen concentration below 2 × 10−8wt%, the magnetite layer will first be completely removed from the surface of the fuel rods at the core outlet. The liquid LBE oxidation corrosion model and coupled CFD method presented in this paper can be used for numerical simulation of oxidation corrosion in different units of LBE cooling system. [ABSTRACT FROM AUTHOR]

During Loss of Coolant Accident (LOCA) in Pressurized Water Reactor (PWR), the oxidation of zirconium alloy cladding has a significant impact on the core degradation process. However, empirical parabolic rate correlations applied in accident analysis codes only provide oxide thickness obtained by the hypothesis of stoichiometric zirconia, which has limitations for simulating complicated oxide behaviors under accident conditions. The diffusion model with moving boundaries proposed in previous studies can give the distribution of oxygen concentration. However, due to simplified assumptions cladding expansion was not properly considered. In this study, an oxidation model for zirconium alloy cladding considering oxygen diffusion, zirconium conservation and mass density variation with temperature and oxygen concentration is developed. The model is validated by experimental data and shows good agreement. To analyze cladding oxidation during LOCA transient, QUENCH-05 and QUENCH-SR test are simulated. The model can give reasonable simulation results. The simulation results of QUENCH-05 test confirm the fact that β-Zr plays an important role in maintaining the integrity of cladding under quenching process. The simulation results of QUENCH-SR test indicate that steam starvation can still lead to cladding embrittlement due to the consumption of β-Zr matrix and thus increase the risk of cladding failure. [ABSTRACT FROM AUTHOR]

• A finite element method (FEM) model for FCM fuel thermo-mechanical performances analysis was presented. • The thermo-mechanical behaviors, coupled with irradiation behaviors were developed. • A 3D modeling of FCM fuel pellet with typical packing fractions was realized by an optimized stochastic strategy. • Effects of thermal boundaries, burnup, particle spacing, no fuel zone width and other key parameters were well investigated. Fully ceramic microencapsulated (FCM) fuel is a promising ATF concept with extraordinary fission product inclusion capacity, excellent oxidation and corrosion resistance, and high thermal conductivity. The thermo-mechanical performances are of great significance when evaluating the safety and applicability of a nuclear fuel. In this study, a finite element method (FEM) model for FCM thermo-mechanical performances analysis was presented. The thermo-mechanical behaviors, coupled with irradiation behaviors (including irradiation-induced dimensional change (IIDC), heat generation, conduction, thermal expansion, creep, burnup, and gap/plenum pressure, etc.), were developed to investigate the thermal and mechanical characteristics of TRISO-based FCM Fuel. A simplified model of TRISO fuel particles was established and validated. After that, a full 3D modeling of FCM fuel pellet with typical packing fractions was realized by an optimized stochastic strategy. A numerical method was validated and successfully applied to realize the simulations of a FCM fuel pellet. Two different thermal boundaries, namely fixed particle power and fixed pellet power, were performed to investigate the irradiation behavior of FCM fuel. Moreover, the effects of burnup, particle spacing, no fuel zone width and other key parameters were well studied and investigated. The performed calculation showed that temperature and stress in matrix and TRISO are both positively correlated with the packing fraction. It's worth noting that compared with the cases of fixed particle power, cases of fixed pellet power may lead to higher fuel temperature and higher stress. The results also implied that from the perspective of probability evaluation, most of the TRISO particles in FCM fuel can maintain the structural integrity under different packing fractions, but some of the particles with higher stress may crack or damage, especially in the cases of high burnup and high packing fraction. [ABSTRACT FROM AUTHOR]

Displacement cascades in FeCrAl alloys are simulated by molecular dynamics using a recently fitted embedded atom method (EAM) potential. The threshold displacement energy of the atoms in FeCrAl is firstly computed and then the evolution of displacement cascades is quantitatively analyzed with the recoil energy ranging from 0.5keV to 15keV, and the results are compared with Fe. The influence of temperature and the direction of the primary knock-on atom (PKA) are also investigated. The results show that temperature decreases the number of surviving Frenkel pairs, but the PKA direction has very little effect on them, similar to other materials. The cluster fraction and size for interstitials increase with both irradiation temperature and PKA energy, but the increase of vacancy clusters is smaller due to their lower mobility. Compared to the case of Fe, the presence of Cr and Al in FeCrAl slightly increases the surviving FPs in FeCrAl but reduces the number of clusters formed at the annealing phase. Cr can reduce the mobility of nearby Fe and thus the surviving interstitials are mostly in the form of relatively stable Fe-Cr dumbbells. Al has little influence on surviving FPs but will restrain the formation of clusters. The proportion of Cr in clusters is largely in excess of its atomic fraction in FeCrAl, while Al takes a tiny proportion. [ABSTRACT FROM AUTHOR]