Design of an overmoderated fuel and a full MOX core for plutonium consumption in boiling water reactors

The use of uranium–plutonium mixed oxide fuel (MOX) in light water reactors (LWR) is nowadays a current practice in several countries. Generally 1/3 of the reactor core is loaded with MOX fuel assemblies and the other 2/3 with uranium assemblies. Nevertheless the plutonium utilization could be more effective if the full core could be loaded with MOX fuel. In this paper the design of a boiling water reactor (BWR) core fully loaded with an overmoderated MOX fuel design is investigated. The design of overmoderated BWR MOX fuel assemblies based on a 10×10 lattice are developed, these designs improve the neutron spectrum and the plutonium consumption rate, compared with standard MOX assemblies. In order to increase the moderator to fuel ratio two approaches are followed: in the first approach, 8 or 12 fuel rods are replaced by water rods in the 10×10 lattice; in the second approach, an 11×11 lattice with 24 water rods is designed with an active fuel length very close to the standard MOX assembly. The results of the depletion behavior and the main steady state core parameters are presented. The feasibility of a full core loaded with the 11×11 overmoderated MOX fuel assembly is verified. This design take advantage of the softer spectrum comparable to the 10×10 lattice with 12 water rods but with thermal limits comparable to the standard MOX fuel assembly.     

固体燃料超燃冲压发动机燃烧室的数值仿真

基于一种固体燃料超燃冲压实验发动机的实验数据,使用数值模拟软件分别对超燃冲压发动机燃烧室的初始状态以及启动后的燃烧流动进行数值模拟。采用用户自定义函数方式给定PMMA燃料进口边界。数值模拟结果显示:燃烧室流场特性分布符合理论分析;燃烧室固体燃料壁面的燃料退移速率与实验数据有一定差异,但是整个燃面沿轴向的燃速分布规律与实验值近似;沿轴向的燃面附近的压力分布与实验结果较为吻合。研究结果表明:该数值计算模型较为合理,对固体燃料超燃冲压发动机的理论研究具有一定参考价值。     

Failure mechanism of PEM fuel cell under high back pressures operation

Proton exchange membrane fuel cells (PEMFCs) are expected to function under relatively higher back pressures for targeting higher outpower. Under this condition, the durability of fuel cells will be a huge challenge for commercialization. In our study, a 1000-h durability experiment was performed on a PEMFC to investigate the durability under high back pressures. A semi-empirical fuel cell polarization curve model was used to separate the activation and concentration losses, and study their changes with testing time at different current densities. In addition, the charge transfer resistance (R_ct) related to oxygen reduction reaction (ORR) in the catalyst layer was also investigated. Additionally, the mass transfer resistance (Z_d) was investigated using electrochemical impedance spectroscopy (EIS). Moreover, the contact angle and energy-dispersive X-ray spectrum (EDX) of carbon paper surface were characterized. The results indicated that the increase in mass transfer resistance was the biggest contributor to the loss of cell voltage with testing time. The decrease in contact angle of carbon paper surface implied that the weakening of hydrophobicity contributed to an increase in the mass transfer resistance, which comes from the PTFE loss observed from EDX. This may result from the aggravated corrosion of carbon fiber or physical erosion induced by flooding in fuel cell under high back-pressure.     

Magnetic field effect on the heat of reaction in metal-hydrogen systems

The effects of magnetic fields were generally formulated on the thermodynamic properties of chemical reactions. The magnetic field-induced change in the heat of reaction was calculated for the metal–hydrogen system, especially for LaCo₅-H₂, on the basis of the general formulation. Independently of this, the heat of reaction under the influence of magnetic fields was estimated from the temperature dependence of the equilibrium hydrogen pressure. The agreement between both the results indicated that magnetic fields influence the heat of reaction in metal–hydrogen systems.     

Evolution of recrystallisation texture and microstructure in low alloyed titanium sheets

The evolution of microstructure and crystallographic texture in low alloyed titanium sheets, initially deformed by 80% cold rolling, are investigated at different stages of the recrystallisation process. Optical and transmission electron microscopies, as well as X-ray diffraction and EBSD are used to provide information about recrystallisation mechanisms and kinetics. Orientation Density Function (ODF) differences are used to quantitatively compare recrystallised and deformed states. The main texture features of the deformed state evolve only slightly during the primary recrystallisation. The major changes in texture result from secondary recrystallisation or grain growth. Primary recrystallisation can be roughly separated into two stages. The first one is very fast and corresponds to the appearance of new grains in about 80% of the material volume. The second stage is more sluggish. It corresponds to the disappearance of the so-called “white grains”, which did not twin during deformation due to their stable orientation near {ϕ₁=0°, φ=45°, ϕ₂=0°}. Recovery is an important mechanism throughout the process and deformation heterogeneities must be taken into account for a good understanding of the recrystallisation in titanium.     

Strain-induced precipitation of NbC in Nb and Nb–Ti microalloyed HSLA steels

The precipitation start time (P_s) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel. The delay of precipitation of strain-induced NbC carbides is attributed to both the insufficient solution of Nb during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides.     

Transverse tension fatigue life characterization through flexure testing of composite materials

The transverse tension fatigue life of S2/8552 glass–epoxy and IM7/8552 carbon–epoxy was characterized using flexure tests of 90-degree laminates loaded in 3-point and 4-point bending. The influence of specimen polishing and specimen configuration on transverse tension fatigue life was examined using the glass–epoxy laminates. Results showed that 90-degree flexure specimens with polished machined edges and polished tension-side surfaces had lower fatigue lives than unpolished specimens when cyclically loaded at equal stress levels. The influence of specimen thickness and the utility of a Weibull scaling law were examined using the carbon–epoxy laminates. The influence of test frequency on fatigue results was also documented for the 4-point bending configuration. A Weibull scaling law was used to predict the 4-point bending fatigue lives from the 3-point bending curve fit and vice versa. Scaling was performed based on maximum cyclic stress level as well as fatigue life. The scaling laws based on stress level shifted the curve fit S–N characterizations in the desired direction, however, the magnitude of the shift was not adequate to accurately predict the fatigue lives. Furthermore, the scaling law based on fatigue life shifted the curve fit S–N characterizations in the opposite direction from measured values. Therefore, these scaling laws were not adequate for obtaining accurate predictions of the transverse tension fatigue lives of heterogeneous, fiber reinforced, polymer matrix composites.     

Liquid crystalline polymers and elastomers

The results of experiments and theory correlate increasingly well in the investigation of liquid crystalline polymers. Current work focuses on rheology and the behavior of blends of thermotropic LC polymers, due to reemerging economic interest in these materials. Research on liquid crystalline elastomers concentrates on potential applications such as artificial muscles and lasing.     

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers. The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test. The form of the spectrum changed sensitively, as the delamination length increased. Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum. The calculated result reproduced the change in the measured spectrum very well. Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length. This indicator has a potential to be applied to other laminate configurations.