| With the development and progress of society,human beings'demand for energy is also increasing.Nuclear energy has become more and more important as a clean energy source.High-performance nuclear fuel is an important guarantee for the safety of nuclear reactor operation and the sustainable development of nuclear energy industry.Spherical nuclear fuel particles are widely used in various types of nuclear reactors due to their excellent fluidity,good mixing and large specific surface area.In particular,it is possible to directly prepare a Sphere-Pac Fuel for use as a nuclear fuel for fast reactors,and it is also the most important material for preparing ADS enthalpy nuclear fuel.At the same time,spherical nuclear fuel is the most important fuel form for high temperature gas cooled reactors and the candidate form of Accident Tolerant Fuel(ATF).In these applications,nuclear fule microspheres have the properties of precise and controllable particle size,very narrow particle size distribution,excellent sphericity and smooth spherical surface.These are also properties that must be possessed for engineering use.The sol-gel method has attracted much attention for its ability to directly convert nuclide from liquid to solid to achieve dust-free preparation of nuclear fuel microspheres.The conventional sol-gel nuclear fuel microsphere preparation method required the raw material liquid to be placed in a low temperature environment(0?5?)to ensure the stability of the sol solution.But an additional refrigeration system increases the probability of failures in the preparation of strong radiactive nuclear fuels.At the same time,the low temperature environment also limits the choice of nuclear fuel microsphere surface modification.Therefore,a variety of microfluidic systems based on the fluid properties were designed and constructed in this paper to control the particle size and morphology of nuclear fuel microspheres,which breaks through the need to prepare nuclear fuel microspheres under low temperature conditions in order to fabricate nuclear fuel microspheres in a normal temperature environment.In addition,diffused fuel is the most important type of accident-tolerant fuel.It needs to disperse uranium oxide microspheres in a dispersion medium with high therma!conductivity and low neutron absorption cross section.However,due to the difference between the physical properties of uranium or cerium oxide and the dispersing medium,the surface of dispersed fuel pellets prepared by high-temperature sintering may not fully fuse perfectly with the dispersing medium.This will result in a significant reduction for the thermal conductivity,physical and mechanical properties of fuel pellets.Therefore,based on electrostatic attraction and LBL technology,the surface modification of fuel microspheres loaded with dispersion medium nanoparticles is realized,which improved the interfacial compatibility of nuclear pellet.The main research work and results of the thesis are as follows:(1)The design and construction of a series of microfluidic control systems,which could be used to fabricate single or complex topography(such as Janus or core-shell)structures.The prepared microspheres have a very narrow size distribution and good monodispersity,and the coefficient of variation CV is less than 5%.The device's handling performance on droplet size and morphology was verified by experiments.Detailed study for the effects of experimental parameters(such as viscosity,flow rate,capillary size,etc.)on the size of the microspheres and how to prepare complex morphological microspheres through different microfluidic devices have also been down.These parameters are combined with an empirical formula,Here,Dcap refers to the diameter of capillary,K is a dimensionless coefficient,?c and?d represents the viscosity of continuous phase and dispersed phase,respectively,while Vc and Vd represents the velocity of continuous phase and dispersed phase.(2)A novel flow-focusing assembled microfluidic device was designed to prepare small particle size particles that could not be realized by conventional microfluidic devices.Due to the smaller the size of the commercial capillary,the greater the difficulty of processing,the size of capillary cannot be processed to a level below 90 ?m.In turn,the conventional Co-flow capillary-based microfluidic control system cannot produce droplets and microspheres with a particle size of less than 100 ?m.The Flow-Focusing based microfluidic device designed in this paper changes the flow field characteristics of the continuous phase fluid.A controllable preparation of droplets and microspheres of less than 100 ?m is achieved.(3)A microfluidic device is combined with a sol-gel method to prepare the fuel microspheres having a porous structure by adding a water-soluble polymer pore former to the dispersed phase of raw material liquid.The regulation for the density of porous microspheres is achieved by adjusting the content of pore-forming agent.When the content of polyethylene glycol/cerium ammonium nitrate increased from 0 to 30%,the density of porous cerium oxide reduced from 93%to 26%of theoretical density.(4)Two technical routes for the preparation of nuclear fuel microspheres at room temperature are designed and implemented,namely micro-mixer method and micro-reaction method based on interfacial tension regulation.Firstly,an active micro-mixer based on piezoelectric bimorph was designed and constructed to realize the rapid on-line mixing of ADUN solution(uranyl nitrate)with HMUR solution(urea,hexamethylenetetramine)to form raw material liquid.Then nuclear fuel microspheres were prepared by sol-gel method.In addition,by adjusting the interfacial tension of droplets,the morphology transformation of droplets from the Janus/core-shell structure to the core-shell structure is achieved,resulting the ADUN solution originally on the sides of the Janus/core shell droplets can be fully contacted and mixed with the HMUR solution.Thus,the gel preparation of the fuel microspheres at room temperature is realized from another path.(5)A method for surface modification of microspheres based on electrostatic attraction and LBL(Layer-By-Layer)self-assembly was designed.This is applied to the surface modification of fuel microspheres to realize adsorption of matrix powder nanoparticles on the surface of microspheres and the improvement of interfacial compatibility of fuel microspheres in matrix powder.It provides an important research basis for the development of high performance ATF nuclear fuel pellet.In summary,this paper applies microfluidic control technology to the preparation of nuclear fuel microspheres by sol-gel method,which realizes the precise control of size and morphology in the preparation of nuclear fuel particles,and the gel of raw material liquid under normal temperature.The research and exploration of nuclear fuel microsphere surface modification can provide reference and support for improving the preparation level of ATF nuclear fuel in China. |
PDF Full Text Request