Study On The Conformation Of Uranium Adsorbents And Phase Structure Of Uranium Based On Neutron Scattering Technology

Uranium plays an extremely important role in the utilization of nuclear energy and is critical to the sustainable development of nuclear energy.In the uranium fuel cycle,on the one hand,the resources of uranium ore will be depleted within 100 years.But seawater contains 1,000 times as much uranium as ore.Uranium extraction from seawater has become a strategic direction in many countries.The key is development of excellent adsorption materials.At present,amidoximated adsorbents is widely accepted by researchers.There is already been a large amount of studies on improving the performance of such materials,including the optimization of the trunk material structure and the functional molecular chain components and structure.However,the researchers paid little attention to the conformation of poly?acrylamidoxime?in water,which is one of the most important factors affecting the performance of uranium adsorption.On the other hand,a lot of research has also been done on phase diagram,phase transformation,etc.,which affect the design and use of uranium fuel.But most of work is limited to the study of depleted uranium.It is not known the difference between depleted and enriched uranium.Additionally,the impurity,the heat treatment conditions and processes of each fuel production are not the same,resulting in large differences in the structure and phase transformation of the fuel.Uranium niobium alloys are potential candidates for the fourth-generation reactors,it is also a focus of research.In order to study two kinds of structure problem in the uranium fuel cycle with neutron scattering,this thesis developed neutron scattering mainly neutron reflection and neutron diffraction techniques.The conformation of amidoximated adsorbents,structure and phase transformation of U-18at%Nb alloy were studied.A method for the preparation of poly?acrylamidoxime?brushes on the surface of quartz by Atom Transfer Radical Polymerization was developed.The process of preparation was characterized and optimized.The results show that the polymerization is a controllable and first-order kinetic process.The temperature effect on the polymerization agrees with Arrhenius equation,The apparent rate constant of polymerization is 2.56×10^-5s^-1 at 65?.Negative correlation between molecular weight distribution and sacrifice initiator concentration was found.When preparing self-assembling monolayer,by adding a silane coupling agent with no terminal halogen atom in the initiator,it can easily adjust the graft density of the polymer on the quartz surface.The thickness of the poly?acrylamidoxime?brushes is positively related to the initiator density on the substrate and reaction time.The thickness of the polymer brushes is linearly related to the monomer conversion and the molecular weight of the polymer,and the growth kinetics of the polymer brushes accords with the exponential equation h=a ×(1-e^-bt).For the first time,the conformation of poly?acrylamidoxime?brushes in water and solutions were studied by in-situ neutron reflectivity.It has been proved the speculation that the complexation of UO₂²⁺ions with the AO group leads to the transformation of the polymer brusher into a collapsed or more compact conformation.Meantime,the water fraction in the polymer brushes decreases,which will be unfavorable to the further diffusion and adsorption of uranyl ions.The results of the dry film adsorbed uranyl ions show that the collapse conformation did not completely prevent uranyl adsorption.This also reveals that the adsorption process deviates from the first order kinetic.The temperature makes the conformation of polymer brushes more stretched,which is conducive to the adsorption of uranyl ions.Na+,Mg2+,and VO2+make the conformation of polymer brushes collapsed,which is not conducive to the adsorption of uranyl ions.Their influence mechanisms are different.In heavy water,the conformation of polymer brushes with smaller grafting density is more stretched and higher swelling ratio.The polymer brushes with higher grafting density is the opposite.The design of uranium adsorbents needs to balance the conformation and adsorption sites of different grafting density.Because the uranium will fuel absorb neutron,the absorption correction of the flat-plate specimen for neutron diffraction was established,and the transmission factor,diffraction intensity and diffraction center offset of the flat-plate and annular specimen were compared.The results show that there is singular point of transmission factors and dffraction intensity in the reflection measurement and penetration measurement mode of flat-plate specimen.It is not favourable for the experiment.This phenomenon does not exist in the rotation measurement mode.The diffraction intensity of the reflection measurement mode of the flat-plate specimen is also strengthened in the large diffraction angle region when the thickness is the same as the width.Under the same conditions,the annular specimen is suitable for diffraction experiments with low absorption coefficient.The flat-plate specimen reflection measurement mode makes full use of the reflective geometric light path,and is more suitable for samples with large absorption coeficient.Simulated absorption samples and ²³⁵U samples confirmed the accuracy of the absorption correction model and the feasibility of neutron diffaction experiments in uranium fuel samples.The structure and phase transformation of U-18at%Nb alloy under temperature loading were studied by neutron diffraction.The results show that the transformation of the a-U phase to the ?1-U phase is occured between 630? and 640?,which is lower than the phase diagram The phase transformation is also related to the thermal history of the alloy.The ?2-U phase during the phase transformation is not completely dissolved immediately.The mass fraction of ?2-U phase gradually becomes smaller when the temperature increases,and it is completely dissolved when the temperature rises to the uranium niobium intersoluble zone.During the heating process,the lattice constants of a and c for the a-U phase increase,and the constant b value decreases.The cell volume has been swelling with temperature,and the swelling ratio has reached 3.5%.These changes are consistent with the a-U to y-U phase transformation paths of pure uranium.The diffraction peak width of the y2-U phase is large before phase transformation,and it decreases with temperature increase after phase transition.It is mainly caused by grain size and stress.It is speculated that this stress is one of the reasons that affect the decrease of phase transformation temperature.This thesis developed neutron scattering techniques which applied in structure studying of uranium extraction and enriched uranium in the uranium fuel cycle.It fills the gap of neutron scattering technology such as sample preparation,environmental loading and data correction.The conformation of poly?acrylamidoxime?brushes,which have a significant influence on the adsorption performance of the uranium absorbing material,and the structure and phase transformation of uranium fuel are studied.The research results offer a reference for design of uranium extraction material and uranium material.