Study On Preparation Of Ceramic Nuclear Fuel Microspheres By An Improved Non-cooling Instant Mixing And Microwave-assisted Rapid Internal Gelation Process

According to the demands of rapid economic development and environmental protection,it is an essential strategy to develop clean energy such as nuclear energy.The rapid development of nuclear power will inevitably produce large amounts of spent nuclear fuels?SNF?.In order to improve the utilization rate of uranium resources and achieve the sustainable development of nuclear energy,the closed nuclear fuel cycle must be developed accordingly.There are many strategies for handling of SNF,among which transmutation is considered as the most effective and environment-friendly solution.Sub-critical Accelerator Driven System?ADS?is a potential way to transmute the minor actinides?MAs?and long-lived fission products?LLFPs?into short-lived or stable nuclides by fast neutrons,and consequently reduce the difficulty of geological disposal.Based on the ADS project,Chinese academy of Sciences proposed an advanced Accelerator Driven closed Nuclear Fuel cycle?ADRUF?concept,which is a process of removing some of the fission products and neutron poisons by an advanced dry head-end processing technology while retaining U,Pu and MAs,and then refabricating the SNF into regenerated nuclear fuels.Therefore,it is of great significance to realize how to reprepare regenerated nuclear fuels from SNF in the ADRUF cycle.Because of high radioactivity and biotoxicity,an improved rapid internal gelation process combining non-cooling instant mixing with microwave-assisted heating is proposed to prepare regenerated ceramic nuclear fuel microspheres from SNF.Firstly,two different methods?viscosity change and optical value change?are adopted to investigate the chemical kinetics of the mixed solutions with different compositions at different temperature in gelation process.It indicates that the gelation time could be effectively adjusted by the chemical composition of the mixed solutions at low temperature,while the mixed solutions solidified instantly via gelation reactions at high temperature.These results motivate us to propose an improved rapid internal gelation process combining non-cooling instant mixing with microwave-assisted heating.Accordingly,a set of experiment platform coupling this process with shielding glove box is developed.This improved process can not only reduce the radiolysis effects of SNF on gelation agents,but also avoid the generation of second organic wastes,simplifying the process of reproducing regenerated ceramic nuclear fuel microspheres from SNF.Secondly,study on preparation of uranium dioxide?UO₂?ceramic nuclear fuel microspheres is carried out via the improved rapid microwave-assisted internal gelation process.The influence of[HMTA]/[U]molar ratio on the hardness,specific area and microstructure of the gelled microspheres is studied and it is determined that1.3 is the optimized molar ratio to prepare perfect uranium oxide hydrate gelled microspheres.Then the influence of sintering atmosphere,temperature and time on the phase composition and morphology of the final ceramic microspheres is investigated and it is determined that crack-free ceramic UO₂ microspheres with a density of 98%TD could be obtained under the condition of 3?/min,1500?in 4%H₂/Ar for 5 h.In order to simulate the SNF in ADRUF cycle,the preparation of multi-element metal mixed oxides?MOX?ceramic simulated nuclear fuel microspheres is also carried out in this thesis.Under the condition of 3?/min,1500?in 4%H₂/Ar for 5h,U+Ce and U+Ce+Nd oxides ceramic microspheres with a density of 95%TD are successfully prepared.The final microspheres are solid solution of faced centered cubic MO₂?M=U,Ce and Nd?,and the lattice parameter decreases with increasing content of doped elements.In addition,the oxidation behavior of the ceramic MOX microspheresisalsoinvestigated,anditisfoundthatthe doped element has certain stabilizing effect on the faced centered cubic MO₂ under oxidation ambient.When the content of doped element is less than 10%,the faced centered cubic MO₂ can be oxidized to rhombic phase M₃O₈ and the microspheres are pulverized due to obvious volume expansion above 500?.However,when the content of doped element is more than 20%,the cubic phase MO₂ can only be oxidized to the similar cubic phase M₄O₉ even up to 1000?,and the morphology of the microspheres keeps unchanged during the oxidation process.In the end,the preparation of uranium carbide and multi-element metal mixed carbide?MC?ceramic nuclear fuel microspheres is carried out via the improved rapid microwave-assisted internal gelation process.The nano-scale carbon black is uniformly dispersed in the HMUR solution by ultrasonic method,and carbonaceous uranium hydrate?C-UO₃·2H₂O?gelled microspheres are then prepared.The effects of dispersion conditions of carbon black,molar ratio of[C]/[U],calcination temperature and calcination time on the phase composition and morphology of the final ceramic microspheres are studied and it is found that crack-free UC and MC?M=U,Ce and Nd?ceramic microspheres can be obtained in Ar atmosphere under 1500?for 5 h with an initial[C]/[M]molar ratio of 3.5.The final microspheres are solid solution of faced centered cubic MC.In conclusion,the improved rapid internal gelation process combining non-cooling instant mixing with microwave-assisted heating is successfully applied to prepare perfect UO₂,UC,MOX and MC ceramic microspheres and it is reasonable to predict that this method could potentially be used to prepare regenerated ceramic nuclear fuel microspheres from the SNF for Chinese initiative accelerator driven system?ADS?project..