Preparation Process Of Simulated Uranium Nitride Powder Based On Nitriding Route Of Ammonium Fluoride Compound

Uranium mononitride is an important candidate for Accident Tolerant Fuel(ATF)and space reactor fuel due to its inherent high uranium atomic density,high proliferation ratio,good thermal conductivity and good chemical compatibility with sodium coolant.At present,there is no absolute leading route for the synthesis of uranium mononitride nuclear fuel,which mainly includes carbon thermal reduction route,metal nitriding route,sol-gel route and ammonium fluoride compound nitriding route.Carbon thermal reduction and metal nitriding are mainly studied in China,but they are still in the laboratory research stage.In this paper,the synthesis of uranium mononitride powders and the nitriding route of ammonium fluoride compounds were systematically explored and studied,and the following innovative achievements were achieved:1.The reaction process of fluorination of CeO2 and NH4HF2 to cerium ammonium fluoride compound((NH4)4CeF8)was studied by replacing UO2 with CeO2.The influence of reaction container material,raw material ratio,reaction temperature,reaction time and other factors on the preparation process of(NH4)4CeF8 was discussed.It was found that:(1)When the molar ratio of CeO2 to NH4HF2 was 1:6,the target product(NH4)4CeF8 could be obtained at room temperature,but the time of the reaction took two months.Reaction time was too long,resulting in low reaction efficiency;(2)By the reaction against 390 K for 4 h,the target product(NH4)4CeF8 could be obtained,which shortened the reaction time and improved the reaction efficiency.It provided reference to the efficient synthesis of uranium fluoride compounds.2.Nitriding reaction was studied by using(NH4)4CeF8 as raw materials and ammonia as nitrogen source,and phase pure cerium nitride(CeN)powders were successfully prepared.Two kinds of nitriding reaction processes were studied:(1)nitriding reaction was carried out directly with(NH4)4CeF8 as raw material,and the effects of reaction time and temperature on the nitriding reaction were studied.XRD analysis of the product showed that the phase pure CeN powders could be obtained after 70 h of reaction at 1000℃.(2)Nitrizing reaction was carried out with(NH4)4CeF8 thermal decomposition product CeF3 as raw material,and the influence of reaction time and temperature on the product was discussed.XRD,SEM,TEM and other means were used to characterize the product.Phase pure CeN powder could be obtained at 1000℃ and reaction time 60 h.A scheme to obtain uranium mononitride powder by direct reaction of uranium isotope enrichment product uranium hexafluoride with ammonia gas was put forward,to shorten the current preparation process of uranium mononitride.3.Based on first-principles calculation method,the influence of O and F impurities on uranium nitriding were studied by Materials Studio and VASP.By analyzing the crystal structure,transition state,electron state density and binding energy,the influence of O impurity on the whole of UN cell can be ignored.The bonding energy of O and U was-5.04 eV,which indicated that O could exist stably in UN lattice.The length of U-O bond was smaller than that of U-N bond and the bond was closer.The gap transfer barrier of O was 2.17 eV.For F impurity,the lattice constants in the a,b and c directions increased slightly after N-substitution incorporation.The binding energy of F to UN was-6.78 eV,indicating that F could exist stably in UN lattice.The migration potential energy of F gap was 2.32 eV,which was slightly larger than that of O.Moreover,according to the density of electronic states of UN cells before and after O and F doping,it could be found that the stability of the system doped with F was slightly worse than that of the system doped with O,but the overall structure tended to be stable.The influence of O and F on uranium mononitride was small,so it was preliminarily determined that the impurities O and F might be introduced into the nitride route of ammonium fluoride compound would been not a great influence on uranium mononitride products.In summary,in this paper,the element Ce was used as an analogue of U,and the preparation process of uranium mononitride powder based on the nitriding route of ammonium fluoride compound was simulated and studied by simulation experiment and theoretical calculation.It was found that this route was high reaction efficiency,good product purity,small influence of impurities,and has a good application prospect in the preparation of advanced uranium mononitride nuclear fuel.