Study On N,C-doping By Pulsed Laser And Its Corrosion Resistance On Uranium Surface

Uranium is widely applied in national defense and nuclear energy field because of its excellent nuclear properties.However,uranium has rather active chemical properties,resulting in corroding quickly in the natural environment.Surface treatment technology has been widely concerned in dealing with uranium corrosion because it does not change the overall properties of the material.The corrosion of uranium will affect the performance of uranium components,so it is very important to make it have excellent corrosion resistance by doping on the uranium surface.The pulse laser doping technology is chosen as the N,C doping method on uranium surface because it offers several advantages such as high doped elements concentration,low matrix temperature,fast treatment,simple vacuum chamber and precise position control,which is favorable for radioactive material passivation.At the same time,the microstructure and oxidation behavior of the modified layer are studied.The N/C-doped modified layers on uranium surface is a composite layer with a gradual transition structure from UO2-xNy/UO2-xCy and uranium oxy-nitride(UNxOy)/oxy-carbide(UCxOy)to UN/UC and eventually to metallic U from surface to interior,respectively,and the proportion of UN/UC in the modified layer is the largest.The results of the initial oxidation behavior of O-rich surface and O-poor surface illustrate that UNxOy/UCxOy in the O-rich region is inert under oxygen exposure.The distribution of products after UN and UC oxidation indicates that the O concentration has an important influence on the oxidation corrosion reaction.UN/UC react with O to form UNxOy/UCxOy and O/N or O/C in UNxOy and UCxOy increases when oxygen concentration increases.Of course,UNxOy and UCxOy can continue to be oxidized,and the O/N or O/C will continue to increase,but the oxidation rate of UNxOy and UCxOy is extremely low.When the O content is high to a certain extent,the structure of UNxOy and UCxOy may transform into oxides similar to UO2.The UC1-xNx solid solution modified layers were prepared by pulsed laser irradiating the surface uranium in the mixed atmospheres where the partial pressure ratios of CH4 to N2 are 1:2,1:1 and 2:1,respectively,and the C and N contents in the UC1-xNx solid solution can be adjusted by the respective partial pressure of CH4 and N2.The results of the initial oxidation behavior of O-rich surface and O-poor surface illustrate that U(N/C)xOy in the O-rich region is inert under oxygen exposure.The oxidation behavior of UC1-xNx and the distribution of the oxidation products in the environment of O2 and H2O were studied,respectively,and the results indicated that the UC0.55N0.45 prepared in an atmosphere where the ratio of CH4 to N2 at 1:1 have excellent corrosion resistance.The study found that oxidation inert U(N/C)xOy was formed on the surface of UC0.55N0.45 after oxidation,which further delayed the oxidation process.The corrosion resistance of the N,C doped modified samples prepared pulsed laser irradiating the surface uranium is obviously improved.The physical process of laser doping is as follows:first,laser acts with U to form liquid molten pool,and the N/C dissolves in liquid U.Next,the UN,UC,UCi-xNx with the highest melting point formed rapidly during the temperature drop of the liquid molten pool.The UNxOy,UCxOy,U(C/N)xOy,which are formed in the oxidation process of UN,UC,UC1-xNx,respectively,is the key reason for the corrosion resistance of N,C doped samples.