Study On The Preparation And Properties Of Large-grain Composite Accident-tolerant Fuels

Accident Tolerant Fuel（ATF）is a new nuclear fuel concept proposed after the Fukushima nuclear power accident in Japan in 2011 which,in comparison with the standard uranium dioxide/Zircaloy system,can tolerate loss of active cooling in the core for a considerably longer time while maintaining or improving the fuel performance during normal operations.Uranium dioxide,as the fuel in current commercial light water reactors,suffers a relatively low thermal conductivity,which can cause internal high temperatures during operation and therefore aggravates the thermal gradients within the fuel.This will increase the detrimental temperature dependent effects,such as fission product release,pellet cracking and relocation,pellet and the cladding interaction（PCI）.Therefore,a fuel with enhanced thermal conductivity has been regarded as a major priority in the research of ATF fuel.The thermal conductivity of fuel pellets can be improved by two methods,i.e.,increasing the intrinsic thermal conductivity and adding a second phase with high thermal conductivity.The intrinsic thermal conductivity of UO₂ crystals is mainly affected by crystal defects,stoichiometric ratio and grain boundary effect.The current way to improve the intrinsic thermal conductivity of UO₂ is mainly to increase the grain size of UO₂.Since the thermal conductivity of UO₂ is related to the crystal orientation,the existence of grain boundaries will reduce the thermal conductivity,therefore,increasing the grain size can improve UO₂ intrinsic thermal conductivity,and it will also increase the diffusion distance of fission gases to grain boundaries,thereby reducing its release rate and improving the safety performance of fuel pellets.The key point of developing composite fuel pellets is to select the appropriate additives.Silicon carbide（β-Si C）is considered as a promising additive material for its high thermal conductivity,low neutron cross section,and high melting point.Zirconium is also used due to its excellent mechanical,thermo-physical,and radiation resistance properties.In this research,large-grained UO₂ particles,which was used as the fuel phase,were prepared by electrochemical deposition;Si C and Zr were used as the second additive phases,and high thermal conductivity composite fuel pellets was prepared by spark plasma sintering technology（SPS）.Compared with the traditional sintering process,the SPS process can produce pellets with high density,good interface contact and less chemical reaction under the premise of maintaining a lower sintering temperature.The main work is as follows:First,the electrochemical deposition process was used to obtain large-grained UO₂particles by galvanostatic electrolysis.The UO₂Cl₂（2.5wt%）-Li Cl-KCl mixed molten salt was electrolyzed with Pt as the working electrode,graphite as the counter electrode and Ag/Ag Cl as the reference electrode.When the deposition temperature was 500℃,the current density was 5m A/cm² and the electrolysis time was 20h,the obtained UO₂particles had a grain size of 100～400μm,without twins and other defect.It has good crystal characteristics,and the stoichiometric ratio of uranium and oxygen is 1:2,which avoids the decrease of thermal conductivity caused by the superstoichiometric effects.The thermal conductivity of UO₂ particles was obtained by using the Time Domain Thermal Reflection measurement system,ranging from 7.10 to 7.68 W m^-1 K^-1,which is basically consistent with the theoretical thermal conductivity of UO₂.Secondly,different kinds of large-grain composite fuel pellets were prepared by spark plasma sintering technology.It was found that the relative density of composite fuel pellets increased first and then decreased with the increase of UO₂ particles.The thermal conductivity of UO₂-Si C composite fuel pellets increased obviously,especially at higher temperatures,the maximum thermal conductivity increased more than 60%,based on a density normalized to 95%TD.The mechanical properties of UO₂-Zr composite fuel pellets improved more obviously,and the compressive strength at high temperature was greater than 250 MPa.The thermal conductivities of the two composite fuel pellets are basically consistent with the results calculated by the Maxwell-Eucke model.The densification and sintering behavior of different fuel pellets was studied.The densification process of composite fuel pellets was divided into five stages.Among them,the critical temperature range for densification of UO₂-Si C composite fuel pellets is 1150～1500℃,and the densification rate reaches 2.33（?）10^-2mm/s,while the critical temperature range for densification of UO₂-Zr composite fuel pellets is 900～1300℃,and the densification rate reaches 1.466（?）10^-2mm/s.Finally,the performance of the composite fuel pellets at high temperature was evaluated.From room temperature to 1200°C,there was no significant interfacial reaction and element diffusion between the two phases of the composite fuel,and no element diffusion occurred between it and the zirconium alloy cladding,which proved the stability of the composite fuel pellet under normal working conditions.In the high-temperature air oxidation experiments,it was found that cracks appeared in the UO₂-Si C composite pellets at 350°C,and the oxidation first occurred at the interface of the two phases;UO₂oxidation plays a leading role,while Si C plays a blocking role in the initial oxidation.The UO₂-Zr composite fuel pellets were pulverized on the surface at300°C,and the pellets were completely cracked at 350°C.The oxidation first occurred in the Zr phase.Zr oxidation and UO₂ oxidation were took place at the same time,which did not improve the antioxidant capacity of the pellets.In the high-temperature steam oxidation experiments,it wasfound that the UO₂-Si C composite fuel pellets oxidized very limitedly over the entire temperature range,and even at a temperature of 1200°C,the weight change was negligible,and the steam oxidation was dominated by Si C oxidation,resulting in the preventing of the pellets Oxidation.The UO₂-Zr composite fuel pellets were completely pulverized at 700℃.Therefore,the oxidation isas before dominated by Zr oxidation,which also fails to improve the water corrosion resistance of the pellets.In this thesis,we focus on the methods to improve the thermal property of fuel pellets,two kinds of large-grain composite fuel pellets were fabricated by increasing the UO₂intrinsic thermal conductivity and adding a second phase with high thermal conductivity,and their thermodynamic properties and high temperature performance were evaluated.The results show that compared with the traditional UO₂ fuel pellets,the large-grained UO₂-Si C composite fuel pellets have significantly improved the thermal conductivity and the high temperature oxidation resistance,so it is expected to be a good candidate fuel for ATF thermal conductivity enhancement.