Thermal-mechanical Coupling And Damage Mechanism Of The Fuel Rod Of Lead-cooled Fast Reactor

As an efficient and clean energy source,nuclear energy has many special advantages over the other energy sources.Therefore,all the countries in the world are paying close attention to the development and application of nuclear energy.When the nuclear reactor is in operation,the temperature inside the fuel rod is very high,causing the structure of the fuel rod to have a large deformation and stress.Large deformation or high stress may cause damage to the fuel rod structure.Lead-cooled fast reactor uses Lead-Bismuth Eutectic(LBE)as the coolant.Because LBE has low melting point,high boiling point and good thermal stability,the reactor can operate at low pressure,reducing the probability of uncontrollable chemical reactions,and giving a high safety factor.The superior multifunctional characteristics of the LBE have also made it possible to miniaturize the nuclear power plant,which has attracted much attention in recent years.The finite element method is used in this paper to simulate the temperature distribution and mechanical behavior in nuclear fuel rods and analyze the cracking mechanism of the fuel pellet to provide a basic calculation method for the subsequent nuclear fuel rod design.First,a simplified analysis model of nuclear fuel rod is established,and the thermo-mechanical coupled iterative calculation process of nuclear fuel rod is set up.Then the temperature distribution,stress,and deformation of nuclear fuel rod are obtained using the method of two-dimensional axisymmetric analysis.After that the effect of several geometrical parameters on the temperature and stress of nuclear fuel rod is studied.It is found that the maximum radial deformation of fuel pellets designed as many short pieces is greater than that of one single fuel pellet.However,the maximum radial deformation of fuel pellets almost does not change when the number of the fuel pellets continues to increase.The thickness of the clad has a significant influence on the clad stress,but has a small influence on the stress of fuel pellets.The gas gap has a great influence on the temperature field and deformation of the fuel pellets.The temperature field on fuel pellets is analyzed using the heat transfer theory,and the thermal stress on fuel pellets is calculated using elasticity theory.It is found that under a certain heat source,the surface of fuel pellets may give rise to cracks because of the large tensile stress along circumferential direction.In practice,it is found that the surface of fuel pellets has a large number of cracks.In the last chapter,a two-dimensional fuel pellet model is used to simulate the surface cracking process.An analysis method to release the constraints of cracked boundaries is proposed.The damage behavior of the fuel pellet is simulated and the radial cracks of fuel pellets under different heat sources is estimated.It is found that with the increase of heat source,the number of radial cracks of the fuel pellets increases,which is in good agreement with the actual engineering observation.