Irradiation Swelling And Creep To The Research On The Effects Of Dispersion Type Fuel Mechanical Behavior

Compared with the traditional nuclear fuel rods, the dispersion nuclear fuel elements have higher burn-up and higher thermal conductivity, so they have been extensively used in the research reactors, and they have good prospects in the nuclear vessels, nuclear waste disposal and commercial reactors.The dispersion nuclear fuel element consists of the metal cladding and the dispersion nuclear fuel meat. The fuel meat is distinguished by having the nuclear fuel particles dispersed through the matrix. Inside the demanding environment of the reactors, the fuel particles generate heat by nuclear fissions, the fission product accumulation results in irradiation-induced swelling of the fuel particles. At the same time, fast neutron and fission products release from the fuel particles. Under the long-time-work, the in-pile materials creep under the attack of fast neutron and fission products. Because of the strong penetrability of fast neutron, the distribution of fast neutron flux in the matrix can be considered as uniform. But the fission products are only distributed near the fuel particles. So, the matrix materials near the particle get enhanced creep. The inhomogeneous creep in the matrix can result in different in-pile mechanical behaviors. In order to ensure the safety and reliability of the dispersion fuel rod, it is necessary to study the in-pile irradiation behaviors, especially the effects of irradiation-swelling and irradiation-creep on the in-pile irradiation behaviors.Firstly, mechanical behaviors of the sparsely dispersed nuclear fuel are investigated through analytic solutions, considering the thermal effect, irradiation swelling of the fuel particle and the inhomogeneous irradiation-creep of the surrounding matrix. Analytic solutions of the stress fields are obtained with the method of solving the Eshelby’s inclusion problem in a three-phase, spherically concentric solid. The result indicates that the maximum of the first principal stress in the matrix appears at some distances away from the interface between the matrix and the inclusion instead of on the interface, and that is induced by a locally enhanced creep of the matrix. When the ratio of the volume modulus or shear modulus between the matrix and the inclusion is smaller, the matrix is more difficult to perform fracture considering the same inclusion or matrix.Subsequently, mechanical behaviors of the densely dispersed nuclear fuel are investigated through the finite element method. Two classical computing models, considering the design feature of the elements and the practical boundary conditions, are adopted to simulate the dispersion fuel plate and the dispersion fuel rod separately. The results indicate that:compared with the situation of a homogeneous distribution of the creep rate in the matrix, different distributions of stress and strain perform in the matrix, when considering the effect of fission product:the creep rate increases in the matrix, and the creep rate increases more intensely when the point has a closer distance from the particle; Accordingly, the Von Mises stress decreases in the matrix, and the Von Mises stress decreases more intensely when the point has a closer distance from the particle. Because of the distance between the cladding and particle, the similar distributions of stress and strain in the cladding perform both on the conditions of homogeneous creep rate and inhomogeneous creep rate in the cladding.This study is useful to the optimal design of the dispersion nuclear fuel elements, and it could provide numerical reference basis for the actual operation of the fuel element.