Optimization of a nuclear fuel spacer grid spring using homology constraints

Abstract

The springs in a spacer grid support the fuel rods in a nuclear fuel system. The spacer grid is part of the fuel assembly. Since the spring has repeated contact with the fuel rod, fretting wear occurs on the surface of the fuel rod. Design is usually performed to reduce the wear while the functions of the spring are maintained. The design process for the spring is defined by using the Independence Axiom of axiomatic design and the design is carried out based on the direction that the design matrix indicates. For a detailed design, an optimization problem is formulated. In optimization, a homologous design is employed to reduce the fretting wear. The deformation of a structure is called homologous if a given geometrical relationship holds for a certain number of structural points before, during, and after the deformation. In this case, the deformed shape of the spring should be the same as that of the fuel rod. This condition is transformed to a function and considered as a constraint in the optimization process. The fretting wear is expected to be reduced due to the homology constraint. The objective function is minimizing the maximum stress to allow local plastic deformation. Optimization results show that contact occurs in a wide range. The results are verified by non-linear finite element analysis.