Keywords: spacer grid spring, finite element analysis, characteristic curve, boundary condition, maximum load, shape change.

The spacer grid, a structural component of the nuclear fuel assemblies for the pressurized light water reactors, is composed of the crossed straps, and its primary role is to provide both lateral and vertical support for fuel rods and to provide a flow channel between the fuel rods [1]. Each grid is an interconnected array of slotted grid straps that has springs and dimples, to form an egg crate structure.

In this paper, three-dimensional finite element analysis is performed to obtain the characteristic curve of the spacer grid spring uniquely designed by KAERI [2,3]. Several boundary conditions are considered to simulate the characteristic test. The appropriate boundary condition and the analysis procedure for obtaining the characteristic curve of the grid spring are proposed by comparing the finite element results with the test results. The developed analysis procedure will be adequate for predicting the characteristic curve of the grid spring. The minor shape change is also made to overcome a shortcoming of the new grid spring that its maximum load is relatively small for the established springs. A good shape design is found and the characteristics of the final design are compared with those of the initial design.

The unit cell specimen was prepared to perform the characteristic test of the spacer grid spring. The load-displacement curve (characteristic curve) of the spring is made, and the stiffness and maximum load of the grid spring are determined by the characteristic curves obtained from several tests.

Three-dimensional finite element analysis is performed for simulating the characteristic test of the spacer grid spring. In order to predict the characteristic test results well by finite element analysis, several boundary conditions are considered to represent the test condition and the appropriate boundary condition is chosen. The characteristic curve of the spacer grid spring obtained from the finite element analysis agrees well with the test result in the linear range while there is a little difference between two results in the nonlinear and plastic range. Therefore, it can be stated that the present finite element analysis is verified for predicting the characteristic curve of the spacer grid spring within the small range of error.

Despite the superior wear characteristic, the spacer grid spring newly designed by KAERI has a shortcoming that it may lose the role as a spring for the application of the sudden load due to its small maximum load. The grid spring supports the fuel rod mainly by the lower part of spring. The lower part is the configuration of the beam with both ends fixed, and thus it is expected that the maximum supporting load can be increased if the length of the lower part of spring is decreased.

The initial design may not support the fuel rods effectively for the sudden load, but the shortcoming can be overcome if the maximum load is increased over 50N by decreasing the length of the lower part below 16mm. A good shape design for which the maximum load is increased with satisfying the stiffness guideline and not decreasing the linear range is found.

1

Y.H. Jung et al., "Development of Fuel Performance and Thermal/Hydraulic Technology", Korea Atomic Energy Research Institute, KAERI/RR-2015/99, 1999.

2

H.K. Kim, H.S. Kang, K.H. Yoon, K.N Song, "Experimental Investigation on the Parameters Affecting Fuel Fretting Wear", SMiRT 16, Paper 1548, 2001.

3

K.H. Yoon et al., "Spacer Grid with H-spring for Fuel Rods for Use in Nuclear Reactor Fuel Assemblies", US Patent 6167105, 2000.

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