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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 83
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Paper 190

Estimation of the Dynamic Buckling Strength of a Spacer Grid Assembly for PWRs Using a Finite Element Model

K.N. Song and S.H. Lee

Development of a High Performance Spacer Grid Assembly, Korea Atomic Energy Research Institute, Daejon, Korea

Full Bibliographic Reference for this paper
K.N. Song, S.H. Lee, "Estimation of the Dynamic Buckling Strength of a Spacer Grid Assembly for PWRs Using a Finite Element Model", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 190, 2006. doi:10.4203/ccp.83.190
Keywords: crush strength, spacer grid assembly, nuclear fuel assembly, PWR.

Application of this study is to estimate the dynamic buckling strength of a spacer grid assembly, which is an interconnected array of slotted grid straps and welded at the intersections to form an egg crate structure. The spacer grid assembly is one of the core structural components of the nuclear fuel assemblies of a pressurized light water reactor (PWR). In a PWR, the fuel assembly consists of several spacer grids, lots of fuel rods, one top nozzle, one bottom nozzle; several guide tubes, and an instrumentation tube as shown in Figure 1. The primary function of the spacer grid assembly is to support and protect the fuel rods from external impact loads in an abnormal operating environment such as an earthquake or a loss-of-coolant accident (LOCA). And the spacer grid assembly must maintain the guide tubes (or fuel skeleton structure) straight so as not to impede a control rod insertion under any normal or accidental conditions. Moreover, the spacer grid assembly must maintain the instrumentation tube straight so that a plant's neutronic instrumentation can be freely inserted and removed from the tube even after design lateral loading conditions. Therefore a plastic deformation of the spacer grid assembly needs to be limited and it must be designed to have enough impact lateral strength [1].

In this study, both dynamic crush strength tests and a finite element analysis of the spacer grid assembly specimens as shown in Figure 1 were carried out. Impact analyses and tests are carried out both on the Korea Atomic Energy Research Institute (KAERI) designed spacer grid assembly and on a commercial spacer grid assembly. A finite element model for the spacer grid assembly is constructed with four-node shell elements for the grid straps and 4-node tetrahedral solid elements for the welding points at the intersection of the grid straps. The non-linear dynamic analysis is performed using ABAQUS/EXPLICIT [2]. The impact hammer is modelled as a rigid element, which has an equivalent mass of the hammer, in the finite element analysis. The external impact load is modelled for the initial impact velocity at the reference node of the upper rigid surface, which is located in the centre. The pendulum type impact tester, which was developed at KAERI [3], is used for the dynamic buckling test on the spacer grid assembly. The dynamic crush strength tests were performed both at room temperature and at 320oC by using a pendulum-type impact tester. A comparison of the results between the impact test and the finite element analysis is carried out.

As a result of the comparisons, the finite element analysis results were in good agreement with the test results to within an 8% difference range. Therefore, we could estimate the dynamic behaviour of a spacer grid assembly in advance before performing the dynamic crush strength test. We have proposed an impact analysis model for the spacer grid assembly of PWRs by using shell elements for the straps and solid elements for the welding points.

Figure 1: PWR fuel assembly and spacer grid assembly.

H.J. Kunz, R. Schiffer, and K-N Song, Fuel Assembly Mechanical Design Manual, Siemens/KWU Work-Report U6 312/87/e326, 1987.
H.D. Hibbit, G.I. Karlsson and E.P. Sorensen, ABAQUS/Explicit Users' Manual Version 6.5, 2005.
K-H Yoon, Korea Patent 0380885, 2000.

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