تجزیه و تحلیل مقاومت فشاری شبکه جداکننده و رفتار سازه ای تجمع سوخت

In this work, finite-element models were proposed to evaluate the spacer grids compression strength and structural behavior of fuel assemblies, mainly in terms of their natural frequencies. Firstly, a three-dimensional model was developed to provide consistent predictions of 16 × 16-type spacer grids compression strength. Regarding their original geometry and some possible design variations, the models were submitted to compression conditions to calculate the maximum compression force and they were validated for comparison with experimental predictions. Secondly, fuel assembly models were proposed with the aim at to correct its natural frequencies. For that, two distinct three-dimensional finite element approaches for the spacer grids, called total mesh and inner mesh, were adopted, respectively. For each model, the maximum and minimum fuel assembly lateral stiffness was determined. Also, by adopting the correction factor β, the natural frequencies were corrected by a View the MathML sourceβ value that was characteristic of each model and compared to experimental results. The procedure used in the present work permitted a good agreement between numerical and experimental natural frequencies results with the total mesh model.

Fuel assemblies are defined by Gouvêa et al. (2000) like arrangements of fuel rods containing uranium, mounted like a spaced and reticulate bundle. The fuel assembly and its components studied in this work can be observed in details in Fig. 1. Their most important structural components are the spacer grids due its great strength against static and dynamic loads, during manufacturing and operation inside nuclear reactors. Thus, in order to prevent breakdown of spacer grid failure, it is important to know the structural behavior of the fuel assemblies and to estimate their lateral strength, as well as their lateral stiffness and natural frequencies, as observed in the pioneer work developed by Lee et al. (1890) by as pointed out by Medeiros (2005). Likewise in a most recent research, Yoon et al. (2001) proposed a numerical method for predicting the buckling strength of the spacer grids by adopting a nonlinear dynamic finite element model. In the same way, Yoon et al. (2001) have performed a physical test and a numerical simulation to predict the buckling behavior on the spacer grid structure. Following these thoughts, Kang et al. (2001) also studied the strength of spacer grids by evaluating their interaction with fuel rods.The work proposed by Jhung et al. (1992) revealed experimental and numerical results of spacer grids structural behavior by reproducing the actual nuclear reactor operational conditions. In the same context, Park et al. (2003) evaluated an axiomatic design method to achieve a new shape for the spacer grid. However, in these studies the effect promoted by the loads during manufacturing of this component was neglected. It is well known that the control of the spacer grid straightness and the fuel assembly vibrations during the nuclear energy generation process plays an important role on its structural integrity. Therefore, the determination of strength limits for spacer grids deformation, before and after the process conditions, and the fuel assembly natural frequencies are necessary. In this sense, finite element-based simulations represent a reasonable alternative for that when compared to the high costs associated to experimental tests. In this context, distinct three-dimensional finite element models have been developed in the present work. Initially, it is proposed a model able to evaluate spacer grids behavior during fuel assembly manufacturing. This assessment is done by calculating the maximum compression force which it structural component undergoes. After that, a second model is idealized to analyze the fuel assembly structural behavior and extract its natural frequencies.

After the numerical model calibration, the results presented are coherent with the behavior and the value is quite similar to the experiment one. Thus, it could be stated that: • Deformed geometry after compression: the deformed shape and the location of the displaced cells of the numerical model are consistent and very similar to what occurs in reality. • The maximum critical force obtained by numerical simulation shows a very good agreement with the physical test once it was only 0.4% higher than experimental prediction. • When the force applied in the model for each step increase due to the displacement of the upper bar, the graph shows a deviation when compared to what actually occurs, although the behavior of the curves is consistent with reality, since the numerical model is more idealized, i.e. the model is constructed without any imperfections that a real physical system presents, due to some features related to the manufacturing and assembly process of the grid spacer. • The correction of fuel assembly natural frequencies by considering fuel rod sliding permitted a good agreement with experimental results by using the total mesh model. The inner mesh model provides low quality results due to the high flexibility observed when the Inconel 718 straps were not considered. In the case of the inner mesh, the corrected natural frequencies were lower than experimental predictions due to the greater flexibility. Therefore, this model does not reproduce correctly the fuel assembly structural behavior.