Contact Author:
Dr. Young-Ho Lee
leeyh@kaeri.re.kr
P:+82-42-868-8761
F:+82-42-863-0565
150 Dukjin-dong Yuseong-gu
Daejeon 305-353 KOREA
Daejeon, 305-353
KOREA
The Necessity of a New Type Test Rig for the Development of an Evaluation Method in Grid Fretting Problems
Young-Ho Lee, Hyung-Kyu Kim (KAERI)
The advanced nuclear fuel assemblies have been investigated and developed by many fuel vendors and utilities in order to improve the safety and reliability of the pressurized water reactors (PWRs). As a result, the major degradation mechanisms such as crud/corrosion, debris, manufacturing error, etc. are almost disappeared. Among these damages, however, a grid fretting problem has not been investigated enough to apply the examination results of fretting experiments to the development and design of spacer grid structures. This is because it is difficult to develop the fretting wear model for the grid fretting problem due to the various wear mechanisms according to the mechanical and environmental variables, the contact condition with spring/dimple and the material properties. In addition, demands for a higher burn-up, plant power uprates and longer operating cycles result in the severe operating conditions and finally it is expected that the grid fretting is one of the main root causes in PWR fuel damages.
In general, it is well known that the change of the contact condition between the fuel rod and spring is relatively easy to apply for the development of a wear-resistant spacer grid with the consideration of the economy and the development period. As a result, a number of the spring shapes was developed in KAERI and their performance tests have been carried out from a part unit to a full assembly scale. In order to evaluate the wear resistance of the developed spring shapes, in the previous study, fretting wear experiments have been performed with a part unit of various types of spacer grid spring/dimple in room temperature air and distilled water. One of the noticeable results is that the contacting force (normal load) was gradually decreased with increasing fretting cycle due to the depth increase and this behavior closely related to the contacting spring shape. Also, it is found that a spring shape with a relatively higher stiffness value shows a rapid decrease of normal load because an amount of elastic deformation is in inverse proportion to the spring/dimple stiffness.
When considering the actual contact condition between fuel rod and spring/dimple, if fretting wear progress due to the flow-induced vibration (FIV) under a specific normal load exerted on the fuel rod by the elastic deformation of the spring, the contacting force between the fuel rod and dimple that were located in the opposite side should be decreased. Generally, an excess wear due to gross slip progresses under a relatively small normal load. Consequently, the evaluation of developed spacer grids against fretting wear damage should be performed with the results of a cell unit experiments because the contacting force is one of the most important variables that influence to the fretting wear mechanism. Therefore, it is necessary to develop a new type of fretting test rig in order to simulate the actual contact condition. The objective is to examine the difference between the fretting wear behaviors of the fuel rod according to the part unit test and the cell unit one with a concave shape spring. The discussion was focused on the development procedure of a new test rig, the variation of the contacting forces during the fretting wear tests and fretting wear behavior by using both the part unit test rig and the cell unit one.