Research Papers: Nuclear Power

Density Change of an Oxidized Nuclear Graphite by Acoustic Microscopy and Image Processing

[+] Author and Article Information
Se-Hwan Chi

Nuclear Hydrogen Development and Demonstration Project, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Daejeon, 305-353 South Koreashchi@kaeri.re.kr

Cristian I. Contescu

Materials Science and Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831contescuci@ornl.gov

Timothy D. Burchell

Materials Science and Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831burchelltd@ornl.gov

J. Eng. Gas Turbines Power 131(5), 052904 (Jun 10, 2009) (4 pages) doi:10.1115/1.3098415 History: Received November 05, 2008; Revised November 18, 2008; Published June 10, 2009

The strong correlation between the density and the physical and mechanical properties of graphite suggests that the method of nondestructive density evaluation could be developed into a characterization technique of great value for the overall improvement of the safety of graphite moderator reactors. In this study, the oxidation-induced density changes in nuclear graphite for very high temperature reactor were determined by a conventional destructive bulk density measurement method (BM) and by a new nondestructive method based on acoustic microscopy and image processing (AM). The results were compared in order to validate the applicability of the latter method. For a direct comparison of the results from both measurements, two specimens were prepared from a cylindrical graphite sample (1 in. diameter and 1 in. height, oxidized to 10% weight loss at 973 K in air for 5 h). The specimens were used for characterization by BM and AM methods, respectively. The results show that, even with a large standard deviation of the AM, the density changing trend from both methods appeared the same. The present observation may be attributed to the fact that AM images reflect characteristic density changes of the graphite sample through the acoustic impedance changes. This study demonstrates the possibility of using AM as a nondestructive technique for the evaluation of density changes in graphite when a database is prepared through a systematic series of experiments.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Schematics of specimen preparation for AM and BM

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Figure 2

Schematic of scanning acoustic microscopy. Acoustic waves generated by the transducer pass through the sapphire rod. The curvature at the end of the rod focuses the plane waves to a point on or below the sample surface. Blue ray is incident at Rayleigh angle (θR) on the sample. These rays generate Rayleigh waves, which travel along the blue line to the transducer. The axial ray (green) is a normally reflected ray. These two rays together contribute to the impedance information. The AM microstructural image is produced from the interference patterns between normally reflected longitudinal waves and the Rayleigh waves (8).

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Figure 3

Schematics of the acoustic microscopy plan on the AM specimen cross section for density estimation

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Figure 4

Density gradient determined by BM and AM machining methods in the 10% burn-off NBG-10 nuclear graphite in air at 973 K

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Figure 5

(a) Acoustic microscope images obtained at 180 deg surface (W0) and (b) at the center of specimen in Fig. 3. Differences in the intensity are observed. It is seen that (b) of the center is brighter (higher intensity) than (a) of the surface.

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Figure 6

Reproduced Fig. 1 with a large standard deviation for AM data



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