Masters Theses

Date of Award

12-1998

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl J. McHargue

Committee Members

Joseph E. Spruiell, Thomas T. Meek

Abstract

The advent of third generation synchrotrons and the ongoing revolution in x-ray focusing optics have contributed to the development of x-ray probes with micron and submicron resolution. As a result, it is now possible to perform micro-structural analysis of materials with techniques previously not possible. In this thesis, x-ray fluorescence microtomography, a comparatively new technique, coupled with a synchrotron x-ray microprobe of micron resolution were used to study the trace element distributions in a SiC shell of an advanced nuclear fuel particle, known as TRISO. The TRISO fuel particles contain a small kernel of nuclear fuel encapsulated by alternating layers of C and a barrier layer of SiC. The SiC shell provides the primary barrier for radioactive elements in the kernel and metallic fission products. The performance of this barrier under adverse conditions is key to containment.

The tomographic measurements were made with an~ 1x3 μm2 x-ray probe on beamline 2-ID at the Advanced Photon Source (APS). The distribution of trace elements in the SiC shell was reconstructed after correcting the data for artifacts arising from absorption and scattering off the kapton tape used to encapsulate the sample. The tomographic reconstructions were carried out, both with and without absorption correction. The images obtained in both reconstruction cases show that the SiC shell is ~38 μm thick, and the observed trace elements are distributed in small <1 μm regions through the SiC shell. The results also indicates that most trace elements have localized distributions within the SiC shell. The trace elements can be attributed to radiation enhanced diffusion of elements in the kernel or to trace elements introduced during fabrication. The investigation of the reconstructed images with absorption correction indicates that for low energy fluorescence lines the effect of SiC absorption is rather significant in estimating the concentration of trace elements.

The overall results of this research study verifies that X-ray fluorescence microtomography is an ideal tool for detecting high Z trace elements in a low Z matrix because it is a sensitive and penetrating nondestructive probe and because it provides a picture of the elemental distribution with micron spatial resolution. The advantages of x­-ray fluorescence tomography for the non-destructive 3-D characterization of materials will continue to grow as x-ray source brilliance increases.

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