The characterization of a tri-isotropic (TRISO) coated particle by using a Focused Ion Beam Scanning Electron Microscope (FIB SEM) is illustrated. TRISO particles are used as fuel in high temperature gas reactors and the quality of the coating layers surrounding the kernel is important for the safety of the reactor. It is shown that the FIB SEM is an ideal instrument for the preparation and investigation of TRISO particles.

Introduction

Current high temperature gas reactor designs use tri-isotropic TRISO-coated particles as fuel. The TRISO-coated particle consists of a fuel kernel and coating layers of porous pyrolitic carbon (PyC), inner high-density PyC (IPyC), silicon carbide (SiC) and outer high density PyC (OPyC). These coating layers act as a pressure vessel for the fission product gases as well as a barrier to the release gaseous and metal fission products [1]. The release of metallic fission products from TRISO-coated particles during postirradiation heating tests has been reported by Minato and co-workers [2] and Bullock [3].

During irradiation the gas pressure increases in the particle, however the inner PyC layer starts to shrink during irradiation, pulling radially inwards on the SiC layer [1]. If debonding between the inner PyC and SiC layers occurs, it can lead to the early failure of the fuel particle resulting in the rapid release of fission products. Since the safety of gas-cooled reactors depends on the quality of the layers in the TRISO-coated particles, the characterization of the coating layers in the TRISO particles is important for the quality control of the fuel. There is an interest to use the TRISO-coated particle fuel also in the fuel rods of pressurized water reactors (PWRs) which will improve the safety of the PWR technology by preventing the release of fission products [4].

The investigation of the structural integrity of the coated particles may conveniently be done by using scanning electron microscopy (SEM) after removing one hemisphere of the particle by mechanical grinding and polishing. However, very often the grinding procedure may introduce damage such as debonding of the layers or surface cavities in the polycrystalline SiC layer resulting in the inconclusive evaluation of the coated particle.

This paper presents results obtained from the investigation of a TRISO particle, with a ZrO₂ kernel, by using a Helios NanoLab focused ion beam (FIB) SEM.

Results

Figure 2 shows a cross-sectional FIB SEM image of the TRISO-coated particle with a FIB milled channel running from the SiC layer (outer layer) to the kernel (centre of particle). In this particle the outer PyC layer was lost during the grinding/polishing process. The three coating layers starting from the inside buffer layer, followed by the inner PyC and finally the SiC layer are indicated by the arrows. The cracks extending through the layers were most likely introduced by the mechanical grinding and polishing of the TRISO particle prior to FIB SEM analysis.

In figure 3 an image of the interface region between the buffer layer and kernel are shown (concentric backscattered mode) with the cavities in the buffer layer and ZrO₂ grains in the kernel clearly visible. The separation of kernel and buffer layer can also be seen.

Figure 4(a) shows the interface between the SiC and IPyC (concentric back scattered mode) indicating significant intergrowth of the two layers, which would result in strong bonding between the different layers. However, the high density of cavities at the IPyC/SiC interface is not ideal since it could act as stress concentrators leading to the initiation of cracks. The interface between the SiC and PyC may also be imaged in dark field (DF) scanning transmission electron microscopy (STEM) mode, as shown in figure 4(b), which reveals more microstructural detail of the PyC and SiC. Microtwins in SiC have usually been observed by transmission electron microscopy.

However, the availability of the STEM detector in the Helios NanoLab allows the imaging of microtwins in the FIB SEM as shown in figure 5 which is a high magnification bright field (BF) STEM image of the SiC grains with microtwins in the SiC layer clearly visible.

Conclusions

In this paper, the usefulness of a FIB SEM for the initial characterization of a coated particle has been illustrated. The investigation of the structural integrity of the coated particles may conveniently be done by FIB SEM without introducing structural damage by conventional grinding/polishing procedures.

Acknowledgements
The author wishes to thank Dr D. Wall from FEI Company in Eindhoven for preparing the FIB specimens and for recording the images.

References
[1] Miller G.K. et al.: J. Nucl. Mater. 317, 69-82 (2003)
[2] Minato K.et al.: J. Nucl. Mater. 202, 47-53 (1993)
[3] Bullock R.E. 1984. J. Nucl. Mater. 125, 304-319 (1984)
[4] Hussain A. et al.: Progr. Nucl. Energ. 52, 531-535 (2010)