Tristructural Isotropic Research Articles

Optical anisotropy measurements of TRISO nuclear fuel particle cross-sections: The method

The analysis of two-modulator generalized ellipsometry microscope (2-MGEM) data to extract information on the optical anisotropy of coated particle fuel layers is discussed. Using a high resolution modification to the 2-MGEM, it is possible to obtain generalized ellipsometry images of coating layer cross-sections with a pixel size of 2.5 μm and an optical resolution of ∼4 μm. The most important parameter that can be extracted from these ellipsometry images is the diattenuation, which can be directly related to the optical anisotropy factor (OAF or OPTAF) used in previous characterization studies of tristructural isotropic (TRISO) coated particles. Because high resolution images can be obtained, the data for each coating layer contains >6000 points, allowing considerable statistical analysis. This analysis has revealed that the diattenuation of the inner pyrocarbon (IPyC) and outer pyrocarbon (OPyC) coatings varies significantly throughout the layer. The 2-MGEM data can also be used to determine the principal axis angle of the pyrocarbon layers, which is nearly perpendicular to the TRISO radius (i.e., growth direction) and corresponds to the average orientation of the graphene planes.

06/02129 Recursively coupled thermal and mechanical FEM–analysis of lower plenum creep failure experiments: Willschütz, H. -G. et al. Annals of Nuclear Energy, 2006, 33, (2), 126–148.

A conceptual design study for a small-pressurized water reactor (PWR) core has been presented in this research paper that utilizes carbon coated fuel particles with a novel composition. The utilization of novel tristructural-isotropic (TRISO) fuel particles can completely eliminate the use of soluble boron system and requirement of burnable poison if adequate number of control rods is used. Apart from the simplification of the design by excluding such as complex systems from the design, the utilization of TRISO fuel particles in PWR technology can enhance the design integrity by confining the radioactive fission products within fuel itself during reactor operation. Reactor simulation computer codes WIMS-D/4 and CITATION were used for neutronics design and RELAP5 was used for thermal hydraulic analysis. The designed core is small which has only 25 MWth power out put with more than 2 EFPY operating life. The values of Doppler, moderator and void reactivity coefficients were found negative. The Soluble Boron Free (SBF) and Burnable Poison Free (BPF) concepts are more viable in small and medium size reactors (SMRs) because it makes the plant simpler and uniform burnup can also be achieved throughout the core life.

Optical characterization of tristructural isotropic fuel particle cross-sections using generalized ellipsometry

One particularly important measure of quality of tristructural isotropic (TRISO) fuel particles is the degree of preferred orientation of crystallites in the polycrystalline pyrolytic carbon coatings. Excessive crystallographic anisotropy leads to unwanted anisotropic dimensional changes during irradiation that can cause the TRISO coatings to fail. Early optical methods were developed in the 60s and 70s to measure this anisotropy by taking advantage of the large optical anisotropy of graphite. Since that time, there have been significant improvements in both the theoretical understanding and experimental techniques in the understanding of optical anisotropy. Here we discuss a new method, based on the two-modulator generalized ellipsometer (2-MGE) to measure the optical anisotropy. This technique has been demonstrated to measure the optical diattenuation to an accuracy from ±0.001 to ±0.005 and the preferred direction of the crystallites to an accuracy of better than ±2° with a spatial resolution of better than 5μm. Diattenuation ‘pictures’ of the nuclear fuel cross-sections reveal that the inner pyrocarbon layer (IPyC) is far from uniform both in the degree of diattenuation and in the direction of the principal axis. The 2-MGE technique is faster, more accurate, and collects considerably more data than previous optical anisotropy measurements of TRISO fuel particles.

96/03844 Viscosity of coal-derived liquids. 1. A group contribution method for pure model compound liquids

96/03844 Viscosity of coal-derived liquids. 1. A group contribution method for pure model compound liquids

Permeability of refractories to air

Very High Temperature Reactor (VHTR) fuels consist of the bistructural-isotropic (BISO) or tristructural-isotropic (TRISO)-coated particles embedded in a graphite matrix. Management of the spent fuel generated during VHTR operation would most likely be through deep geological disposal. In this framework we investigated the alteration of BISO (with pyrolytic carbon) and TRISO (with SiC) particles under geological conditions simulated by temperatures of 50 and 90 °C and in the presence of synthetic groundwater. Solid state (scanning electron microscopy (SEM), micro-Raman spectroscopy, electron probe microanalyses (EPMA) and X-ray photoelectron spectroscopy (XPS)) and solution analyses (ICP-MS, ionique chromatography (IC)) showed oxidation of both pyrolytic carbon and SiC at 90 °C. Under air this led to the formation of SiO2 and a clay-like Mg–silicate, while under reducing conditions (H2/N2 atmosphere) SiC and pyrolytic carbon were highly stable after a few months of alteration. At 50 °C, in the presence and absence of air, the alteration of the coatings was minor. In conclusion, due to their high stability in reducing conditions, HTR fuel disposal in reducing deep geological environments may constitute a viable solution for their long-term management.