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[en] This paper presents historical results of graphite irradiation-induced creep experiments that were performed at Oak Ridge National Laboratory from the 1950's to the 1970's. These experiments were performed at temperatures from 150°C to 1000 °C, and bend stresses ranging from 500 to 5000 psi (~3.3–34.5 MPa). The experimental setup utilized in-situ measurement of specimen displacement, on-line applied stress control, and the ability to change stress during the experiment. The different stress conditions showed that the primary creep strain and the steady-state creep rates both have a linear stress dependence. The temperature range used in this work resulted in trends that have not be previously presented in the literature: 1) a linear dependence of primary creep strain on temperature, and 2) the shape of steady state creep rate versus temperature (see graphical abstract). The maximum dose in the specimens was 0.9 dpa, which is sufficient to achieve steady-state creep without the structural changes that alter the observed creep behavior. The results from this experiment provide evidence that dispels that the pinning-unpinning model describes the mechanism of irradiation creep in graphite. Instead these results suggest a dislocation climb mechanism is the probable mechanism for creep within the crystalline regions.
[en] Graphite is used as a moderator of fast neutrons in some types of nuclear reactors and for other industrial applications. The influence of smaller pores on the mechanical and physical properties of graphite remains to be fully understood. In this work, focused ion beam-scanning electron microscopy (FIB-SEM) tomography was applied to characterise the porosity of AGX graphite – an electrode material. FIB-SEM tomography consists in alternating the ion milling and SEM imaging at an area of interest with the objective of creating a 3D reconstruction. Regions containing a filler and a mixture of filler and binder were selected as the areas of interest. Resolutions of a few nanometers were achieved in volumes up to 1400 μm3 for both regions. Typical porous structures were detected in the filler and binder regions such as thermal cracks, gas evolution porosity and lenticular pores. The resolution achieved with these experiments made possible detection of pores smaller than 150 nm in diameter, that is of the length scale of voids generated by neutron irradiation, and an improvement in spatial resolution of traditional x-ray tomography studies. The resolution achieved by FIB-SEM tomography may be essential for the study of the microstructure of graphite, providing complimentary data to x-ray tomography experiments.
[en] Highlights: • Annealing of SiC via continuous dilatometry to determine irradiation temperature. • Wrote a program to analyze dilatometry results to determine irradiation temperature. • Dilatometry results are consistent with results from a historical technique. • Computer program was written in an open-source language and is available for others. - Abstract: Silicon carbide is used as a passive post-irradiation temperature monitor because the irradiation defects will anneal out above the irradiation temperature. The irradiation temperature is determined by measuring a property change after isochronal annealing, i.e., lattice spacing, dimensions, electrical resistivity, thermal diffusivity, or bulk density. However, such methods are time-consuming since the steps involved must be performed in a serial manner. This work presents the use of thermal expansion from continuous dilatometry to calculate the SiC irradiation temperature, which is an automated process requiring minimal setup time. Analysis software was written that performs the calculations to obtain the irradiation temperature and removes possible user-introduced error while standardizing the analysis. This method has been compared to an electrical resistivity and isochronal annealing investigation, and the results revealed agreement of the calculated temperatures. These results show that dilatometry is a reliable and less time-intensive process for determining irradiation temperature from passive SiC thermometry.
[en] This objective of this work was to develop an experimental facility that can perform in situ high temperature proton irradiation-induced creep experiments on a range of materials. This was achieved by designing an irradiation chamber and stage that allows for load application and removal, provides a method for controlling and monitoring temperature and proton flux, and a means to make in situ measurement of dimensional change of the samples during the experiment. Initial experiments on POCO Graphite Inc. ZXF-5Q grade ultra-fine grain samples irradiated at 1000 °C at a damage rate of 1.15 × 10−6 dpa/s exhibited a linear dependence of measured creep rate on applied stress over a range of stresses from 10 MPa to 40 MPa
[en] Obtaining accurate diffusion kinetics in materials representative of those found in tristructural-isotopic (TRISO) coated particle fuel is needed to predict the diffusive release of the same fission products in reactor. Planar diffusion couples with representative pyrocarbon (PyC) and silicon carbide (SiC) layers are being produced using the same fluidized-bed chemical vapor deposition (FB-CVD) technology used to produce TRISO particles from the first irradiation experiment of the Advanced Gas Reactor Fuel Qualification and Development Program (AGR). The layer properties of the planar diffusion couples are tailored to meet the specified PyC density and microstructure of the SiC layer as defined by the AGR program. The influence of these variables on diffusion is also being explored by producing PyC and SiC variants. The pathway to producing the diffusion couples is discussed.