High Temperature 2-D Millimeter-Wave Radiometry of Micro Grooved Nuclear Graphite
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High Temperature 2-D Millimeter-Wave Radiometry of Micro Grooved Nuclear Graphite Paul P. Woskov1 and S. K. Sundaram2 1 MIT Plasma Science and Fusion Center, 167 Albany Street, Cambridge, MA 02139, U.S.A. 2 Alfred University, Kazuo Inamori School of Engineering, 2 Pine Street, Alfred, NY, 14802, U.S.A. ABSTRACT A dual 137 GHz heterodyne radiometer system was used to study grooved nuclear grade graphite (SGL Group NBG17) inside an electric furnace from room temperature to 1250°C. The millimeter wave radiometer views were collinear with the electric field of one polarized parallel, and the other perpendicular, to the grooves. The anisotropic emissivity was readily detected for 100 μm wide grooves of various depths with a spacing period of 0.76 mm. The emissivity in the 500 – 1250°C temperature range was found to be 5.1 ± 0.5% when the E-field was parallel to the grooves and a factor of 2 – 4 higher, depending on groove depth, in the perpendicular direction. The parallel surface emissivity which was identical to ungrooved surface emissivity corresponded to a 137 GHz surface resistance of 5.3 Ohms, which is about 2.5 times higher than the value predicted from frequency scaling dc surface resistance. The perpendicular emissivity had a modulation with groove depth at odd integral multiples of ¼Ȝ SUHGLFWHG E\ electromagnetic finite difference time domain analysis. INTRODUCTION Future high temperature nuclear reactors with potential temperature excursions to 1200qC will present new challenges for hot fuel element and structural materials in a high neutron irradiation environment. Graphite, much used for reactors in the past, will continue to be an important material for new reactors because of its very-high-temperature strength, thermal conductivity, and neutron irradiation resistance capabilities [1, 2]. Extending its use into the more extreme environments of future reactors is motivating more studies of graphite properties and the development of new diagnostic tools. A new high temperature measurement capability is needed for resolution of anisotropic properties that can occur either from a material structure that is asymmetric such as a fiber-reinforced matrix composite or from an initial isotropic composition that evolves under non uniform thermal, neutron, and/or structural stresses. For example, fractures will grow approximately linearly in the direction of the least stress [3, 4] causing a dynamically varying anisotropy in the material. Current analysis methods such as xray imaging [3] or nondestructive ultrasound [5] are used in laboratory fracture studies, but are not generally applicable to high temperature in situ measurements. Millimeter-wave (MMW) radiometry can provide direction-resolved remote measurement of material emissivity and submillimeter dimensional changes that can be directly correlated with material temperature, resistivity, and swelling [6, 7]. MMWs refer to the electromagnetic wavelength range of 10 – 0.1 mm that can be guided efficiently at high temperature to remote locations. MMW coherent
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