SiC Based Neutron Flux Monitors for Very High Temperature Nuclear Reactors
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0929-II03-04
SiC Based Neutron Flux Monitors for Very High Temperature Nuclear Reactors Wolfgang Windl1, Behrooz Khorsandi2, Weiqi Luo1, and Thomas E. Blue2 1 Materials Science and Engineering, The Ohio State University, Columbus, OH, 43210 2 Nuclear Engineering, The Ohio State University, Columbus, OH, 43210 ABSTRACT The Gas Turbine-Modular Helium Reactor (GT-MHR) and the Very-High-Temperature Reactor (VHTR) are next-generation high-temperature reactor types that are being designed to operate under normal conditions with primary coolant outlet temperatures in the range of 850 °C and 1000 °C, respectively. A new type of silicon carbide based diode neutron detector is currently under development in order to monitor the neutron flux in this environment. An important problem, in this context, is the long-time reliability of the diodes under continuous irradiation at high temperatures. In this paper, we discuss a computational methodology to study the accumulation of radiation damage in the detectors as a function of temperature and its influence on the electrical properties. INTRODUCTION The Gas Turbine-Modular Helium Reactor (GT-MHR) is designed to operate with an outlet coolant temperature of approximately 850 oC. The Very-High-Temperature Reactor (VHTR) is being designed to operate with even higher outlet coolant temperatures. If a detector were to be placed within the GT-MHR or VHTR cores, it would be exposed to a very harsh environment, in the sense that the detector would be exposed to high temperatures and high neutron fluences. If neutron detectors are to be placed in environments that are as harsh as these, then novel detectors need to be developed to monitor the neutron flux. Because of its superior materials properties, we are investigating a hexagonal polytype of silicon carbide, 4H-SiC, as the base material for diode detectors. It is our hope that such detectors may be designed to survive long-time irradiation (at least one refueling cycle, i.e. 15.7 months for GT-MHR) without too much degradation in their electrical properties. This might allow them to be placed inside the reactor core, where they can be used to gather information regarding the neutron flux (and power) spatial distributions during reactor operation. With a more detailed knowledge of the neutron flux and power spatial distributions, engineering margins may be decreased, thus allowing for increases in the power level at which the reactor may be safely operated and thus reactor profitability. In comparison to silicon (Si), SiC is a wide-band gap (3.2 eV for 4H-SiC) and radiation-hard semiconductor material. However, like Si, the properties of SiC change with irradiation by energetic neutrons, which cause displacement damage. As the neutrons collide with the atoms of the semiconductor, they may transfer enough energy to Primary Knock-on Atoms (PKAs) to displace them from their original sites and may, if sufficient energy is transferred to the PKA, initiate the creation of a damage cascade as the PKA collides with other atoms and moves them f
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