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The Idaho National Engineering Laboratory is actively involved in the development and application of fiberoptic-based sensor technology for use in nuclear reactor research. One such sensor is a minature fiberoptic probe which senses steam-to-water transitions in a high pressure, high temperature coolant circuit.' Hundreds of these probes have been utilized in non-nuclear reactor simulation facilities and efforts are underway to adapt these and other fiberoptic techniques to full-scale nuclear applications. This paper describes work recently completed to characterize fiberoptic attenuation induced by gamma and neutron radiation in a reactor. The specific goal was to make in-situ attenuation measurements of promising waveguide materials during radiation exposure at elevated temperatures characteristic of reactor environments. Testing was done in a swimming-pool-type reactor generating about one kilowatt thermal power and producing fluxes of about 109n/cm2/sec fast neutrons (>1 MEV), 1010 on /cm2/sec thermal neutrons, and dose rates of 6 X 106 rad/hr gamma radiation. Samples of state-of-the-art radiation-resistant waveguides, received from five manufacturers, were tested simultaneously. One set of waveguides was held at 180°C temperature and the other set remained at 20°C. Attenuation was monitored con-tinuously in several spectral bands in the 600-1050 nanometer region. Total reactor exposure time was about ten hours and the change in attenuation at this time for the heated samples was as low as 5 db/km at the longer wavelengths and as high as 3000 db/km at the shorter wavelengths. Attenuation in this range is acceptable for many instrumentation or sensor applications because transmission distances would be relatively short in the radiated region. The annealing effect at elevated temperature was found to be significant for all waveguide samples. A ten-fold decrease in attenuation was observed for one heated sample in comparison with its counterpart at ambient temperature (i.e., for 160oC temperature difference). Additional testing is underway in a 26-megawatt reactor facility. Higher dose rates, exposure times, and temperatures are planned.
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