[en] The Idaho National Engineering Laboratory (INEL) is actively involved in the development and application of fiberoptic-based instrumentation for use in nuclear reactor research. To aid in this development effort, a testing program has been undertaken to quantify optical losses in promising waveguide materials during radiation exposure at elevated temperatures characteristic of reactor environments. This paper reports results of testing conducted during the fuel-conditioning phase of the Severe Fuel Damage Test 1-1A performed at the INEL's Power Burst Facility. Radiation-resistant fiberoptic samples were obtained from five manufacturers: the Spectran Corporation, the Quartz Products Corporation, Dainichi-Nippon Cables, Ltd., Fiberguide Industries, and Hughes Research Laboratories. Two identical sets of samples were placed in a specially designed test fixture and positioned approximately 60 centimeters above the reactor core. One was held at a temperature of 200⁰C, while the second remained at 20⁰C. The radiation-induced optical attenuation was monitored in several spectral bands in the 600 to 1100 nanometer region. The in situ measurements were recorded over a 125 hour period during which the reactor power varied between 0.1 to 22 MW thermal. The samples were exposed to average fluxes of 2.8 x 10⁹ n/cm²/sec thermal neutrons, 4.4 x 10⁸ n/cm²/sec fast (> 1 MeV) neutrons, and dose rates of approximately 10⁶ rad/hr gamma radiation. The growth and recovery of radiation-induced losses as a function of wavelength, temperature, and reactor power level are presented