Cerenkov Light Production in a Water Moderated Nuclear Reactor

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An experimental investigation of the production of Cerenkov radiation in a water moderated nuclear reactor is conducted using a photomultiplier as a light sensor. The variations in light intensity are studied during various phases of reactor operation, namely: startup, steady state and shutdown. The relevant theory is presented as an aid in interpreting and extrapolating the experimental results. It is found that for transients such as startup, the light signal is directly related to reactor power for periods (e - folding time) faster than about 20 seconds. Additional transient data acquired from measurements performed on a TRIGA pulsed-type reactor illustrate the excellent agreement between the Cerenkov detector and a conventional ionization chamber for measuring pulse characteristics such as peak power, pulse half-width, and prompt period. The proportionality between reactor power and Cerenkov signal is no longer valid for whole core measurements made at steady state power level because of the gradual increase of the Cerenkov signal as a result mainly of fission product contributions. Selective scanning of the Cerenkov spectrum through the use of interference filters over the wavelength range 3500 Å to 5530 Å results in a lower buildup fraction. Indications are that measurements further into the short wavelength region may yield a light sensor, and hence a good power detector, independent of any fission product buildup. The decrease in the Cerenkov light intensity after shutdown is measured for reactor operating times from 20 minutes to 4 hours. Comparison of the empirical data with theoretical considerations results in good agreement for shutdown times ranging from 500 seconds to 10,000 seconds. Spectral measurements made through 17 feet of water with a Hilger quartz spectrograph show a spectral distribution ranging from 2500 Å to 6000 Å. A calculated spectral distribution is compared with the measured spectrum after correcting for water attenuation.