Fuel Rod Scanner for Reactor Power Density Distribution Measurement

概要

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A fuel rod scanner with an automatic decay correction system was manufactured to measure the reactor power density distribution by the γ-activity from an irradiated fuel rod. The detection head is a scintillation counter surrounded with Pb shield. A fuel rod travels on this head by the feed mechanism operated by compressed air force. Main advantages are that this mechanism does not give any oscillation to the fuel rod in moving and settles the measuring point accurately within ±0.01 mm. It is hardly possible to calculate the correction for decay of the fission product because it includes many kinds of decays and it has decay constant varying with time. Therefore, an automatic correction for decay time is highly desirable. A system of automatic decay correction, combining an automatic fuel rod scanner and electronic circuits, is used which effects the remote and automatic counting with minimal personnel. The effect of background counts in measuring and monitor channels and the pulse pile-up at high counting rate in electronic circuits are discussed in detail. In case of predominant background counts, it is preferable to make a measurement so as to satisfy the following equation, BM/B=P/Nprii, where BM, B are background counts in monitor and measuring channels respectively and P, NiPri a preset count in monitor channel and an observed count in measuring channel.
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