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The problems associated with flux mapping in the Plum Brook Research Reactor, which employs an 18 plate Materials Testing Reactor type fuel element, are discussed fully. Design of a flux survey instrument is presented which, utilizing four solid state fission probes, makes it possible to map the complete Plum Brook core in a little over 6 hours, with the data presented in a form immediately available for analysis. Mapping a fuel element (from even 30 ft. above the core) does not require an accurate above water positioning system, yet a high degree of accuracy (Â±0.1 in.) in referencing in-core measurements has been obtained. The effect of radiation on the semiconductor detectors and the operating characteristics of the instrument, along with typical flux profiles obtained in mapping the Plum Brook Reactor are presented.