An In-Core Movable Detector Thimble Handling System for Nuclear Power Plants

J.-K. Lee, S.-H. Byun, B.-H. Cho, and J.-Y. Park (Korea)


Detector thimble, thimble handling machine system


The in-core neutron flux mapping system in a pressurized water reactor yields information on the neutron flux distribution in the reactor core at selected core locations by means of movable detectors. The detector cables run through guide tubes (thimbles). For reactor operation, the thimbles are inserted into nuclear fuel assemblies through conduits connected from the bottom of the reactor vessel to a seal table. During the plant refueling outage period, the thimbles are withdrawn up to 4 meters from the seal table, the height of fuel assemblies. Thimble handling work has been performed by maintenance personnel. Manual handling efficiency is low due to narrow and high-radiation working environment around the seal table. Besides, manual force exerted on the thimbles is not unidirectional, which may cause serious damage to the thimbles. To overcome these difficulties and maintain the thimbles safely, an automatic thimble handling system that is composed of a thimble handling machine system, a control system, a thimble tensioner and so on, has been developed. The user friendliness of a control system and the robustness of thimble handling machine are particularly emphasized to guarantee minimum radiation exposure to maintenance personnel and efficient thimble maintenance.

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