Development of Thimble Handling System for Neutron Flux Mapping

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 obtained data are used to verify the reactor core design parameters. The detector cables run through guide tubes (thimbles), and typically thirty six to fifty thimbles are allocated in the reactor depending on the number of fuel assemblies. 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 During the plant refueling outage period, the thimbles are withdrawn up to 4 meters from the seal table, the height of fuel assemblies.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 constant and usually does not have unique direction, which may cause serious damage to the thimbles. To overcome these difficulties, automatic handling system for in-core neutron flux mapping thimbles has been developed, and the validity of the developed system is shown via experiments