Due to vibration of flux thimbles in certain reactor vessels, it becomes necessary to reposition the thimbles as an economical fix. BCP shortens the thimble by withdrawing the thimble from the core and cutting off a 1″ to 3″ length, then re-establishing a new high pressure seal at the seal table. This type of repair has been performed many times when a thimble demonstrates wall loss or a defect due to flow induced vibration, fretting or even erosion.
The defect areas are usually just above the lower core support plate and near the lower nozzle of the fuel assembly. In this area the thimble is unshielded from the high velocity coolant flow against and past it. When the defect occurs in this area, the thimble can be pulled back a few inches so that the defect location on the thimble tube wall is retracted downward into the lower core support plate and therefore is protected and no longer exposed. This retraction then places a new area of the thimble tube that has full wall integrity in the high axial and transverse flow position between the lower core support plate and the fuel assembly lower nozzle.
Description of Service
To effect this repair, the designated thimble is pulled up at the seal table, the front and back ferrules are cut off the thimble, an appropriate length of thimble tubing (approximately 1″ to 3″) is cut off with a tubing cutter. The tube end is deburred, being careful to use a chip catcher in the thimble bore. As necessary, (on nominal .301″ OD thimble tubing), the upper 1-1/2″ of the thimble is expanded to allow the OD to accept the standard Swagelok 5/16″ fittings for the new high pressure middle and upper seals. The new thimble sealing surface area is then polished to receive the high pressure seal ferrule set. The thimble height is reset to its previous position (this height setting is very important) above the Seal Table and the new high pressure seal is made up using new fittings.
This type of repair is very useful as plants are finding thimble wall loss or thinning in this high flow area and especially in situations where the plant is not ready or willing at that time to replace damaged thimble(s). BCP personnel have performed this work over 1700 times, at 24 different sites.