Background
After a few years of operation, many plants experience difficulty in pulling (or retracting) the thimbles back to allow for the fuel shuffle and reload. When this problem is caused by debris falling into the bore of the thimble conduits (or guide tubes), BCP’s process of conduit cleaning draws the debris away from the reactor vessel and flushes the loose debris out at the seal table. The debris is caught in replaceable filters before returning the reactor coolant to the vessel inventory, typically flushing only a few gallons to the plant radwaste sump.
Description of Service
The incore flux thimble conduit (or guide tube) cleaning equipment provided by BCP consists of hoses, pumps, filter housing, filter cartridges, valves, fittings, etc., necessary to water flush the individual conduit bores outwardly from the reactor vessel for the full length of each conduit (i.e. from the reactor vessel to the seal table).
The Conduit Cleaning usually takes one shift to perform, and can be done with the Thimbles in place (as shown in this figure), or during Replacement services when the Conduit is empty and the best flow is obtained.
The process is applied by equipment fitted directly to the Seal Table end of each conduit. No strong solvents or mechanical devices are used that could hang up in the conduit or damage the inner surfaces. A supervising engineer and three technicians are provided for this work.
The conduits are flushed clean by BCP equipment located only at the Seal Table. Using this equipment, RCS water is drawn from the reactor vessel at a high velocity to flush out the loose crud, debris, and metal particles, thus cleaning the conduit bore of loose debris prior to incore flux thimble withdrawal or insertion. Small micron water filters are used at the Seal Table suction/discharge outlet of a single conduit to collect the loose debris 
and solid material withdrawn from each individual conduit using the filtered pump vacuum-cleaning process.
As applicable, the pump discharges filtered RCS water from the first conduit cleaned into the plant liquid radwaste system.This involves a loss of only a few gallons from the RCS. For the remaining conduits, the filtered RCS water is pumped back into the reactor vessel through the first conduit cleaned, thereby causing no further change in the RCS water inventory. The conduit water filter is temporarily shielded, as necessary, with plant supplied lead blankets to reduce local radiation levels. Filter cartridges are changed out by the BCP crew.
This detail shows the flow path of the
suction side of the Conduit Flushing Tee
and illustrates the operation with the
thimble still in place.
Experience
BCP conduit cleaning equipment has been used many times for several different purposes, including debris removal, foreign object retrieval and dose rate reduction in the “keyway” below the Seal Table and reactor vessel. The system has been used successfully at several operating plants.
BCP is the only vendor able to perform this cleaning process with the flux thimble still in place in the conduit, as shown in the above figure.
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