Skip Navigation LinksReactor Core Inspections _ Graphite Trepanning Tool (GTT)

 

2. Cutter Assembly and Sample Break Off

The cutter is a thin wall cylindrical hollow steel tube with a thicker section at the cutting end. This allows room for the cutter to move during the sample break-off process. The cutting face is coated in an abrasive grit, and is driven by a lubrication free air motor in an anti-clockwise direction, looking in on the cutter.

The cutter drive assembly is mounted on a plain bearing slide assembly and is fed outward from, and returned into, the GTT body by a pneumatic cylinder. The whole feed assembly is mounted on a secondary linear side arrangement, which, in case of primary drive failure, can be retracted using either of two similar but separate electric drive units.

The depth of cut is measured via a linear transducer and the final depth is determined by the cutter length. During the cutting process a constant air supply is blown through the cutter to clear the debris and prevent it entering the GTT. Acoustic monitoring of the cutting process is also provided to give the operator secondary feedback on the progress of the cutting process.

To enable the sample to be broken from the graphite brick the cutter can be moved about the cutter tip, in a vertical plane, using a pneumatic cylinder. Once the cutter is at full depth it can be levered up and down until the sample snaps off at the root of the cut. The sample then remains in a cutter which is returned to the horizontal for retrieval.

3. Air Knife

To ensure that all the graphite dust caused by the trepanning process stays in the channel being sample an “air knife” is incorporated in the GTT. An annular air knife is mounted at the bottom of the GTT generates a film of high velocity air which is directed down the fuel channel and uses momentum exchange to overturn the upward flow of air in the fuel channel and induce a downward flow past the tool. This flow entrains the graphite dust and takes it away from the GTT preventing contamination of the tool and components.

The air knife is turned on before the cutter is started and is only turned off once the GTT is clear of the graphite core. Transducers allow the pressure above and below the GTT to be measured thus giving an indication of the direction of gas flow.

The standpipe valve is opened and the GTT is lowered to the first designated sample height, usually brick four. Having lowered the GTT to the required sample level, the clamps are deployed to stabilise the GTT in the channel. The rotational position is measured using the laser stripe and polarised light units, and the azimuth drive operated to achieve the correct cutting angle. Readouts on the Control Console allow the height and cutting angle to be recorded.

To restrict the trepanning dust to the area below the GTT, the air knife is switched on and adjusted until the channel flow is reserved. A differential pressure readout on the console confirms when the optical flow has been achieved.

7. Sample Cutting and Retrieval

The cutter feed is operated, without rotating the cutting head, unit the cutter touches the channel wall where the linear transducer reading is noted and recorded. The cutter drive is then started and the cutter automatically feeds until the pre-set depth has been achieved. This is confirmed both audibly from a change in cutting noise, and from the transducer reading on the console panel. 

The sample break off mechanism is then cycled to break the sample from the brick before retracting the cutter into the body of the GTT and driving the azimuth back to the datum position.

Releasing the clamps leaves the GTT hanging freely in the channel ready for retrieval. The air knife is kept on until the GTT has been raised beyond the graphite core to prevent the trepanning dust blowing up past the tool. Once clear of the core the air knife is turned off and the tool is raised up the standpipe into the containment where it docks with the radial alignment keys. A photoelectric sensor indicates that the cutter is accurately aligned at the correct height for sample transfer to proceed.

8. Sample Transfer to the Sample Transfer Flask (STF)

The cutter is driven out and a vacuum is used to move the sample from the extended cutter to the inspection station where it is stopped in a rigid transparent tube and visually inspected using a remotely operated camera. Its activity is measured before being allowed to pass into the flask.

The cutting sequence is repeated at different heights until the required number of samples for that channel has been taken.