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Contents
 
Reactor Core Inspections – Channel Bore Measurement Unit (CBMU)
 
1 The CBMU Device
2 Control Console
3 CMBU Pre-Deployment
4 CBMU Deployment and Scan
 

 

 

 

A device known as the Channel Bore Measurement Unit (CBMU) has been used on stations to examine a number of fuel channels at each reactor periodic shutdown. The data used from the CBMU is used to assess fuel brick diameters, brick ovality and channel distortion and to monitor the ageing of the cores through development of the brick shapes and levels of channel distortion with time. THE CBMU can also be used to detect significant cracks and provide information on the influence of cracking on brick and channel distortion.

1. The CBMU Device

The CBMU is a stainless type cylindrical device weighing approximately 230 kg consisting of two sets of four wheels, one set at the top and the other at the bottom: these are geared up to keep the CBMU centred in the channel. The CMBU is deployed into fuel channels using a delivery hoist.

Feelers are arranged at every 90° and measurements at taken at one-millimetre interval height intervals along the channel. The feelers operate as Linear Variable Displacement Transducers that produce an electrical signal proportional to the feeler movement. Each feeler is capable of resolving measurements to within 0.02mm. To prevent the fellers snagging as the CBMU is lowered down a channel, they are held within the body of the CBMU and only deployed by the feeler drive motor once the CBMU has been lowered into position

CBMU Wheels and Feelers

The CBMU also contains two tilt transducers; arranged 90° apart which can be used to determine the tilt of the brick the CBMU is deployed in. Tilts as little as 0.1° can be resolved, and these measurements provide an indication of the bowing within a fuel channel.

A rotary joint on the upper section of the CBMU enables rotation of the body of the CBMU about a vertical axis to allow a series of scans to be carried out at 30° increments. These scans eliminate systemic errors caused by equipment manufacturing tolerances.

2. Control Console

The CBMU has its own dedicated Control Console located at pilecap level. The console consists of the circuitry associated with the onboard equipment on the CBMU, and a computer for handling the data generated by the CBMU. The Delivery Hoist controls have also been incorporated into the CBMU console.

3. CMBU Pre- Deployment

The CBMU is calibrated along with its delivery hoist prior to deployment – this induces the feelers and tilt transducers. This is achieved by calibrating the feelers and tilt transducers separately, and then deploying the CBMU on a sample brick stack in the Test and Training Facility, with a known series of tilts and diameters to ensure the CBMU is functioning correctly.

Once the CBMU and its hoist have been set up at the appropriate channel at pilecap level, a Pre-Scan Reference check is carried out. The feelers are fuller extended and a reference reading taken, and then retracted.

 

Evolution in CBMU Design

4. CBMU Deployment and Scan

The relevant standpipe value is opened and the CBMU is lowed to the bottom of the channel. This is shown by a low load indication on the control panel and by comparing the height encoded with the known depth of the channel. The CMBU is raised until the load comes back on the chains, and is then ready to carry out a channel scan.

Now the CBMU is ready to commence the Scan. The feelers are extended to make contact with the inner surface of the graphite and the computer is set to start logging the feeler and tilt readings. As the CBMU is raised to the top of the graphite core, the feelers measure the diameter of the channel bore, in two planes, against the measured height of the CBMU to give a graphical output of the profile of the channel. The CBMU is stopped at the top of the core and the feelers retracted. Unless a repeat scan in the same orientation is required the CBMU is raised into the hoist containment. On some scans the tilt data continues to be logged pas the CBMU passes up the standpipe.

A Post Scan reference check is carried out once the CBMU is returned to its reference height. The feelers are fully extended and a reference reading is taken.

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