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This information is assessed to ensure that feeler and tilt transducer noise is within acceptable limits and that the readings are consistent with other measurements within the channel, other channels measured in the reactor and are following the evolution in shape predicted from previous outage data.

The CBMU data is then further processes to produce measurements of channel bow, channel tilt and brick bow. These are assessed for any significant safety implications, for example, for fuel stringer insertion and removal. The following values are typical of readings taken to date: channel bows of approximately 6mm, channel tilts in the order of 10mm, and brick bows of less than 1mm.

Further detailed assessment of CBMU data from all stations are conducted periodically to analyse the evolving state of reactor cores. This ensures an up-to-date understanding of evolving core behaviour.

1. Measuring Dimensional Change and the Onset of Stress Reversal

The average diameter from the CBMU readings are used to measure the shrinkage of graphite fuel bricks. In general, all channels are expected to exhibit the same rate of shrinkage, although there is scatter in the data due to dose variations across the core and differences in virgin properties between the bricks. Monitoring of repeat channels gives the opportunity to measure changes more closely.

Although the CBMU analysis monitors against a minimum diameter for fuel movements, the data can also be used to monitor against the onset of stress reversal. A cylinder that is subjected to greater shrinkage stresses at the bore compared to the outside will tend to “barrel in” at the ends as shown by Figure 61. The extent of barrelling will increase while the rate of shrinkage at the bore remains above at the outside. This differential shrinkage results in a tensile stress at the bore. Once stress reversal begins, the rate of increase in barrelling will fall until a peak is reached at the point when shrinkage rates are equivalent across the bore. However several factors complicate this analysis:

• The geometry is not a pure cylinder as the bricks contain full or part length keyways, depending on the station;
• The bricks have fuel end dose depressions within their length;
• There will be differences in cross-brick shrinkage rates resulting from power differences in the subject and neighbouring channels.

2. Measuring Channel Distortion

From the tilt transducers in the CBMU the overall channel profile (defined by the channel bow and channel tilt) can be determined.

Channel bow is the measure of the distance between the axis traced from the top of the bottom brick to the top of the top brick, and the top of each brick relative to this axis. If sufficient channels evenly spaced across the core are measured, the levels of distortion in the remaining channels can be estimated.

Channel tilt along has no implication for fuel movement. Channel tilts have only recently been calculated and require more information to eliminate zero-offsets. The available data so far suggests that channel tilts are not changing significantly with increasing irradiation, however there is a significantly larger scatter in the data and increased uncertainty in the measurements compared with channel bows. The channel tilt data collected from inspections is currently being used as inputs in whole core modelling.

In summary, channel bow and tilt data is used to characterise core distortion. The levels of distortion across a core can be bounded from an evenly spaced inspection regime.

3. Detecting and Analysing Defects

As discussed the CBMU can indicate the presence of cracks within bricks and provide further information beyond that of a visual image. The additional information provided by the CBMU allows cracks to be fully characterised to include their influence on bore distortion. The response of the CMBU differs depending on the type of crack within a brick. This is illustrated by Figure 62 below whereby the changes in geometry can be used to infer the type of brick crack.