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Defence in depth: reactors




  • Overpower detectors calibrated for 96% power rather than 100%
  • Alternative trip parameter available
  • Provides protection against 'expected' initiators


During a routine calibration of the regional overpower detectors for shutdown systems 1 and 2, an incorrect calibration factor was applied. The calibration factor used was for 96% power, although the reactor was at 100% power. This error in calibration was discovered approximately six hours later, at which time all detectors were recalibrated to the correct value for operation at full power. The trip effectiveness of this parameter for both shutdown systems was, therefore, reduced for approximately six hours. An alternative trip parameter with redundancy was available throughout for all overpower faults and expected initiators.

INES procedure (click the highlighted flowchart items to follow the procedure)

Figure 9, page 150 of the Manual

Determine the safety function operability

The operability of the protection system was reduced. The operability was less than the minimum allowed by OL&C but:

  • A second trip parameter with redundancy remained available
  • The wrongly calibrated detectors would also have provided protection for most fault conditions

So, the operability was adequate.

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Determine the initiator frequency

The initiator frequency required for overpower transients was expected.

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Determine the basic rating

Box C1 of Table 10 is appropriate, giving Level 1 or 2. Level 1 was chosen for the basic rating, as the operability was considerably more than just adequate.

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Reduce rating by one level?

The fault only existed for a short time, so the basic rating of Level 1 could be reduced by 1 to Level 0/Below the scale.

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Consider additional factors

When considering additional factors, there were deficiencies in the procedure such that the rating would be increased by 1.

Check the answer


The final rating for the miscalibration of overpower detectors would be Level 1.