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DEFENCE IN DEPTH
Defence in depth: reactors

WORKED EXAMPLE: FAILURE OF DIESEL GENERATOR DURING ROUTINE TEST

 

Summary:

  • A diesel generator failed a routine test and was taken out of service.
  • The other two diesel generators were not tested as required by Technical Specifications.
  • These two diesel generators were subsequently tested and found to be operational.

Scenario

The unit was operating at nominal power. During the routine testing of one diesel generator, a failure of the diesel generator control system occurred. The diesel was taken out of service for maintenance. The Technical Specifications require that if one diesel generator is taken out of service, the other two safety system trains should be tested. This testing was not carried out at the time. Subsequently, the other safety system trains were tested and shown to be available.

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 was not less than the minimum allowed by OL&C, as two trains remained available. The additional testing that was eventually carried out, did show that two trains were available.

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Determine the initiator frequency

There was no initiator. The diesel generators were required for a loss of off-site power (expected).

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Determine the basic rating

Box B1 of Table 10 is appropriate, giving a basic rating of Below scale/Level 0.

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Consider additional factors

Workers violated the Technical Specifications without justification, so the event was uprated to Level 1.

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Answer

The final rating for the diesel generators which failed a routine test would be Level 1.