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DEFENCE IN DEPTH
Defence in depth: facilities

WORKED EXAMPLE: LOSS OF VENTILATION AT FUEL FABRICATION PLANT

 

Summary:

 

  • The normal ventilation system lost electrical power at a fuel fabrication plant
  • The emergency ventilation system failed to start
  • Investigation showed a common mode link between their power supplies
  • Procedures required work to be stopped immediately, but work continued
  • No increase in atmosphere contamination occurred

Scenario

Following a loss of normal and emergency ventilation and non-compliance with procedures, the operating personnel worked for over an hour without dynamic containment (the creation of a slight negative pressure gradient in the room in order to avoid the transfer of radioactivity into other areas).

The event started with the loss of electrical power supply to the normal ventilation system. The emergency ventilation system, which should have taken over, did not start up. Subsequent investigation indicated that the breakdown of the normal ventilation system and the failure of the emergency ventilation system to come into operation were linked to the presence of a common mode between the electrical power supplies to these ventilation systems. The alarm was signalled in the guard post, but the information reached neither the supervisory staff nor the operating personnel. The procedures stated that work should have stopped if both ventilation systems failed, in order to minimize the risk of a fire caused by machinery in the facility. The operating personnel were only informed that the alarm had been triggered one hour after a new shift had started. Had there been a coincident event (such as a fire) leading to pressurization, and a failure of the fire fighting systems, some radioactivity which should otherwise have been discharged through a filtration system would have been discharged to the plant operating area and then to the atmosphere without the same degree of filtration. The maximum potential consequence would be Level 4 based on the potential release to the atmosphere.

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

Figure 10, page 151 of the Manual

Determine maximum potential consequences

The maximum potential consequence would be Level 4 based on the potential release to atmosphere.

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Determine number of safety layers and basic rating

The remaining safety layers, not including emergency procedures, were:
- Automatic firefighting systems;
- The building structure that provided both containment and decontamination to reduce exposures to less than 0.1 mSv

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Consider additional factors

  • Investigation showed a common mode link between their power supplies
  • Procedures required work to be stopped immediately but work continued

The maximum defence in depth rating is Level 2
However, the event was not one step from:

  • A fire
  • Failure of fire fighting systems
  • Containment problems

 

Check the answer

Answer

The basic rating is not increased by 1 due to the additional factors.

The loss of ventilation at the fuel fabrication plant would be rated at Level 1.