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DEFENCE IN DEPTH
Defence in depth: facilities

Additional factors - summary

 

Finally, we need to be clear about how these additional factors can be used to increase the rating.

The basic rating can be uprated by a maximum of one level

The total increase cannot be more than one level. It is not a question of adding one for each indicator found or of adding two for an event identifying a serious deficiency in safety culture. Click here for examples

Take care not to double count

Particularly in areas like common cause failure, it's important not to double count. If all the components have already failed due to a common cause, there is no reason to increase the rating further.

There is an upper limit to the rating under defence in depth

Depending on the maximum potential consequences from a facility, we saw earlier that there is a maximum level for defence in depth ratings, click here for a reminder

Facility / activityRating of an event of the maximum potential consequencesMaximum defence in depth rating  
Power reactorsLevel 7Level 3

For power reactors, clearly the worst accident can reach Level 7.

Maintenance cellsLevel 4Level 2

For events associated with maintenance cell entry interlocks, the maximum potential consequences are likely to be related to unplanned worker exposure. If the radiation levels are sufficiently high to cause deterministic effects or death, if the cell is entered and no mitigative actions are taken, then the rating of the maximum potential consequences is Level 3 or 4 (from the individual dose criteria).

Small research reactorsLevel 4Level 2

For events on small research reactors (power of about 1 MW or less) although the physical mechanisms exist for the dispersal of a significant fraction of the inventory (either through criticality events or loss of fuel cooling), the limited inventory means that the rating of the maximum potential consequences could not be higher than Level 4, even if all the safety provisions fail.

Reprocessing facilitiesLevel 7Level 3

For reprocessing facilities and other facilities processing plutonium compounds, the inventory and physical mechanisms which exist for the dispersal of a significant fraction of that inventory (either through criticality events, chemical explosions or fires), are such that the rating of the maximum potential consequences could exceed Level 4, if all the safety provisions fail.

Accelerators and irradiatorsLevel 4Level 2

For accelerators, the maximum potential consequences are likely to be related to unplanned individual exposure. If the radiation levels are sufficiently high to cause deterministic effects or death in the event of entry into restricted areas, then the rating of the maximum potential consequences is Level 3 or 4 (from the individual dose criteria).

For irradiators, most events will be associated with unplanned radiation doses. If the potential radiation levels, in the event of failure of all the protective measures, are sufficiently high to cause deterministic effects or death, then the rating of the maximum potential consequences is Level 3 or 4 (from the individual dose criteria).

Low level waste storagesLevel 2Level 1

For waste stores, it depends on what exactly is being stored but for some facilities the maximum potential consequences could reach Level 2.

Category 1 sourceLevel 5Level 3

For events at facilities with Category 1 sources that have safety systems intended to prevent dispersion of radioactive material (e.g. fire protection systems), the potential release may be large enough to give maximum potential consequences rated at Level 5.

Category 2 or 3 sourceLevel 3/4Level 2  
Category 4 sourceLevel 2Level 1  

 

You can't increase the rating above this maximum level. Also this maximum level only applies if the event was truly a 'near miss' i.e. once further failure would have led to consequences for people and the ennvironment.

Additional factors feature in many rating decisions, so it is important to understand how they are applied.

Violation of operational limitis and conditions (OL&C) including:

  • public exposure > annual limit
  • Worker cumulative exposure > annual limit

Violation of a procedure, without justification

Deficiency in the quality assurance process

An accumulation of human errors

A failure to maintain proper control over radioactive materials, including

  • releases into the environment
  • a failure in the systems of dose control

Repetition of an event, indicating lessons not learned or corrective actions not taken