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DEFENCE IN DEPTH
Defence in depth

Maximum potential consequences - examples

 

In the table below you can see what this means for a range of facilities.

Facility / activity Rating of an event of the maximum potential consequences Maximum defence in depth rating    
Power reactors Level 7 Level 3

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

Maintenance cells Level 4 Level 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 reactors Level 4 Level 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 facilities Level 7 Level 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 irradiators Level 4 Level 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 storages Level 2 Level 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 source Level 5 Level 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 source Level 3/4 Level 2    
Category 4 source Level 2 Level 1