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Radiological barriers and controls

Why have these criteria


Think about what you already know about release and dose critiera.

Was the release from Three Mile Island (TMI) as significant as that defined for Level 4?

(the lowest level for events described as 'accidents')

'No' is the right answer; the release was way below that required for Level 4.

Were the doses enough to reach accident levels?

'No' is the right answer, Level 4 would have required hundreds of people to receive more than 10 mSv, or that lethal deterministic effects have occured.

You also know that defence in depth criteria only go to Level 3 on the scale, a serious incident.

Why was TMI significant?
All the primary barriers had failed:

  • fuel
  • cladding
  • primary circuit

Only the containment prevented a significant release.

Three Mile Island (28 March 1979).
Level 5.

A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. Radioactive gases were released into the atmosphere, no deaths have been attributed to this accident.

The event was rated based on the impact on radiological barriers and controls.