An explanation of Avertable Dose
ICRP-103 suggests that countermeasure strategy should be optimised to reduce residual dose
where as UK advice has traditionally been based on averted dose. Both of these make sense in planning countermeasure
strategies but are hard to use to determine the optimum countermeasure strategy on the day. We need to understand what these terms mean.
Projected Dose The dose a particular person would be expected to receive in the course of an
emergency without any countermeasures being applied.
Residual Dose The dose a particular person would be expected to receive in the course of an
emergency if countermeasures were applied.
Averted Dose The difference between projected and residual dose. That is, the dose saved (averted)
by the countermeasures.
Public Health England (formerly NRPB) advice is that countermeasures should be applied if the averted dose
is greater than the Upper Emergency Reference Level (ERL) and not applied if the averted dose is below the Lower ERL.
For averted doses between these two values a judgement is needed on the difficulties that imposing the countermeasure
will entail.
| Dose Equivalent (mSv) | |||
| Countermeasure | Body organ | Lower | Upper |
| Sheltering | Whole body Thyroid, Lung, Skin |
3 30 |
30 300 |
| Evacuation | Whole body Thyroid, Lung, Skin |
30 300 |
300 3000 |
| Stable Iodine | Thyroid | 30 | 300 |
Example - Consider an accidental release that would last for 10 hours and give a dose of
10 mSv/hr at Site-Perimeter Cottages. Assume that we can ask the inhabitant to shelter, which would be effective after
three hours delay and reduce the subsequent dose rate by 50%, or evacuate, which would be effective after six hours
and reduce the subsequent dose rate to zero. What would the projected, residual and averted doses be?
The answer is shown in the table and graph below. The projected dose is 100 mSv. Shelter gives a
residual dose of 65 mSv and an averted dose of 35 mSv. Evacuation gives a residual dose of 60 mSv
and an averted dose of 40 mSv. The two countermeasures combined give a residual dose of 45 mSv and
an averted dose of 55 mSv of which 30 mSv can be attributed to shelter and 25 mSv to evacuation.
.
If we compare the averted doses in these situations to the Emergency Reference Levels (ERLs) we find
that both shelter and evacaution are indicated as useful countermeasures. Shelter averts 35 mSv and evacuation 40 mSv
compared to their Lower ERLs of 3 mSv and 30 mSv. Interestingly in the case of applying both, sheltering
only provides 20mSv which is below the Lower ERL. However, in the interest of optimising averted dose,
both countermeasures are indicated.
Further dose could be averted by making the countermeasures effective more quickly which is why there are off-site
plans and these are tested regularly and why some countermeasures are applied by default on declaration of an
off-site nuclear emergency.
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