A Detailed Emergency Planning Zone (DEPZ) is the area around a nuclear licensed site where the operator's risk assessments suggest that it is worth having a special emergency plan to ensure that Protective Actions such as shelter, Stable Iodine Tablets and evacuation can be agreed upon, communicated and executed in a timely manner.
If you live in a DEPZ you should have received prior information and, if near a fixed reactor site, stable iodine tablets from the local authority.
The DEPZ is defined in the REPPIR Regulations and ACOP thus:
"Detailed emergency planning zone” means a zone determined in accordance with regulation 8 and covered by the local authority’s off-site emergency plan.
The regulations require that the Operator of the site undertake a Hazard Evaluation of the site with the intent of identifying the hazards from their work with ionising radiation, evaluating risks and preventing emergencies. Where a hazard that could potentially lead to a radiation emergency is identified its potential consequences and likelihood should be estimated and these used to determine if detailed or outline planning is required.
REPPIR Regulation 4(1) - 4(4)
(1) The operator of any premises to which these Regulations apply must make a written evaluation before any work with ionising radiation is carried out for the first time at those premises.
(2) The evaluation required under paragraph (1) must be sufficient to identify all hazards arising from the work undertaken which have the potential to cause a radiation emergency.
(3) Where the evaluation required under paragraph (1) does not reveal any hazards having the potential to cause a radiation emergency, reasons for such a conclusion should be set out in that evaluation.
(4) Where the evaluation required under paragraph (1) does reveal the potential for a radiation emergency to occur, the operator must take all reasonably practicable steps to—
(a) prevent the occurrence of a radiation emergency; and
(b) limit the consequences of any such emergency which does occur.
The clearest explanation (at least to a RPPPIR expert) of the required size of the DEPZ is found as paragraph 700 in the REPPIR ACOP "The largest of the distances recommended for the urgent protective actions identified against the lower ERL should be selected as the recommended distance for the minimum geographical extent of the detailed emergency planning zone".
In summary this is saying that the operator has to do a significant body of work to determine a range of accidents that might occur and determine how much radioactivity they might release, then estimate the doses the general public might receive at a range of distances downwind for each accident sequence (allowing for a range of weather conditions). The minimum range of the DEPZ is then the largest distance downwind that the 7.5 mSv trigger point for shelter being a worthwhile protective action (assuming it averts 60%) for any of the event sequences considered to have a likelihood greater than 1 x 10 -5 per year.
Having been informed of this value in the Operator's Consequence Report the Local Authority have to set a DEPZ no smaller than the recommendation. They can increase the size of the DEPZ (within reason) and can add "lumps and dumps" to include communities or vulnerable people that would be just outside the DEPZ if it were circular if they think the result would be more acceptable to the public.