The 1990 recommendations of the International Commission on Radiological Protection introduced the concept of the dose constraint. The concept of a generalised derived constraint (GDC) was introduced in the NRPB response to the recommendations.
GDCs apply to discharges of radionuclides to the environment and have been calculated for discharges to atmosphere, rivers and sewers for isotopes of strontium, ruthenium, iodine, caesium, plutonium, americium and curium. The GDCs in this document are intended as convenient reference levels against which proposed discharges can be compared.
The GDCs are calculated using deliberately cautious assumptions and are in terms of annual discharges, assuming that the releases to the environment are continuous. If a proposed discharge is greater than about 30% of the GDC, the doses should be examined more closely, taking account of site-specific factors.
Generalised derived limits (GDLs) are intended for use as convenient reference levels against which the results of environmental monitoring can be compared. Generalised derived limits have been calculated in various environmental materials for the radiologically significant isotopes of polonium, lead, radium and uranium, derived from the annual effective dose limit of 1 millisievert. These radioisotopes are all found naturally in the environment and the GDLs relate only to possible increases in activity concentrations resulting from human activities.
GDLs are calculated using deliberately cautious assumptions and are based on the assumption that the level of environmental contamination is uniform over a year. If a measured environmental concentration exceeds about 10% of the GDL, the doses should be examined more closely, taking account of site-specific factors and the length of time for which the measured level is likely to be maintained.