Exposure levels should be controlled through dose limits, dose constraints and reference levels for representative individuals (ICRP, 2007a) and mainly throughout the full application of the principle of optimization of protec­tion. ICRP has provided ample guidance for the implementation of optimi­zation (ICRP, 1973, 1980, 1990, 2006b).

A dose limit is an individual-related dose restriction defined as the value of the effective dose or the equivalent dose to individuals from planned exposure situations that shall not be exceeded.

Dose constraints are prospective and source-related restrictions on the individual dose from a given source, which provide a basic level of protec­tion for the most highly exposed individuals from that source, e. g. a NPP in toto or any of its systems, and serve as an upper bound on the dose in opti­mization of protection for that source. For occupational exposures, the dose constraint is a value of the individual dose used to limit the range of options considered in the process of optimization. For public exposure, the dose constraint is an upper bound on the annual doses that members of the public should receive from the planned operation of the NPP.

Reference levels are used in emergency or existing controllable exposure situations, and represent the level of dose or risk above which it is judged to be inappropriate to plan to allow exposures to occur, and below which optimization of protection should be implemented. The chosen value for a reference level will depend upon the prevailing circumstances of the expo­sure under consideration. They shall be used in the emergency planning of any NPP.

Figure 11.6 contrasts dose limits with dose constraints and reference levels for protecting workers and members of the public (ICRP, 2007a).

Table 11.8 illustrates the use of dose limits, dose constraints and reference levels within the system of protection for occupational and public exposures

at NPPs (ICRP, 2007a). The recommended dose limits are as follows (ICRP, 2007a):

• For occupational exposure in planned exposure situations, the limit should be expressed as an effective dose of 20 mSv per year, averaged over defined five-year periods (100 mSv in five years), with the further provision that the effective dose should not exceed 50 mSv in any single year.

• For public exposure in planned exposure situations, the limit should be expressed as an effective dose of 1 mSv in a year. However, in special circumstances a higher value of effective dose could be allowed in a single year, provided that the average over defined five-year periods does not exceed 1 mSv per year.

The limits on effective dose apply to the sum of doses due to external exposures and committed doses from internal exposures due to intakes of radionuclides. Occupational intakes may be averaged over a period of five years to provide some flexibility. Similarly, averaging of public intakes over a period of five years would be acceptable in such special circumstances where averaging of the dose to members of the public could be allowed.

Finally, Table 11.9 presents the framework for recommended source — related dose constraints and reference levels with examples of constraints for workers and the public exposed to NPPs.