With respect to operational radiation protection the following structure of “Levels of task” has been proposed by the IDEAS Guidelines [49]:
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Level 0: Annual dose (committed effective dose from intakes of radionuclides that occur in the accounting year) <0.1 mSv. No evaluation of dose needed.
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Level 1: Simple, “reference” evaluation, with ICRP defaults used for all parameter values, except where there is better a priori information available, e.g. for inhalation intakes information on the particle size distribution (dose from the intake typically 0.1 – 1 mSv).
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Level 2: Sophisticated evaluation using additional information to give more realistic assessment of dose: typically a special assessment of an accidental intake. Comparisons are made of the model predictions (“the fit”) with the data, to choose between alternative parameter values, or to find optimum parameter values (a posteriori). At this Level, the parameters adjusted typically relate to the material (for inhalation intakes the AMAD and absorption Type), and the time of intake if unknown (dose from the intake typically 1 – 6 mSv).
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Level 3: More sophisticated evaluation, which applies to cases where there are comprehensive data available, as would be the situation after an accident. The evaluation is an extension of Level 2, typically to parameters relating to the subject (e.g. for inhalation intakes the HRTM particle transport rates). The fundamental approach at this Level is to adjust the model parameter values systematically, in a specific order (“step-by-step” approach), until the goodness of fit is acceptable (i.e. the fits obtained to all the data are not rejected by the specified criteria) (dose typically > 6 mSv).
Level 0 is the lowest level and it refers to cases where the effective annual dose would be most likely below 0.1 mSv, even if there should be similar intakes in each monitoring interval of the year. At this level there is no need to evaluate the measured values explicitly, and the effective dose can be set to zero in analogy to the rounding of doses in external dosimetry. However, the measured value should be recorded with respect to further assessments in the future.
According to the definition on the left a measured quantity M can be allocated to Level 0, if M < MC, where
where,
Mc is the “critical” monitoring quantity,
T is the monitoring interval (in days) for the monitoring quantity considered,
m(T/2) is the corresponding retention or excretion function (per unit intake) for the monitoring quantity at time t = T/2,
and e(50) is the effective dose coefficient (in Sv/Bq).
The critical monitoring quantity MC defined by the equation (1) is typically above, or close to, the lower limit of detection (LLD) for the fission and activation products whereas it is below the LLD for the actinides considered. So in the case of the actinides, any significant monitoring value is likely to result in a dose of more than 0.1 mSv and thus has to be evaluated. In the case of the fission and activation products, however, there might be significant monitoring values which result in a dose of less than 0.1 mSv. Thus, Level 1 applies typically to those radionuclides, which are easy to measure and which have low effective dose coefficients (i.e. 3H, 137Cs etc).