Foundation docs Optimisation; Dose to Individual

Draft document: Foundation docs Optimisation; Dose to Individual
Submitted by Prof Dr W-U Müller, SSK (German Comm. on Rad. Prot).
Commenting on behalf of the organisation

In 2004 the SSK submitted to ICRP its comments on the “Draft for Consultation” [1] of the 2005 Recommendations of the ICRP. Regarding topic 2.2 “Characterisation of the Individual” the SSK made the following statement: “The characterisation of the individual, e.g. of members of the public, has always been a key issue in radiological protection. Moving from the utilitarian approach of the 'greatest good for the greatest number' to one with more concern for the 'individual' (6) the need for such a characterisation becomes even more important. It is unfortunate, that the indications in the draft report on how such a characterisation could be done are very vague. For public exposure the application of the concept of the critical group (not the individual!) which has previously been proposed by ICRP is re-emphasised. There is, however, no general concept, which is accepted at international level, to define the critical group in various situations. It would be helpful if ICRP could give guidance on how to define the critical group. In (173) an additional concept of using age-averaged effective dose coefficients and ageaveraged habit data for the individual in the case of continuing exposures of the public is mentioned. It is very unfortunate that the present draft of the 2005 recommendations does not give any details about this concept. It rather states: “Methods to assess such doses will be addressed by a Task Group of ICRP Committee 4”. There is fundamental concern that the use of age-averaged effective dose coefficients and ageaveraged habit data for the individual might not be suitable for practical application for the following reasons: the dose limits and dose constraints as defined by ICRP are fixed values and they are independent of the age of the individual. International legislation requires that the dose estimation has to be performed in 6 age-specific groups. The age groups <1 year and between >1 year and <2 year are of particular importance in this context because the age dependent dose coefficients vary by more than an order of magnitude over the lifetime with the highest values at young ages. For a given intake of certain radionuclides such as Pb-210 or I-131 the age-dependent values of the effective dose would be more than an order of magnitude higher for the age groups <1 year and between >1 year and <2 year than the values for adults. The application of age-averaged effective dose coefficients and age-averaged habit data would result in a substantial underestimation of the dose in these age groups. From a radiation protection point of view this can not be accepted.” Defining the representative individual The present ICRP Draft for consultation “Assessing Dose of The Representative Individual for the Purpose of Radiation Protection of The Public” gives guidance on how to define the critical group. The Draft distinguishes between three situations: Practices, existing situations and emergency situations. For every situation two types of assessment are defined: Prospective and retrospective dose assessment. The given overview is helpful to identify the adequate type of assessment and its background when the concept is applied. For retrospective dose assessments the recommended procedure is relatively clear, because it is based on real situations, real exposures and real individuals. There is no need to define a group or a representative individual, but to collect the available data about the exposure situation and adequate parameter values to assess the individual doses. Regarding prospective assessments, the ICRP clearly states that the dose should not be assess for a “critical group”, but for a “representative individual”: “(S9) … The representative individual is the hypothetical individual receiving a dose that is representative of the most highly exposed individuals in the population” and “(S14) … Ultimately, a group is identified that is expected to receive the highest exposure. The average characteristics of this group are used to estimate dose to the representative individual.“ If the value of dose to the representative individual meets the dose constraint established by the Commission, then the Commission’s goal is achieved (S10). The methodology to identify the group and its average characteristics is described in the ICRP Draft. The recommendation permits a lot of different possibilities to define the representative individual: “(S13) Dose to the representative individual may be calculated using several different approaches that range from simple deterministic to probabilistic (Monte Carlo) methods.” One mentioned approach are deterministic methods, which involve the direct multiplication of selected point values of parameters and environmental concentrations. The simplest form of deterministic method would be screening, where very conservative assumptions are made to estimate dose using concentrations of radionuclides at the point of discharge to the environment. In a prospective probabilistic assessment of dose to hypothetical individuals, whether from a planned facility or an existing situation, the Commission recommends that the representative individual be identified such that the probability is less than about 5% that a person drawn at random from the hypothetical population will receive an annual effective dose exceeding the dose constraint. This hypothetical individual should be representative of, at most, a few tens of people who are the most highly exposed (S16). The ICRP also states that mixtures of determinist and probabilistic models can be used to assess the dose to the representative individual. In this context the ICRP puts the responsibility on the authorities: “(S7) … The Commission believes that the regulatory authority should make the final decision on how to include uncertainties in the estimation of dose for compliance purposes.” The conclusion is that Draft recommends a lot of different methodologies to assess the dose to the representative individual. As a consequence, the recommendations are of limited value for practical execution. Age categories Age-averaged effective dose coefficients are not used to assess the dose of the representative individual in the present ICRP Draft for consultation “Assessing Dose of The Representative Individual for the Purpose of Radiation Protection of The Public”. Therefore, the related criticism in the former comment of the SSK on the basis of [1] is now baseless. In the new Draft the ICRP recommends the use of three age categories for estimating annual dose to the representative individual in prospective assessments (instead of six age categories in former recommendations). These categories are 0 to <6 years (infant), 6 to <16 years (child) and 16 to 70 years (adult). For practical implementation of this recommendation, dose coefficients and habit data for the 1-year-old infant, 10-year-old child, and the adult should be used as representing the three age categories. The reasons for this restriction are inherently large uncertainties associated with estimating dose to the public and identification of the representative individual. In Appendix A the ICRP presents ratios of doses from ingestion of milk, ingestion of green vegetables, ingestion of beef and inhalation for the age categories 1 year/3 months, 1 year/5 years and 10 years/15 years. From these tables the conclusion is drawn that the differences are small and that the restriction to three age categories is sufficient. But in some cases the differences can reach about an order of magnitude. ICRP argues that in these cases a satisfactory limitation would result from other pathways or other age categories. This is made plausible for alpha-emitting transuranium nuclides. But the tables in Appendix A and the related texts in the ICRP Draft are no sufficient bases to support these arguments for the use of only three age categories generally. To proof the adequacy of the recommendation much more comparing dose calculations, considering complete release scenarios and radioecological models, would be necessary. There is another fundamental weakness of the concept: On the one hand ICRP allows the application of probabilistic dose assessments to show that the 95th percentile is covered. On the other hand factors of 2 or 3 are neglected in connection with the choice of age categories. The limitation to three age categories is sufficient when the models and parameters used in the dose assessment give a result that is pessimistic enough to cover a factor of 2 or 3. But in a stringent probabilistic assessment it would be necessary to calculate doses for all the age categories, because the assessment otherwise would be to inaccurate to determine the 95th percentile. The contradictions mentioned above should be eliminated. Stakeholder involvement In section 4.3 the ICRP Draft discusses the “Value of stakeholder input to characterising the representative individual”. ICRP states, that in most cases, the role of stakeholders is to aid in the decision-making process: “(101) In the case of defining characteristics of the representative individual, stakeholder involvement can play an important role. Stakeholders can provide valuable input regarding habit data that are specific to their location. In particular, stakeholders can be helpful in determining reasonableness, sustainability, and homogeneity of data. Collaboration with stakeholders can significantly improve the quality, understanding, and acceptability of characteristics of the representative individual, and also strengthen support from stakeholders in the compliance and decision-making process.” The SSK also is convinced that the stakeholder involvement is an adequate instrument to support decision making processes in radiation protection. Recommendations The present ICRP Draft should be basically revised. The general concept to determine the representative individual for prospective dose assessments should be made homogenous. The full set of age categories should be retained, especially for prospective probabilistic assessments. References: [1] ICRP (2004) 2005 Recommendations of the International Commission on Radiological Protection, Draft for Consultation