Concept of collective dose (Sustainability Assessment)

This page is the "Appendix IV" to Environmental Impact of Stressors

The concept of collective dose was found to be very useful for comparative analysis of radioactive emissions from nuclear facilities and systems.

Introduction

Interpretation of the collective dose has been discussed by the ICRP, which, in its recommendations [128], believes that the use of the collective dose concept is closely connected with formal CBA. The product of the average dose to an individual and the number of individuals in a group (i.e. how collective dose has been calculated in the past) is a legalized quantitative value, but it is of a limited application, as it combines redundant information. For decision making, the necessary information has to be presented as a matrix that indicates the number of individuals exposed to a specific dose and the date it was received. This matrix needs to be considered as an auxiliary decision making tool allowing the significance evaluation of individual matrix elements. The matrix approach leads to a more correct estimation of consequences (risks) of irradiation. The ICRP believes that this will avoid misinterpretation of the collective dose, which has resulted in serious errors in the prediction of lethal outcomes. A justification of these ICRP recommendations can be found in Ref. [128].
Collective doses are generally not used by regulators; instead, regulators usually limit the emission of single radionuclides or classes of radionuclides represented by equivalent isotopes on the basis of doses to the critical group. However, collective doses normalized per unit of electricity were used in the ExternE project for the calculation of external costs for comparing different power technologies [110, 111, 119]. Annual collective doses have also been used for comparison of different options of nuclear cycles in Ref. [116]. Recommendations on how collective doses should be calculated in particular cases can be found in Refs [23, 129, 130].
Use of collective doses also presents some disadvantages, e.g. the possibly prevailing importance given to very low doses occurring over very long time periods (applying the linear dose effect relationship without cut-offs), the dependency on key modelling assumptions that are not easily controllable such as population distributions (in place and time) and the application, or not, of discounting.

Simplified determination of collective dose

The IAEA generic models (see also Appendix II) include a simplified method to determine collective doses. In section 7 of Ref. [23], tables contain collective effective dose commitments per unit activity (man Sv/Bq) of radionuclides discharged to the atmosphere, to marine water and freshwater bodies. All contributions from individual radionuclide species and pathways need to be summed.
The simplified conversion of release rates to dose factors has been derived using the results of two approaches: one based on a simple method using generic parameters [98] and the other one based on complex modelling [131], which have been developed by the NRPB, in the United Kingdom. Reference [23] suggests using simplified models with caution, noting that they can only provide order of magnitude estimates, and that collective doses:

“...should be used only as part of a screening or generic assessment procedure, for example to ensure compliance with dose limiting criteria or as input to an optimization exercise to compare options as part of an intuitive, semi-quantitative analysis. They should not be used for more rigorous optimization analyses, such as cost–benefit analyses, nor for other purposes.”

Reference [23] further states that “the site specific discharge conditions and the actual critical group location be taken into account if the predicted doses exceed a reference level of around 10% of the dose constraint.”
Collective doses were estimated for most radionuclides only in local and regional zones, which may extend from the point of release to distances varying from about a hundred kilometres to several thousand kilometres [23]. Four nuclides were considered for global analysis because of their relatively long radioactive half-lives or a high environmental mobility: ¹⁴C, ³H, ¹²⁹I and ⁸⁵Kr; other long lived radionuclides, such as ²³⁷Np or ⁹⁹Tc, may also become globally dispersed following discharge, but they have not yet been accurately addressed. Therefore, an analyst may need to consider these (and others), depending on the technical characteristics and expected performance of the NES.
Collective doses can be used for the purposes of optimization of radiation protection measures. WS-G-2.3 [33] recommends using the estimations of collective dose arising from discharges to avoid spending resources to assess options for reducing discharges in disproportion to the likely improvement in radiological protection. The collective dose from discharges, which can be estimated using Ref. [23], should be added to an estimate of the relevant collective dose from occupational exposure to provide an estimate of the total collective dose. If the result is less than about 1 man Sv/a, an extensive formal optimization study most probably will not be needed [132]. If the value of the collective dose is greater than about 1 man Sv/a, a formal study by the designer (technology developer) is required, with the use of decision aiding techniques such as CBA and multicriteria methods.

See also

• Appendix I
• Appendix II
• Appendix III