Recommendations on Dose Coefficients for Assessing Doses From Accidental Radionuclide Releases to the Environment

Summary Of Recommended Assessment Parameters

Age-dependent breathing rates recommended for the estimation of inhalation doses are given in Table 1 (ICRP 1995b). Recommended dose coefficients for internal and external exposures to those radionuclides that might be expected to be present in significant amounts following a radiological emergency are given in Table 2. This list has been harmonised with that in CSA guideline document N288.1 (CSA 1987), although many of these may not be released in significant quantities during emergency situations. Dose coefficients for inhalation are taken from ICRP Publication 72 (1996) for default lung absorption types, and the six recommended ICRP age groups. Breathing rates given in Table 1 should be used as default values for the assessment of these doses. Adult dose coefficients for cloudshine and groundshine are taken from Eckerman and Legett (1996). These should be multiplied by a factor of 1.5 when assessing doses to the 3-month- and 1-year-old age groups.

In general, dose coefficients do not reflect the contribution due to ingrowth of decay-chain members in the environment, although coefficients for internal exposures do reflect the contribution to dose from ingrowth in the body. However, in providing the short list of radionuclides given in Table 2, the Working Group has included the contributions from decay-chain members in the external dose coefficients for those cases where it is reasonable to assume that the parent and progeny will remain in equilibrium in the environment following release from the source. These include the following decay chains:

¹⁰⁶Ru - ¹⁰⁶Rh ¹³²Te - ¹³²I ¹³⁷Cs - ¹³⁷mBa ¹⁴⁴Ce - ^144/144mPr

In these cases, it is almost always reasonable to assume that secular equilibrium between parent and progeny is maintained both in the plume and following deposition, due to the short half-lives of the daughters (less than a few hours). For these decay chains, external dose coefficients listed for the parent radionuclide in Table 2 were obtained by multiplying the dose coefficient for the progeny by the decay-branching fraction and adding to the coefficient for the parent.

In most cases, however, secular equilibrium between parent and daughter cannot be assumed. Examples include the complex decay chains of long-lived actinides, those involving noble gases and nuclides with non-zero deposition velocities, such as ⁸⁸Kr - ⁸⁸Rb, and those in which the parent and daughter are reasonably long-lived with similar half-lives (e.g. ⁹⁵Zr - ⁹⁵Nb). In these cases, the dose coefficients for the radionuclide and its progeny can be combined only after consideration of the equations describing production and decay of the daughter radionuclides over time, and differences in environmental behaviour (Kocher 1983). Some notes on the assessment of doses arising from decay-chain members are provided in Appendix B .

The Working Group has reproduced, with permission, an extensive list of external dose coefficients from Eckerman and Leggett (1996) in Appendix C that should be applicable to a broad range of accident scenarios. This list differs from Table 2 in that contributions to the external dose coefficients from radioactive progeny have not been included for any radionuclide. Contributions to dose from the ingrowth and decay of chain members in the environment must be accounted for by the user. As with Table 2, only adult values have been given for external dose coefficients. Infant values should be modified as appropriate. Age-dependent inhalation dose coefficients for radionuclides not listed in Table 2 should be obtained from either the ICRP CD-ROM database of dose coefficients (1998, in press), or ICRP Publication 72 (1996).

Dose assessment parameters recommended in this report should be employed as default values for the evaluation of the three critical pathways immediately following a nuclear emergency. However, in applying the parameters recommended in this report, it should be understood that their applicability to a particular exposure situation is influenced by any conditions that differ from those assumed by the modellers in their derivation. For example, the assumption of uniform radionuclide concentrations and semi-infinite or infinite sources may not apply under certain conditions, and thus, modifications to the external dose coefficients may be required.

Although beyond the scope of this report, modifying factors which account for changes in the radiation field, such as non-uniform or finite sources, shielding, or ground roughness, and factors associated with the duration of the exposure, should be used where appropriate in the assessment process. It is the task of the user to determine the adequacy of such factors to a particular exposure scenario.

Table 1: Recommended breathing rates for the six ICRP age groups

ICRP Age Group	Default Breathing Rate (m3 d-1)
3 months (0-1 year)	2.86
1 year (1-2 years)	5.16
5 year (2-7 years)	8.72
10 year (7-12 years)	15.3
15 year (12-17 years)	20.1
Adult (> 17 years)	22.2

(Reference: ICRP Publication 71 (1995b), Table 6)

Table 2: Recommended effective dose coefficients for internal and external exposures to selected radionuclides

To enlarge table

To enlarge table

Reference: ICRP (1996), Eckerman and Legett (1996)