Organ doses due to external γ-irradiation arising from a finite plume in the atmosphere

A procedure has been developed for the calculation of absorbed doses to human tissues or organs, due to external γ-activity from an atmospheric release of radio-isotopes in the form of a finite plume. The procedure is based upon the moments method, which yields a solution of the photon transport equation for infinite air. Analytical expressions have been derived which are applied to these infinite air γ-fluxes to allow for the presence of the ground. Photon spectra and exposures may be calculated for any type of plume composition, release, height and atmospheric stability. By combining the computed spectra with published human organ doses due to external irradiation, absorbed doses to various human tissues have been evaluated. Calculations have also been carried out, on the basis of ICRP recommendations, to predict effective dose-equivalent and effective dose equivalent per unit exposure (rem/R) factors. As examples of the application of the method in safety assessment one hypothetical monoenergetic emitter release and one notional MAGNOX reactor release have been considered. The annihilation radiation contribution to exposure is also discussed. (author)