Cloud gamma modelling in the UK Met Office's NAME III model

In the United Kingdom radionuclides are discharged to the environment from a range of controlled sources, under a system of authorisation and regulation. Radiological assessments are performed for a wide range of controlled release applications, including assisting in the provision of advice and guidance to regulatory bodies responsible for setting discharge limits. Such limits are granted to nuclear licensed sites and non-nuclear operators such as universities and hospitals, many of which discharge to the atmosphere, resulting in a range of potential routes of exposure of an individual or a population. One such route, external exposure to gamma rays irradiating directly from a dispersing plume and referred to as 'cloud gamma', may contribute significantly to dose if the radionuclides discharged are strong gamma emitters or are noble gases (which do not deposit, thus limiting the number of potential exposure pathways). The nature of cloud gamma modelling necessitates full integration within a dispersion model, unlike other exposure pathways which may be considered subsequent to the dispersion modelling. The inclusion of the ability to estimate cloud gamma dose in the Met Office's Lagrangian Particle Dispersion model, NAME III, is necessary for use in comprehensive assessments of the consequences and risks of a radiological release to atmosphere. This paper describes the cloud gamma modelling approaches implemented within NAME III, details the testing and validation performed, and considers potential applications and future work.