Radiation Protection Dosimetry 7:91-94 (1984)
© 1984 Oxford University Press
Modelling Indoor Exposure to Natural Radiation
Models have been developed to enable prediction to be made of the dose incurred indoors from gamma radiation and from inhalation of radon decay products. The gamma model takes account of attenuation and build-up of the photon fluence and involves a three-dimensional point kernel integration. The model is divided into three parts; calculation of the dose rate from the walls, ceiling and floor slab (walls can be multilayered and may also include windows and doors); calculation of that part of the dose rate from the ground which is attenuated by the floor slab, and calculation of that part of the dose rate from the ground which is attenuated by the walls. The radon model assumes that the diffusion pathway is the important route for radon transfer when average air concentrations are to be predicted, and assumes that the rate of diffusion is only significant in the direction perpendicular to the surface of the building element. Thus the radon flux into the room is predicted by solving the one dimensional diffusion equation under the boundary conditions appropriate to the particular wall or floor structure (e.g. cavity wall or multi-layer). The equilibrium decay product concentration is then found by considering the ventilation rate and the concentration of the decay products in the ventilating air. The one dimensional approach is inappropriate for predicting the flux through the floor when there are discontinuities such as cracks and service ducts present, since these introduce a radon concentration gradient in the horizontal as well as the vertical plane. A two-dimensional model using numerical techniques is being developed which will help to overcome this problem. Preliminary results from an experimental programme designed to test the models are presented.