Models for the transfer of radionuclides in the food chain

Abstract

Airborne particulates containing radionuclides like 238U decay with several intermediate products to 226Ra, 222Rn, and thence to 210Po and 210Po which are ubiquitous in soils and consequently available for uptake for animals and plants. After decay of 222Rn or its daughters in air, 210Po deposits on plants. Grazing animals may take up appreciable amounts of 210Po from the environment. A dynamic multi-compartmental model is proposed to describe mathematically the radionuclides transfer to the food chain and predict the activity concentration within each sub-compartment. The dynamic model is defined by a system of linear differential equations with constant coefficients describing the mass balances in the different compartments, taking into account the fluxes in and out and the radionuclides decay inside each compartment. The system of differential equations was written in the standard matrix form used in the space state approach and solved numerically using Matlab software. The model output represents the time variation for the considered radionuclide in the pasture-cow-milk exposure route and also its distribution within the cow. The aim of the model is to evaluate whether this exposure pathway is relevant in radiological terms; it also allows verifying if more detailed investigations are needed in order to evaluate possible exposure consequences in a specific site. A simulation was done for 226Ra, 210Pb and 210Po in a hypothetical case study for different site conditions considering as initial state a contaminated pasture that is consumed by a cow. The simulation results fit within the range of values given in the literature, although many assumptions had to be done due to the limited available data; conservative parameters had to be used in some cases.