TOCATTA: a dynamic transfer model of ¹⁴C from the atmosphere to soil-plant systems

Many nuclear facilities release ¹⁴C into the environment, mostly as ¹⁴CO₂, which mixes readily with stable CO₂. This complete isotopic mixing (equilibrium) is often used as the basis for dose assessment models. In this paper, a dynamic compartment model (TOCATTA) has been investigated to describe ¹⁴C transfer in agricultural systems exposed to atmospheric ¹⁴C releases from nuclear facilities under normal operating or accidental conditions. The TOCATTA model belongs to the larger framework of the SYMBIOSE modelling and simulation platform that aims to assess the fate and transport of a wide range of radionuclides in various environmental systems. In this context, the conceptual and mathematical models of TOCATTA have been designed to be relatively simple, minimizing the number of compartments and input parameters required, appropriate to its use in an operational mode. This paper describes in detail ¹⁴C transfer in agricultural plants exposed to time-varying concentrations of atmospheric ¹⁴C, with a consideration also of the transfer pathways of ¹⁴C in soil. The model was tested against in situ data for ¹⁴C activity concentration measured over two years on a grass field plot located 2 km downwind of the AREVA NC La Hague nuclear reprocessing plant. The first results showed that the model roughly reproduced the observed month-to-month variability in grass ¹⁴C activity, but under-estimated (by about 33%) most of the observed peaks in the ¹⁴C activity concentration of grass. This tends to prove that it is not suitable to simulate intra-monthly variability, and a fortiori, the response of vegetation to accidental releases that may occur during the day. The need to increase the temporal resolution of the model has been identified in order to simulate the impact of intermittent ¹⁴C releases occurring either the day or night, such as those recorded by the AREVA NC plant.