The TOCATTA-χ model for assessing 14C transfers to grass: an evaluation for atmospheric operational releases from nuclear facilities

Abstract Radioactive 14 C is formed as a by-product of nuclear power generation and from the operation of nuclear fuel reprocessing plants like AREVA-NC La Hague (North France), which releases about 15 TBq per year of 14 C into the atmosphere. This article evaluates a recently improved radioecology model (TOCATTA-χ) to assess 14 C transfers to grassland ecosystems under normal operating conditions. The new version of the TOCATTA model (TOCATTA-χ) includes developments that were derived from PaSiM, a pasture model for simulating grassland carbon and radiocarbon cycling. The TOCATTA-χ model has been tested against observations of 14 C activity concentrations in grass samples collected monthly from six plots which are located around the periphery of the reprocessing plant. Simulated 14 C activities are consistent with observations on both intensively managed and poorly managed grasslands, but an adaptation of the mean turn-over time for 14 C within the plant is necessary in the model to account for different management practices. When atmospheric 14 C activity concentrations are directly inferred from observations, TOCATTA-χ performs better than TOCATTA (the root mean square error is decreased by 45%), but when atmospheric 14 C activity concentrations are not known and must be calculated, the uncertainty associated with the TOCATTA-χ model outcomes is estimated to be larger than the standard deviation of the observations.