Continental-scale enrichment of atmospheric ¹⁴CO₂ from the nuclear power industry: potential impact on the estimation of fossil fuel-derived CO₂

Abstract. The ¹⁴C-free fossil carbon added to atmospheric CO₂ by combustion dilutes the atmospheric ¹⁴C/C ratio (Δ¹⁴C), potentially providing a means to verify fossil CO₂ emissions calculated using economic inventories. However, sources of ¹⁴C from nuclear power generation and spent fuel reprocessing can counteract this dilution and may bias ¹⁴C/C-based estimates of fossil fuel-derived CO₂ if these nuclear influences are not correctly accounted for. Previous studies have examined nuclear influences on local scales, but the potential for continental-scale influences on Δ¹⁴C has not yet been explored. We estimate annual ¹⁴C emissions from each nuclear site in the world and conduct an Eulerian transport modeling study to investigate the continental-scale, steady-state gradients of Δ¹⁴C caused by nuclear activities and fossil fuel combustion. Over large regions of Europe, North America and East Asia, nuclear enrichment may offset at least 20% of the fossil fuel dilution in Δ¹⁴C, corresponding to potential biases of more than −0.25 ppm in the CO₂ attributed to fossil fuel emissions, larger than the bias from plant and soil respiration in some areas. Model grid cells including high ¹⁴C-release reactors or fuel reprocessing sites showed much larger nuclear enrichment, despite the coarse model resolution of 1.8°×1.8°. The recent growth of nuclear ¹⁴C emissions increased the potential nuclear bias over 1985–2005, suggesting that changing nuclear activities may complicate the use of Δ¹⁴C observations to identify trends in fossil fuel emissions. The magnitude of the potential nuclear bias is largely independent of the choice of reference station in the context of continental-scale Eulerian transport and inversion studies, but could potentially be reduced by an appropriate choice of reference station in the context of local-scale assessments.