Atmospheric ¹⁴CH₄, ¹⁴CO₂ and ³⁷Ar measurements around a Swiss pressurized water reactor during an annual revision period

Since the 1980s, radiocarbon (¹⁴C) has gained attention as a valuable tool to quantify the amount of fossil and non-fossil emissions of CO₂ and CH₄ in the atmosphere. Since the 1970s, however, important ¹⁴C emissions in the atmosphere also occur through the operation of nuclear power plants. The limited knowledge about these emissions challenges the use of ¹⁴C as a universal source apportionment tool. Depending on the reactor type, ¹⁴C is emitted in different forms; in particular, pressurized water reactors emit ¹⁴C as a mixture of ¹⁴CH₄ and ¹⁴CO₂. However, few atmospheric ¹⁴C measurements close to nuclear power plants are available, which mostly address ¹⁴CO₂ emissions. Argon-37 (³⁷Ar) can also be produced in nuclear reactors; however, its atmospheric measurement is challenging, resulting in limited available data. In this study, we sampled ambient air during 20-75 min into 18 individual bags around the pressurized water reactor in Gösgen, Switzerland, at the beginning of the annual revision period in 2019, when ¹⁴C and ³⁷Ar emissions can be expected due to the depressurization of the reactor. These samples were analyzed for ¹⁴CH₄, ¹⁴CO₂ and partly for ³⁷Ar. About 1 km downwind of the stack, we found background-corrected activities up to 1900, 370, and 93 mBq m^-3 respectively. Considering corresponding background activities of 0.3, 48 and 2 mBq m^-3 for ¹⁴CH₄, ¹⁴CO₂, and ³⁷Ar, this represents an excess of about 6300, 7.4, and 47 times, respectively. Using an atmospheric dispersion model, we satisfactorily simulated the ¹⁴CH₄ and ¹⁴CO₂ activities in the surroundings of the reactor during this event. Our measurements emphasize the importance of nuclear power plants in the interpretation of atmospheric ¹⁴C measurements and show that pressurized water reactors represent a serious limitation in the use of ¹⁴C for source apportionment of CH₄ sources. Our results also provide insights into the approximate magnitude of civilian ³⁷Ar emissions from nuclear facilities specifically during maintenance operations.