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.