Atmospheric CH, CO and Ar measurements around a Swiss pressurized water reactor during an annual revision period.

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