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Since the first nuclear test explosion on July 16, 1945, at least eight nations have detonated 2,056 nuclear test explosions at dozens of test sites, including Lop Nor in China, the atolls of the Pacific, Nevada, Algeria where France conducted its first nuclear device, western Australia where the U.K. exploded nuclear weapons, the South Atlantic, Semipalatinsk in Kazakhstan, across Russia, and elsewhere.

Most of the test sites are in the lands of indigenous peoples and far from the capitals of the testing governments. A large number of the early tests—528—were detonated in the atmosphere, which spread radioactive materials through the atmosphere. The 1963 Limited Test Ban Treaty brought about the end of most, but not all, nuclear test explosions in the atmosphere. However, many underground nuclear blasts have also vented radioactive material into the atmosphere and left radioactive contamination in the soil.

Through nuclear test explosions, the nuclear testing nations have been able to proof-test new warhead designs and create increasingly sophisticated nuclear weapons. The overwhelming majority of the nuclear weapon test detonations were for "weapons development" and "weapons effects" purposes.

https://www.armscontrol.org/factsheets/nucleartesttally

The radionuclide 14C is created by nitrogen (14N)—naturally present in the atmosphere—capturing the neutrons released in excess during nuclear tests. Once formed, it is rapidly oxidized to 14CO and then to 14CO2, and it is then transferred to the global carbon reservoirs (the atmosphere, the ocean, and the terrestrial biosphere), where it is very difficult to remove from, because of its extremely long half-life (5730 years) (Currie et al. 2011)

https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC4165831/#:~:text=However%2C%20the%20large%20number%20of,socially%20destroyed%20sites%2C%20due%20to