In this work, the fission products 90Sr, 129I, and 137Cs and reactor nuclides 3H, and 134Cs were analyzed in the compartment water from the immediate vicinity of the Chernobyl and Fukushima NPPs, and the methods were adapted for these measurements. Both, radioanalytical and mass spectrometrical methods were used, such as liquid scintillation counting, gamma spectrometry via high-purity germanium detector, and accelerator mass spectrometry. The optimized methods were subsequently applied to natural water samples from the Chernobyl exclusion zone with large sample volumes. Further adaptations were made for small sample volumes of Fukushima surface water samples, which were sampled only a month after the accident. Finally, both drinking water as well as a variety of surface water samples from sites of the Tokyo 2020 Olympic Games were tested for their radionuclide content to assess potential risks to athletes and visitors.