Will the drinking water become unfit for human consumption following a radioactive fall-out? Four theoretical models were adjusted and linked to calculate the contamination of radionuclides in drinking water from radioactive fallout. The source term, the dispersion in the atmosphere, the processes in a surface water source, and the effect of the different purification steps in waterworks were calculated and simulated. The scenarios studied were nuclear fallouts following a serious nuclear power plant accident and a nuclear explosion respectively. The activity concentration (Bq/l) for the relevant radionuclides in drinking water was calculated and normalised to different deposition levels of cesium-137, and then compared to the maximum permitted levels set by the Council of the European Union. At a deposition of 100 kBq/m2 cesium-137, the limits for the iodine isotopes might be exceeded for the first days following a nuclear power plant accident. Considering a fallout of more than 100 kBq/m2 cesium- 137 following a nuclear explosion, the EU limits for iodine isotopes, strontium isotopes and radionuclides with half-lives longer than 10 days, would become exceeded for 3-4 weeks up to 3-4 months, depending on the purification steps in the waterworks. Finally, validation of the model using data from the accidents at Chernobyl and Fukushima showed good agreement between calculated data and reality.