Many nuclear power plants are located near international borders. For several years, the construction of a new nuclear power plant near the border with Belarus has been discussed in Lithuania. Based on a retrospective on the discussions about the construction of the Temelin nuclear power plant in the Czech Republic, near the border with Austria, this article considers the question of which cooperation obligations are incumbent on the installing States with regard to the neighboring country. Here, the text first addresses the question whether the establishment of nuclear power plants near international borders is permissible under international law at all. While there is a lack of state practice, which would be necessary for the development of a rule of customary international law rule to this effect, the principle of state sovereignty, which is laid down in the Charter of the United Nations, could preclude the construction of nuclear power plants near the border because the establishment of the power plant would require the neighboring state to take precautionary measures for potential accidents in the power plant across the border.An intrusion into the sovereignty of the neighboring state, which is incompatible with international law, can already exist if a serious danger for the neighboring state is created, even if this danger has not (yet) been turned into a concrete damage. Due to the serious extent of the potential damages, the normal operation of a nuclear power plant already amounts to an ultra-hazardous activity, which can lead to a liability which requires neither the illegality of the damaging act, nor a culpable action on the part of the causing state.The current customary international law already gives the neighboring State a right to be consulted by the constructing State. On the basis of consultations between the Czech Republic and Austria, it will be shown which requirements such consultations must meet. In addition, Lithuania’s obligations under the Espoo Convention will be touched upon shortly.