The Green Deal of the European Union is to be a policy of sustainable transition from the long-term perspective; however, the ambitious plans were confronted with the energy crisis in 2022 and controversial position on two non-renewables: nuclear energy and gas. These have been formally recognized as sustainable energy sources and potential bases for direct investments. This paper deals with the sustainability of nuclear energy in the context of the new taxonomy. It offers a critical review and alternative approaches to the evaluation of the costs and benefits of nuclear energy to illustrate its unsustainability. As a result, a critical evaluation of nuclear energy from the perspective of the key goals of the sustainable energy transition is offered. It determines the most violated areas of the sustainable transition, such as decentralization and civil engagement, safety, and renewability. Additionally, the Polish nuclear program is discussed to illustrate greenwashing and the typical problems in the evaluation of nuclear energy projects. The results suggest that the commercial use of nuclear energy should not be included in the sustainable energy transition programs, although the research on nuclear technologies should be further supported due to the long-lasting consequences of its use since the 1950s.