The establishment and use of nuclear power plants to meet the energy need is a controversial socioscientific issue in all the countries of the world; as in Turkey. In this regard, the current study intended to investigate the effect of the socioscientific issue-based instructional activities related to the Nuclear Energy Plants (NEP) that have been attempted to be made widespread in our country on the pre-service science teachers' decisions, positions and types of informal reasoning they use while making their decisions. The current study employed one of the mixed methods; "the data transformative design model." At the end of the study, it was determined that the pre-service teachers had decided that the establishment of nuclear plants should not continue and that the instructional activities led them to change their positions. Moreover, it was found that while making their decisions, before the application, they were mostly engaged in the ecology-based informal reasoning; after the application, they mostly utilized the social type of informal reasoning. Thus, it was concluded that the pre-service science teachers' engagement in activities related to the sociosicentific issue of nuclear energy plants increased their types of reasoning they resorted to and contributed to their sophistication in reasoning. The more types of informal reasoning are used by individuals while making their decisions, the more conscious and reasonable decisions they can make. In this regard, it can be argued that the socioscientific issue-based instructional activities brought social dimensions related to the establishment of nuclear power plants to the fore in the decisions of the pre-service science teachers. Moreover, the opinions of the pre-service science teachers who will take an important role in giving direction to the future of the society about this issue should be taken into consideration by different social associations such as administrators, politicians, non-governmental organizations and particularly by teacher training programs.