Although researchers have extensively studied student conceptions of radioactivity, the conceptions held by preservice teachers on this subject are largely absent from the literature. We conducted a qualitative content analysis of problem-centered interviews with preservice teachers (N=13) to establish which conceptions are held by preservice teachers and to examine these conceptions' structure in coordination classes. As has already been observed in students, some preservice teachers inadequately differentiate between radioactive matter and ionizing radiation and between fission and decay. We also observed that preservice teachers tend to describe the activation of materials due to ionizing radiation despite having previously denied an activation, thus showing that the conception of activation of materials can reemerge in particular framings. Within the interviews conducted, the concept of energy emerged as a central coordination class regarding radioactivity. This coordination class appeared across contexts and proved fruitful in explaining preservice teachers' conceptions about radioactivity. We will use the results from this study to develop a teaching-learning laboratory for preservice teachers in which they can actively study high school students' conceptions while reflecting on their own. In this way, these findings will contribute to improving the structure of nuclear physics courses at the university.