Nuclear industry often closes itself to research projects of the social sciences and to their realization by means of conventional tools such as questionnaires, interviews and the study of archives. On the basis of the observation covering the work of employees exposed to the risks of radiation, the article shows that an ethnographical procedure, where possible, is not without problems either, this time in terms of interpretation because of the context of uncertainty and risk which weighs on the observer as much as on those observed. The difficulty of observational work in this case, however, is not an absolute hindrance to any kind of knowledge, partly reflecting, as it does, the very difficulty of industrial work which is to be investigated. The participation of the observer in such delicate situations appears, in hindsight, as a kind of initiation into a new way of communicating with colleagues at work, which permits a certain control of the interpretations.