A process and a system for detecting defective nuclear fuel assemblies in situ. The system includes two basic subsystems. The first subsystem is a sample collection system consisting of a hood (2) placed over the fuel. A technique whereby a vacuum is drawn across a submerged nozzle of a degas tank (14) is used to draw up the sample of fluid out of a fuel assembly (4). The second subsystem determines the amount of Kr-85 radio-isotope contained in the fluid sample. This information is used to determine whether the fuel assembly contains a defective, i.e., leaking, fuel rod. The measurement values for Kr-85 in the fluid sample extracted from the fuel assembly are compared to the measurement values for Kr-85 in fluid samples taken from the reactor pool prior to fuel assembly testing and fluid samples taken from non-leaking control cells, the latter values representing the background to which the subject fuel assembly is compared. Based on this comparison, a determination is made concerning whether the subject fuel assembly is defective.