Possible plutonium contamination of the primary loop and potential cooling disturbances, both due to cladding breaches, result in two areas that need to be considered: (a) the behavior of defective fuel subassemblies transferred to in-vessel storage and (b) the occurrence of new defects in the cladding tubes at the end of service life. A response to the resulting requirement of cladding tube monitoring during in-vessel storage is given, and the following solution is proposed: If fuel subassemblies are stored in in-vessel drums, the defect can be temporarily exposed by rotating the drum and exposing the subassemblies to different levels of residual neutron radiation intensity. Fission products escaping during this process are measured by delayed neutron monitors, as applied in normal operation.