Criteria for removal of defective fuel rod from fuel assembly under repair without cladding rupture

During repair of a failed fuel assembly (FA) there is a risk of cladding rupture while a defective fuel rod is forced out of the assembly skeleton. To reduce the corresponding risks, a program of experimental and analytical studies for WWER fuel was performed. It resulted in formulation of criteria for successful removal of the defective fuel rod from the FA under repair. 'Successful' means that no cladding rupture occurs. The paper summarizes the available data of post-irradiation examinations of WWER FAs with leaking fuel rods. A technique for express estimation of hydrogen content in cladding of a defective fuel rod is presented. The degradation of cladding mechanical properties can be estimated with this technique as well. A criterion of severe secondary hydriding involved in the risk analysis is also discussed. Finally, it is shown how the information on operation conditions may be used for prompt evaluation of the limiting force for successful removal of a defective fuel rod during FA repair in the inspection stand. (author)