FCCI behavior of high-strength ferritic/martensitic steel (PNC-FMS) cladding fuel pins. Results of interim post irradiation examinations of FMS-1 irradiation test

As a part of FMS-1 irradiation test, the interim post irradiation examinations of the ferritic/martensitic steel clad fuels, which were irradiated up to about 72 GWd/t, were performed. The FMS-1 is the first irradiation test using the ferritic/martensitic steels for cladding in JOYO. Following results were obtained from the observations of microstructure and the elemental analysis focused on the cladding inner surface. (1) Maximum corrosion depth value of PNC-FMS cladding was almost equal to that of HT-9M cladding, and also comparable to the values of previous austenitic cladding data. (2) The equation for the evaluation of FCCI derived from the database of the corrosion depth of austenitic cladding, which included the parameters of time averaged temperature of cladding inner surface, local burn-up and initial O/M ratio of fuel pellets, was able to apply to the ferritic/martensitic cladding. (3) The corrosion morphology was not the intergranular type but uniform one, which was the FCCI layer similar to PNC316 cladding. (4) The detected elements by SXMA in the FCCI layer of cladding inner surface were Fe, Cr (cladding components), Cs, Te (fission products) and O. This chemical composition of FCCI layer was also similar to that of conventional austenitic cladding. (author)