DESIGN LIMIT FOR CLADDING LOAD DUE TO OPERATIONAL FUEL SWELLING

摘要

The design limit for cladding equivalent plastic strain due to operational fuel swelling can be raised from 2.5 % to a value of 3.5%. The reference value for calculating the equivalent plastic strain of the cladding is the state where the cladding is completely crept down on the fuel pellet (due to the coolant overpressure) and the fabrication fuel-cladding gap is closed.rnFor some fuel rods with very high burnup the fuel rod inner gas pressure may also exceed the coolant pressure due to high fission gas release from the fuel pellets. Then the fuel-cladding gap can remain open. This case is not considered in this paper.rnThe integrity of the fuel cladding is not hampered by the proposed design limit change. This paper justifies the increase in the design limit on grounds of operational experience with fuel rods, theoretical considerations, and experimental investigations.rnFramatome gained operational in-reactor experience with fuel rods on equivalent plastic strain of the cladding up to values of 3.4%.rnHigh cladding ductility is available when the stress sensitivity n approaches values close to unity. The stress sensitivity n reaches low values close to unity when the cladding is loaded by in-reactor swelling of the fuel. Experimental investigations on cladding material demonstrate high cladding ductility in the low n regime. The design limit for equivalent plastic strain due to operational fuel swelling can be raised at least up to a value of 3.5 % which was already attained in operational experience.