Chapter 5 - FeCrAl—iron–chromium–aluminum monolithic alloys

Abstract

The use of monolithic FeCrAl alloys to replace zirconium alloys is the second nearest term for the accident-tolerant fuel claddings after coated zirconium alloys. Currently a single layer of less than 1-mm-thick zirconium alloy metal separates the fuel cavity and its toxic compounds from the coolant water. The simplest and most elegant improvement would be to replace the current alloy for another alloy, obviously as resistant to corrosion as zirconium under normal operation conditions but a thousand times more resistant to attack by steam under severe accident conditions. An alloy down selection was conducted in steam under severe accident scenarios (>1200°C) and the highest environmental accident resistance was always offered by FeCrAl alloys because of the formation of an adherent, thin, and shielding film of alumina, which protected the FeCrAl from steam attack until its melting point. FeCrAl are ferritic materials that have a low corrosion rate under normal operation conditions by the formation of a protective chromium oxide on the surface. Ferritic FeCrAl are also resistant to environmental cracking from the coolant side. FeCrAl alloys can be made into full-length fuel rod tubes with a wall thickness of less than 0.5mm and they can be industrially welded to hermetically seal the fuel cavity by a solid-state method. The thermal neutron absorption cross section of FeCrAl is approximately ten times higher than that of the current zirconium alloys, but this fact can be partially mitigated by making the cladding wall thickness half of the current values for Zircaloy owing to the stronger mechanical properties at temperature of the FeCrAl alloys. The use of FeCrAl cladding may initially increase the tritium release into the coolant but is anticipated that the oxidation of the cladding from the fuel cavity side and the coolant side will be effective barriers for hydrogen diffusion across the tube wall. The use of FeCrAl cladding will eliminate the occurrence of debris fretting and shadow corrosion, two current Zircaloy cladding environmental degradation modes. FeCrAl tube cladding concepts are currently under irradiation both at the advanced test reactor in Idaho National Laboratory and in two BWR commercial nuclear power stations in the United States. Even though FeCrAl materials were never used in light water reactors before, their characterization in reactor anticipated environments is advancing rapidly through international development efforts.