Fabrication of rodlets with coated and lined Mo-alloy cladding for testing and irradiation

Molybdenum alloy fuel cladding with an outer protective layer has been proposed to achieve enhanced tolerance to severe loss of coolant accidents for light water reactors. The accident tolerant fuel cladding will need to meet stringent fuel reliability requirements and significantly delay steam reaction during a severe accident to provide plant operators additional coping time to recover from the accident. A laboratory measurable target for this feasibility study with coated Mo cladding is to demonstrate its capability to survive in steam and maintain fuel rods in coolable geometry at 1200-1500 Celsius degrees for at least 24 hours. Preliminary studies indicate such a capability may provide additional coying of 4-19 hours. A feasibility study undertaken since 2012 has demonstrated feasibility of fabricating long, thin-wall Mo and Mo-ODS tubes, and forming a metallurgically bonded protective coating of FeCrAl or Zr-alloy via physical vapor deposition to provide protection of the Mo cladding in 1200 C. degrees for 24 hours. The diametral ductility of the Mo and Mo-ODS thin wall tubes can be increased to 5-15% pre-irradiation to meet fuel operational requirements. Fabrication of the outer protective layer using thermo-mechanical processing, such as hot isostatic pressing and hot coextrusion, has been under evaluation to achieve high quality lined Mo cladding from commercial scale production. Welding of lined Mo tubes to end-caps are being developed using a combination of solid state welding of laser or electron beam seal welding. Rodlets of lined Mo tubes are being fabricated for further testing to validate its reliability under normal operation and capability of accident tolerance to the 1200- 1500 C. degrees range. Test reactor irradiation is also being planned. (authors)