Design and Material Selection for Leak-Before Break Nature of Double Walled Once Through Steam Generators in Lead-Bismuth Cooled Fast Reactors

Among various types of Gen-IV reactors, lead cooled fast reactors (LFRs) can support a closed fuel cycle to minimize waste burden by managing both fertile elements and transuranic actinides efficiently. LFRs often adapt an External Boiling Bayonet Steam Generator (EBSSG) system instead of conventional once-through high-pressure steam generator. LFR with the EBSSG designs have advantages in terms of thermal efficiency and maintainability during operation. While EBSSG designs have been proven through extensive operating experiences in submarine reactors of the Former Soviet Union, their complex tube geometries invite life-limiting disadvantages ranging from vibration-led fatigue, oxide particles sedimentation and increased resistance to natural circulation under accident conditions. To assure reliable performance of steam generator over long design lives, both structural design and material degradation issues should be solved. In the paper, we present an innovative LFR steam generator design concepts for a non-refueling and hermetically sealed 40-year life micro-modular LFR designated as Micro-Uranus. The requirements on durability, maintainability and accident-tolerance can be met with the new double-walled once-through steam generators (DWOTSG) design. In order to assure Leak-Before-Break (LBB) of steam generator in Micro-Uranus, the inter-tubular gap of double walled heat transfer tubes is filled with engineered materials for both leak detection and conductivity enhancement. Tube materials are also selected to assure adequate resistance to corrosion and liquid metal embrittlement on heavy-liquid metal side by selecting proven austenitic stainless steels and lowering operating temperatures. Additional corrosion resistance over 40 year-life can be delivered by Functionally Graded Composite (FGC) tubes with substrate made of internationally certified materials. Oxide deposition and thermal degradation can be controlled by advanced flow channel structure and maintenance designs. The LBB characteristics, corrosion resistance, thermal-hydraulic performance of the innovative DWOTSG design have been analyzed by using linear elastic fracture mechanics models. Leak detection sensitivities required in LBB qualifications for preventing excessive defect growth to unstable rupture conditions has been computed conservatively. (author)