lives of the fissionable elements present in spent nuclear fuel while generating electricity largely as a byproduct. Lead-cooled fast reactor The lead-cooled fast reactor (LFR) features a fast-neutron-spectrum lead or lead/bismuth eutectic (LBE) coolant with a closed fuel cycle. Proposals include a small 50 to 150 MWe that features a long refueling interval, a modular system rated at 300 to 400 MWe, and a large monolithic plant at 1,200 MWe. The fuel is metal or nitride-based containing fertile uranium and transuranics. The reactor is cooled by natural convection with a reactor outlet coolant temperature of 550-800 °C. The higher temperature enables the production of hydrogen by thermochemical processes. The European Sustainable Nuclear Industrial Initiative is funding a 100 MWt LFR, an accelerator-driven sub-critical reactor called MYRRHA. It is to be built in Belgium with construction expected by 2036. A reduced-power model called Guinevere was started up at Mol in March 2009 and became operational in 2012. Two other lead-cooled fast reactors under development are the SVBR-100, a modular 100 MWe lead-bismuth cooled fast neutron reactor concept designed by OKB Gidropress in Russia and the BREST-OD-300 (Lead-cooled fast reactor) 300 MWe, to be developed after the SVBR-100, it will dispense with the fertile blanket around the core and will supersede the sodium cooled BN-600 reactor design, to purportedly give enhanced proliferation resistance. Preparatory construction work commenced in May 2020. Assessment The GEN IV Forum reframes the reactor safety paradigm from accepting that nuclear accidents can occur and should be mastered to eliminating the possibility of an accident. Active and passive safety systems would be at least as effective as those of Generation III systems and render the most severe accidents physically impossible. Relative to Gen II-III, Gen IV reactors include: Nuclear waste that remains radioactive for a few centuries instead of millennia 100–300x energy yield from the same amount of nuclear fuel Broader range of fuels, including unencapsulated raw fuels (non-pebble MSR, LFTR). Potential to burn existing nuclear waste and produce electricity: a closed fuel cycle. Improved safety via features such as ambient pressure operation, automatic passive reactor shutdown, and alternate coolants. A specific risk of the SFR is related to using metallic sodium as a coolant. In case of a breach, sodium explosively reacts with water. Argon is used to prevent sodium oxidation. Argon can displace oxygen in the air and can pose hypoxia concerns for workers. This was a