superheated fossil fuel fired boilers, also in wide use. 32 organizations in 13 countries are investigating the concept. SCWRs share the steam explosion and radioactive steam release hazards of BWRs and LWRs as well as the need for extremely expensive heavy duty pressure vessels, pipes, valves, and pumps. These shared problems are inherently more severe for SCWRs due to their higher temperatures. One SCWR design under development is the VVER-1700/393 (VVER-SCWR or VVER-SKD) – a Russian SCWR with double-inlet-core and a breeding ratio of 0.95. Fast reactors A fast reactor directly uses fission neutrons without moderation. Fast reactors can be configured to "burn", or fission, all actinides, and given enough time, therefore drastically reduce the actinides fraction in spent nuclear fuel produced by the present world fleet of thermal neutron light water reactors, thus closing the fuel cycle. Alternatively, if configured differently, they can breed more actinide fuel than they consume. Gas-cooled fast reactor (GFR) The gas-cooled fast reactor (GFR) features a fast-neutron spectrum and closed fuel cycle. The reactor is helium-cooled. Its outlet temperature is 850 °C. It moves the very-high-temperature reactor (VHTR) to a more sustainable fuel cycle. It uses a direct Brayton cycle gas turbine for high thermal efficiency. Several fuel forms are under consideration: composite ceramic fuel, advanced fuel particles, or ceramic-clad actinide compounds. Core configurations involve pin- or plate-based fuel assemblies or prismatic blocks. The European Sustainable Nuclear Industrial Initiative provided funding for three Generation IV reactor systems: Allegro: a 100 MWt gas-cooled fast reactor, planned for central or eastern Europe. The central European Visegrád Group are pursuing the technology. GoFastR: In 2013 German, British, and French institutes finished a 3-year collaboration study on the follow-on industrial scale design. They were funded by the EU's 7th FWP framework programme, with the goal of making a sustainable VHTR. Sodium-cooled fast reactor (SFR) Sodium-cooled fast reactors (SCFRs) have been operated in multiple countries since the 1980s. The two largest experimental sodium cooled fast reactors are in Russia, the BN-600 and the BN-800 (880 MWe gross). These NPPs are being used to provide operating experience and technological solutions that will be applied to the construction of the BN-1200 (OKBM Afrikantov first Gen IV reactor). The largest ever operated was the French Superphenix reactor at over 1200 MWe, successfully operating before decommissioning in 1996. In India, the Fast Breeder Test Reactor (FBTR) reached criticality in October 1985. In September