development of Gen III reactors, offering improvements in safety over Gen III reactor designs. Manufacturers began development of Gen III+ systems in the 1990s by building on the operating experience of the American, Japanese, and Western European light-water reactor. The nuclear industry began to promote a nuclear renaissance suggesting that Gen III+ designs should solve three key problems: safety, cost and buildability. Construction costs of US$1,000/kW were forecast, a level that would make nuclear competitive with gas, and construction times of four years or less were expected. However, these estimates proved over-optimistic. A notable improvement of Gen III+ systems over second-generation designs is the incorporation in some designs of passive safety features that do not require active controls or operator intervention but instead rely on gravity or natural convection to mitigate the impact of abnormal events. Generation III+ reactors incorporate extra safety features to avoid the kind of disaster suffered at Fukushima in 2011. Generation III+ designs, passive safety, also known as passive cooling, requires no sustained operator action or electronic feedback to shut down the plant safely in the event of an emergency. Many of the Generation III+ nuclear reactors have a core catcher. If the fuel cladding and reactor vessel systems and associated piping become molten, corium will fall into a core catcher which holds the molten material and has the ability to cool it. This, in turn protects the final barrier, the containment building. As an example, Rosatom installed a 200-tonne core catcher in the VVER reactor as the first large piece of equipment in the reactor building of Rooppur 1, describing it as "a unique protection system". In 2017, Rosatom has started commercial operations of the NVNPP-2 Unit 1 VVER-1200 reactor in central Russia, marking the world's first full start-up of a generation III+ reactor. First reactors The first Generation III reactors were built in Japan, in the form of advanced boiling water reactors. On 5 August 2016, a Generation III+ VVER-1200/392M reactor became operational (first grid connection) at Novovoronezh Nuclear Power Plant II in Russia, which was the first operational Generation III+ reactor. Several other Generation III+ reactors are under late-stage construction in Europe, China, India, and the United States. The next Generation III+ reactors to come online were an AREVA EPR reactor at the Taishan Nuclear Power Station (first grid connection on 2018-06-29) and a Westinghouse AP1000 reactor at the Sanmen Nuclear Power Station