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Class of nuclear reactors with improved safety over its predecessors
Generation III reactors, or Gen III reactors, are a class of nuclear reactors designed to succeed Generation II reactors, incorporating evolutionary improvements in design. These include improved fuel technology, higher thermal efficiency, significantly enhanced safety systems (including passive nuclear safety), and standardized designs intended to reduce maintenance and capital costs. They are promoted by the Generation IV International Forum (GIF). The first Generation III reactors to begin operation were Kashiwazaki 6 and 7 advanced boiling water reactors (ABWRs) in 1996 and 1997. From 2012, both have been shut down due to a less permissive political environment in the wake of the Fukushima nuclear accident. Due to the prolonged period of stagnation in the construction of new reactors and the continued (albeit declining) popularity of Generation II/II+ designs in new construction, relatively few third generation reactors have been built.

The older Gen II reactors comprise the vast majority of current nuclear reactors. Gen III reactors are so called advanced light water reactors (LWRs). Gen III+ reactors are labeled as "evolutionary designs". Though the distinction between Gen II and III reactors is arbitrary, few Gen III reactors have reached the commercial stage as of 2022. The Generation IV International Forum calls Gen IV reactors "revolutionary designs". These are concepts for which no concrete prognoses for realization existed at the time. The improvements in reactor technology in third generation reactors are intended to result in a longer operational life (designed for 60 years of operation, extendable to 100+ years of operation prior to complete overhaul and reactor pressure vessel replacement) compared with currently used Generation II reactors (designed for 40 years of operation, extendable to 60+ years of operation prior to complete overhaul and pressure vessel replacement). The core damage frequencies for these reactors are designed to be lower than for Generation II reactors – 60 core damage events for the European Pressurized Reactor (EPR) and 3 core damage events for the Economic Simplified Boiling Water Reactor (ESBWR) per 100 million reactor years are significantly lower than the 1,000 core damage events per 100 million reactor years for BWR/4 Generation II reactors. An independent analysis conducted by environmental scientist Barry Brook on the greater efficiency and therefore lower material needs of Gen III reactors, corroborates this finding.

Gen III+ reactor designs are an evolutionary 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.

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 (first grid connection on 2018 06 30) in China. In the United States, reactor designs are certified by the Nuclear Regulatory Commission (NRC). as of 2020, the commission has approved seven new designs, and is considering one more design as well as renewal of an expired certification.

Proponents of nuclear power and some who have historically been critical have acknowledged that third generation reactors as a whole are safer than older reactors. Edwin Lyman, a senior staff scientist at the Union of Concerned Scientists, has challenged specific cost saving design choices made for two Generation III reactors, both the AP1000 and ESBWR. Lyman, John Ma (a senior structural engineer at the NRC), and Arnold Gundersen (an anti nuclear consultant) are concerned about what they perceive as weaknesses in the steel containment vessel and the concrete shield building around the AP1000 in that its containment vessel does not have sufficient safety margins in the event of a direct airplane strike. Other engineers do not agree with these concerns, and claim the containment building is more than sufficient in safety margins and factors of safety. The Union of Concerned Scientists in 2008 referred to the EPR as the only new reactor design under consideration in the United States that " appears to have the potential to be significantly safer and more secure against attack than today's reactors." There have also been issues in fabricating the precision parts necessary to maintain safe operation of these reactors, with cost overruns, broken parts, and extremely fine steel tolerances causing issues with new reactors under construction in France at the Flamanville Nuclear Power Plant.

Developer(s) Reactor name(s) Type MWe (net) MWe (gross) MWth Notes General Electric, Toshiba, Hitachi ABWR;US ABWR BWR 1350 1420 3926 In operation at Kashiwazaki since 1996. NRC certified in 1997. It was initially intended to be named the "ACC 1000", but was ultimately named as the "Hualong One" or "HPR 1000". Fangchenggang Units 3–6 will be the first to utilize the HPR 1000 design, with Units 3 & 4 currently under construction as of 2017. OKBM Afrikantov VVER 1000/428 990 1060 3000 First version of the AES 91 design, designed and used for Tianwan Units 1 & 2, which came online in 2007. VVER 1000/428M 1050 1126 3000 Another version of the AES 91 design, also designed and used for Tianwan (this time for Units 3 & 4, which came online in 2017 and 2018, respectively). VVER 1000/412 917 1000 3000 First constructed AES 92 design, used for the Kudankulam.

Developer(s) Reactor name(s) Type MWe (net) MWe (gross) MWth Notes General Electric, Hitachi ABWR II BWR 1638 1717 4960 Improved version of the ABWR. Uncertain development status. Mitsubishi APWR;US APWR;EU APWR;APWR+ PWR 1600 1700 4451 Two units planned at Tsuruga cancelled in 2011. US NRC licensing for two units planned at Comanche Peak was suspended in 2013. The original APWR and the updated US APWR/EU APWR (also known as the APWR+) differ significantly in their design characteristics, with the APWR+ having higher efficiency and electrical output. Westinghouse AP600 600 619 NRC certified in 1999. Combustion Engineering System 80+ 1350 1400 NRC certified in 1997. OKBM Afrikantov VVER 1000/466(B) 1011 1060 3000 This was the first AES 92 design to be developed, originally intended to be built at the proposed Belene Nuclear Power Plant, but construction was later halted. Candu Energy Inc. EC6PHWR 750 2084 The EC6 (Enhanced CANDU 6) is an evolutionary upgrade of previous CANDU designs. Like other CANDU designs, it is capable of using unenriched natural uranium as fuel. AFCR 740 2084 The Advanced Fuel CANDU Reactor is a modified EC6 design that has been optimized for extreme fuel flexibility with the ability to handle numerous potential reprocessed fuel blends and even thorium. It is currently undergoing late stage development as part of a joint venture between SNC Lavalin, CNNC, and Shanghai Electric. Various (see MKER Article.) MKER BWR 1000 2085 A Development of the RBMK nuclear power reactor. Fixes all of the RBMK reactor's design errors and flaws and adds a full containment building and Passive nuclear safety features such as a passive core cooling system. The physical prototype of the MKER 1000 is the 5th unit of the Kursk Nuclear Power Plant. The construction of Kursk 5 was cancelled in 2012 and a VVER TOI whose construction is ongoing since 2018 is being built instead as of 2018. (see RBMK article)

Developer(s) Reactor name(s) Type MWe (net) MWe (gross) MWth First grid connection Notes Westinghouse, Toshiba AP1000 PWR 1117 1250 3400 2018 06 30 Sanmen NRC certified Dec 2005. OKB Gidropress VVER 1200/392M 1114 1180 3200 2016 08 05 Novovoronezh II The VVER 1200 series is also known as the AES 2006/MIR 1200 design. This particular model was the original reference model used for the VVER TOI project. VVER 1200/491 1085 1199 3200 2018 03 09 Leningrad II VVER 1200/509 1114 1200 3200 Under construction in Akkuyu NPP, as Akkuyu 1 & 2. Grid connections due 2023 & 2024. VVER 1200/523 1080 1200 3200 2.4 GWe Rooppur Nuclear Power Plant of Bangladesh is under construction. The two units of VVER 1200/523 generating 2.4 GWe are planned to be operational in 2024 and 2025. VVER 1200/513 1200 3200 Standardized version of the VVER 1200 based in part on the VVER 1300/510 design (which is the current reference design for the VVER TOI project). VVER 1300/510 1115 1255 3300 The VVER 1300 design is also known as the AES 2010 design, and is sometimes mistakenly designated as the VVER TOI design. The VVER 1300/510 is based on the VVER 1200/392M that was originally used as the reference design for the VVER TOI project, although the VVER 1300/510 now serves that role (which has led to confusion between the VVER TOI plant design and the VVER 1300/510 reactor design). Multiple units are currently planned for construction at several Russian nuclear plants. First units under construction at Kursk Nuclear Power Plant. BARC IPHWR 700 PHWR 630 700 2166 2021 1 10 Kakrapar Successor of indigenous 540MWe PHWR with increased output and additional safety features. Under construction and due to come online in 2020. Unit 3 at Kakrapar Atomic Power Station achieved first criticality on 22 July 2020. The Unit 3 was connected to the grid on 10 January 2021.

Developer(s) Reactor name(s) Type MWe (net) MWe (gross) MWth Notes Toshiba EU ABWR BWR 1600 4300 Updated version of the ABWR designed to meet EU guidelines, increase reactor output, and improve design generation to III+. Areva Kerena 1250 1290 3370 Previously known as the SWR 1000. Based on German BWR designs, mainly that of Gundremmingen units B/C. Co developed by Areva and E. ON. General Electric, Hitachi ESBWR 1520 1600 4500 Based on the unreleased SBWR design which in turn was based on the ABWR. Being considered for North Anna 3. Eschews the use of recirculation pumps entirely in favor of a design completely reliant on natural circulation (which is very unusual for a boiling water reactor design). KEPCO APR+ PWR 1505 1560 4290 APR 1400 successor with increased output and additional safety features. Areva, Mitsubishi ATMEA1 1150 3150 Proposed Sinop plant did not proceed OKB Gidropress VVER 600/498 600 1600 Essentially a scaled down VVER 1200. Commercial deployment planned by 2030 at Kola. Candu Energy Inc. ACR 1000 PHWR 1085 1165 3200 The Advanced CANDU Reactor is a hybrid CANDU design that retains the heavy water moderator but replaces the heavy water coolant with conventional light water coolant, significantly reducing heavy water costs compared to traditional CANDU designs but losing the characteristic CANDU capability of using unenriched natural uranium as fuel. BARC IPWR 900 PWR 900 2700 India's first light water reactor, a Gen 3+ design based on the CLWR B1 reactor of Arihant class submarine.