Primary cooling circuits

As stated above, the water in the primary circuits is kept under a constant elevated pressure to avoid its boiling. Since the water transfers all the heat from the core and is irradiated, the integrity of this circuit is crucial. Four main components can be distinguished:

1. Reactor vessel: water flows through the fuel assemblies which are heated by the nuclear chain reaction.
2. Volume compensator (pressurizer): to keep the water under constant but controlled pressure, the volume compensator regulates the pressure by controlling the equilibrium between saturated steam and water using electrical heating and relief valves.
3. Steam generator: in the steam generator, the heat from the primary coolant water is used to boil the water in the secondary circuit.
4. Pump: the pump ensures the proper circulation of the water through the circuit.

To provide for the continued cooling of the reactor core in emergency situations the primary cooling is designed with redundancy.

Secondary circuit and electrical output

The secondary circuit also consists of different subsystems:

1. Steam generator: secondary water is boiled taking heat from the primary circuit. Before entering the turbine remaining water is separated from the steam so that the steam is dry.
2. Turbine: the expanding steam drives a turbine, which connects to an electrical generator. The turbine is split into high and low pressure sections. To boost efficiency, steam is reheated between these sections. Reactors of the VVER-1000 type deliver 1 GW of electrical power.
3. Condenser: the steam is cooled and allowed to condense, shedding waste heat into a cooling circuit.
4. Deaerator: removes gases from the coolant.
5. Pump: the circulation pumps are each driven by their own small steam turbine.

To increase efficiency of the process, steam from the turbine is taken to reheat coolant in the secondary circuit before the deaerator and the steam generator. Water in this circuit is not supposed to be radioactive.

Tertiary cooling circuit and district heating

The tertiary cooling circuit is an open circuit diverting water from an outside reservoir such as a lake or river. Evaporative cooling towers, cooling basins or ponds transfer the waste heat from the generation circuit into the environment.

In most VVERs this heat can also be further used for residential and industrial heating. Operational examples of such systems are Bohunice NPP (Slovakia) supplying heat to the towns of Trnava^[12] (12 kilometres [7.5 mi] away), Leopoldov (9.5 kilometres [5.9 mi] away), and Hlohovec (13 kilometres [8.1 mi] away), and Temelín NPP (Czech Republic) supplying heat to Týn nad Vltavou 5 kilometres (3.1 mi) away. Plans are made to supply heat from the Dukovany NPP to Brno (the second-largest city in the Czech Republic), covering two-thirds of its heat needs.^[13]

Safety barriers

A typical design feature of nuclear reactors is layered safety barriers preventing escape of radioactive material. VVER reactors have three layers:

1. Fuel rods: the hermetic Zirconium alloy (Zircaloy) cladding around the uranium oxide sintered ceramic fuel pellets provides a barrier resistant to heat and high pressure.
2. Reactor pressure vessel wall: a massive steel shell encases the whole fuel assembly and primary coolant hermetically.
3. Reactor building: a concrete containment building that encases the whole first circuit is strong enough to resist the pressure surge a breach in the first circuit would cause.

Compared to the RBMK reactors – the type involved in the Chernobyl disaster – the VVER uses an inherently safer design because the coolant is also the moderator, and by nature of its design has a negative void coefficient like all PWRs. It does not have the graphite-moderated RBMK's risk of increased reactivity and large power transients in the event of a loss of coolant accident. The RBMK reactors were also constructed without containment structures on grounds of cost due to their size; the VVER core is considerably smaller.^[14]

VVER-440

One of the earliest versions of the VVER-type, the VVER-440 manifested certain problems with its containment building design. As it was at the beginning with the models V-230 and older not constructed to resist the design basis large pipe break, the manufacturer added with the newer model V-213 a so called Bubble condenser tower, that – with its additional volume and a number of water layers – has the aim to suppress the forces of the rapidly escaping steam without the onset of a containment-leak. As a consequence, all member-countries with plants of design VVER-440 V-230 and older were forced by the politicians of the European Union to shut them down permanently. Because of this, Bohunice Nuclear Power Plant had to close two reactors and Kozloduy Nuclear Power Plant had to close four. Whereas in the case of the Greifswald Nuclear Power Plant, the German regulatory body had already taken the same decision in the wake of the fall of the Berlin wall.

VVER-1000

When first built the VVER design was intended to be operational for 35 years. A mid-life major overhaul including a complete replacement of critical parts such as fuel and control rod channels was thought necessary after that.^[15] Since RBMK reactors specified a major replacement programme at 35 years designers originally decided this needed to happen in the VVER type as well, although they are of more robust design than the RBMK type. Most of Russia's VVER plants are now reaching and passing the 35 year mark. More recent design studies have allowed for an extension of lifetime up to 50 years with replacement of equipment. New VVERs will be nameplated with the extended lifetime.

In 2010 the oldest VVER-1000, at Novovoronezh, was shut down for modernization to extend its operating life for an additional 20 years; the first to undergo such an operating life extension. The work includes the modernization of management, protection and emergency systems, and improvement of security and radiation safety systems.^[16]

In 2018 Rosatom announced it had developed a thermal annealing technique for reactor pressure vessels which ameliorates radiation damage and extends service life by between 15 and 30 years. This had been demonstrated on unit 1 of the Balakovo Nuclear Power Plant.^[17]

VVER-1200

The VVER-1200 (or NPP-2006 or AES-2006)^[7] is an evolution of the VVER-1000 being offered for domestic and export use.^[18][19] The reactor design has been refined to optimize fuel efficiency. Specifications include a $1,200 per kW overnight construction cost, 54 month planned construction time, a 60 year design lifetime at 90% capacity factor, and requiring about 35% fewer operational personnel than the VVER-1000. The VVER-1200 has a gross and net thermal efficiency of 37.5% and 34.8%. The VVER 1200 will produce 1,198 MWe of power.^[20][21]

The first two units have been built at Leningrad Nuclear Power Plant II and Novovoronezh Nuclear Power Plant II. More reactors with a VVER-1200/491^[22] like the Leningrad-II-design are planned (Kaliningrad and Nizhny Novgorod NPP) and under construction. The type VVER-1200/392M^[23] as installed at the Novovoronezh NPP-II has also been selected for the Seversk, Zentral and South-Urals NPP. A standard version was developed as VVER-1200/513 and based on the VVER-TOI (VVER-1300/510) design.

In July 2012 a contract was agreed to build two AES-2006 in Belarus at Ostrovets and for Russia to provide a $10 billion loan to cover the project costs.^[24] An AES-2006 is being bid for the Hanhikivi Nuclear Power Plant in Finland.^[25] The plant supply contract was signed in 2013, but terminated in 2022 mainly due to Russian invasion of Ukraine.^[26]

From 2015 to 2017 Egypt and Russia came to an agreement for the construction of four VVER-1200 units at El Dabaa Nuclear Power Plant.^[27]

On 30 November 2017, concrete was poured for the nuclear island basemat for first of two VVER-1200/523 units at the Rooppur Nuclear Power Plant in Bangladesh. The power plant will be a 2.4 GWe nuclear power plant in Bangladesh. The two units generating 2.4 GWe are planned to be operational in 2023 and 2024.^[28]

On 7 March 2019 China National Nuclear Corporation and Atomstroyexport signed the detailed contract for the construction of four VVER-1200s, two each at the Tianwan Nuclear Power Plant and the Xudabao Nuclear Power Plant. Construction will start in May 2021 and commercial operation of all the units is expected between 2026 and 2028.^[29]

From 2020 an 18-month refuelling cycle will be piloted, resulting in an improved capacity utilisation factor compared to the previous 12-month cycle.^[30]

VVER-TOI

The VVER-TOI is developed from the VVER-1200. It is aimed at development of typical optimized informative-advanced project of a new generation III+ Power Unit based on VVER technology, which meets a number of target-oriented parameters using modern information and management technologies.^[35]

The main improvements from the VVER-1200 are:^[4]

• power increased to 1300 MWe gross
• upgraded pressure vessel
• improved core design to improve cooling
• further developments of passive safety systems
• lower construction and operating costs with a 40-month construction time
• use of low-speed turbines

The construction of the first two VVER-TOI units was started in 2018 and 2019 at the Kursk II Nuclear Power Plant.^[36][4]

In June 2019 the VVER-TOI was certified as compliant with European Utility Requirements (with certain reservations) for nuclear power plants.^[4]

An upgraded version of AES-2006 with TOI standards, the VVER-1200/513, is being built in Akkuyu Nuclear Power Plant in Turkey.^[37]

Future versions

A number of designs for future versions of the VVER have been made:^[38]

• MIR-1200 (Modernised International Reactor) – designed in conjunction with Czech company ŠKODA JS^[39] to satisfy European requirements^[40]
• VVER-1500 – VVER-1000 with dimensions increased to produce 1500 MWe gross power output, but design shelved in favour of the evolutionary VVER-1200^[41]
• VVER-1700 Supercritical water reactor version.
• VVER-600 two cooling circuit version of the VVER-1200 designed for smaller markets, authorised to be built by 2030 at the Kola Nuclear Power Plant.^[42][43]