VVER


The water-water energetic reactor, or VVER is a series of pressurized water reactor designs originally developed in the Soviet Union, and now Russia, by OKB Gidropress. The idea of such a reactor was proposed at the Kurchatov Institute by Savely Moiseevich Feinberg. VVER were originally developed before the 1970s, and have been continually updated. They were one of the initial reactors developed by the USSR, the other being the infamous RBMK. As a result, the name VVER is associated with a wide variety of reactor designs spanning from generation I reactors to modern generation III+ reactor designs. Power output ranges from 70 to 1300 MWe, with designs of up to 1700 MWe in development. The first prototype VVER-210 was built at the Novovoronezh Nuclear Power Plant.
VVER power stations have been installed in Russia, Ukraine, Belarus, Armenia, China, the Czech Republic, Finland, Hungary, Slovakia, Bulgaria, India, and Iran. Countries that are planning to introduce VVER reactors include Bangladesh, Egypt, Jordan, and Turkey. Germany shut down its VVER reactors in 1989-90, and cancelled those under construction.

History

The earliest VVERs were built before 1970. The VVER-440 Model V230 was the most common design, delivering 440 MW of electrical power. The V230 employs six primary coolant loops each with a horizontal steam generator. A modified version of VVER-440, Model V213, was a product of the first nuclear safety standards adopted by Soviet designers. This model includes added emergency core cooling and auxiliary feedwater systems as well as upgraded accident localization systems.
The larger VVER-1000 was developed after 1975 and is a four-loop system housed in a containment-type structure with a spray steam suppression system. VVER reactor designs have been elaborated to incorporate automatic control, passive safety and containment systems associated with Western generation III reactors.
The VVER-1200 is the version currently offered for construction, being an evolution of the VVER-1000 with increased power output to about 1200 MWe and providing additional passive safety features.
In 2012, Rosatom stated that in the future it intended to certify the VVER with the British and U.S. regulatory authorities, though was unlikely to apply for a British licence before 2015.
The construction of the first VVER-1300 1300 MWE unit was started in 2018.

Design

The Russian abbreviation VVER stands for 'water-water energy reactor'. The design is a type of pressurised water reactor. The main distinguishing features of the VVER compared to other PWRs are:
  • Horizontal steam generators
  • Hexagonal fuel assemblies
  • No bottom penetrations in the pressure vessel
  • High-capacity pressurizers providing a large reactor coolant inventory
Reactor fuel rods are fully immersed in water kept at MPa pressure respectively so that it does not boil at the normal operating temperatures. Water in the reactor serves both as a coolant and a moderator which is an important safety feature. Should coolant circulation fail, the neutron moderation effect of the water diminishes due to increased heat which creates steam bubbles which do not moderate neutrons, thus reducing reaction intensity and compensating for loss of cooling, a condition known as negative void coefficient. Later versions of the reactors are encased in massive steel reactor pressure vessels. Fuel is low enriched uranium dioxide or equivalent pressed into pellets and assembled into fuel rods.
Reactivity is controlled by control rods that can be inserted into the reactor from above. These rods are made from a neutron absorbing material and, depending on depth of insertion, hinder the chain reaction. If there is an emergency, a reactor shutdown can be performed by full insertion of the control rods into the core.

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 : 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 supplying heat to the towns of Trnava, Leopoldov, and Hlohovec, and Temelín NPP supplying heat to Týn nad Vltavou away and České Budějovice away. Plans are made to supply heat from the Dukovany NPP to Brno, covering two-thirds of its heat needs.

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 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.

Fuel cycle extension

In 2024 Rosatom started testing fuel which contains neutron absorber erbium and uranium enriched to 5%. The experiments happen at MIR.M1 research reactor at the Dimitrovgrad Research Institute of Nuclear Reactors. It will allow to extend the current fuel cycle from 12-18 months to 24 months.

Remix Fuel

is used for Remix Fuel experiments. In December 2024 the third final 18-month phase of the pilot program has started with the goal to achieve closed nuclear cycle for VVER reactors. A mixture of enriched uranium with recycled uranium and plutonium received from the used nuclear fuel at other VVER reactors is used instead of a standard enriched uranium. After the first 2 stages of 3, fuel elements were inspected and were approved for the 3rd final stage. The 3rd stage should conclude in 2026 when the fuel will be unloaded and further studied. Remix fuel has a lower plutonium content of up to 5% compared with MOX fuel.

Versions

VVER-440

One of the earliest versions of the VVER-type, the VVER-440 manifested certain problems with its containment building design. As the V-230 and older models were from the outset not built to resist a design-critical 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 – aims 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.
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.
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.