WHAT IT IS
VVER (Russian: ВВЭР) stands for Vodo-Vodyanoi Energetichesky Reaktor — “water-water power reactor.” Both words “water” matter. Light water cools the core. Light water also slows neutrons so the chain reaction can run on slightly enriched uranium. Same basic idea as a Western pressurized water reactor (PWR).
A VVER is not an RBMK. RBMKs are the graphite-channel reactors used at Chernobyl. VVERs keep the fuel inside a thick steel pressure vessel and keep the primary water from boiling. They are the Soviet/Russian standard PWR line, designed mainly by OKB Gidropress in Podolsk, with plant layout by Atomenergoproekt institutes and fuel from TVEL.
The number after the name is roughly the original electrical rating: VVER-440, VVER-1000, VVER-1200, VVER-TOI (about 1,250–1,300 MWe). Each family has factory models (V-230, V-213, V-320, V-392M, V-491, V-510, and so on).
HOW A VVER PLANT WORKS
Two water circuits, kept separate.
Primary circuit (radioactive, high pressure). Water is pumped through the core. Fission heat raises its temperature, but pressure is high enough that it stays liquid. Typical published numbers: VVER-440 about 12.3 MPa, inlet ~268 °C, outlet ~296 °C. VVER-1000 about 15.7 MPa. VVER-1200 about 16.2 MPa, core outlet toward 320 °C-class conditions.
Hot water leaves the vessel through hot-leg nozzles, goes to steam generators, gives up heat, and returns through cold-leg nozzles and the main coolant pumps. A pressurizer sits on one hot leg. Heaters make steam to raise pressure. Spray from a cold leg knocks pressure down. Relief and safety valves protect the circuit.
Secondary circuit (not supposed to be radioactive). Feedwater sits on the outside of the steam-generator tubes. It boils. Saturated steam drives a turbine and generator, then is condensed and pumped back. Fission products stay on the primary side if the tubes hold.
VVER-440 units usually have six primary loops and were often built in pairs sharing a turbine hall. VVER-1000 and later units usually have four loops and one large turbine-generator per unit.
WHAT MAKES A VVER LOOK LIKE A VVER
Horizontal steam generators. Western PWRs use tall vertical steam generators. VVERs use long horizontal cylinders (PGV-4 on 440s, PGV-1000M on 1000s). Primary water flows through thousands of small horizontal U-tubes into vertical hot and cold collectors. Secondary water boils around those tubes with a large water inventory. Russian designers argue this is gentler on the tubes and easier to inspect. The price is bulk and a wide reactor building.
Hexagonal fuel. Western PWR assemblies are square. VVER assemblies are hexagons on a triangular pin lattice.
VVER-440: 349 assemblies, each typically 126 rods, with a hexagonal shroud. Control is by moving whole control assemblies (fuel below, absorber above). About 42 tonnes of uranium in the core.
VVER-1000 / 1200: 163 assemblies, each typically 312 fuel rods plus guide tubes and an instrument tube. No shroud on serial 1000s (Novovoronezh-5 still had one). Control is by rod clusters. V-320 core about 3.16 / 3.53 m diameter/height, uranium load over 70 tonnes, power density around 115 kW per litre.
Fuel is ceramic UO2 pellets in zirconium-niobium cladding (E110 / Zr-1%Nb family), not the Zircaloy-tin alloys common in the West. Burnable absorbers are often gadolinium mixed into some pellets. Modern cycles run 12–24 months.
No penetrations in the vessel bottom. Control rods and instruments enter from the top. The bottom head is solid. In a core-melt accident there are no bottom holes to fail first.
Hot legs above cold legs. Outlet nozzles sit higher than inlet nozzles. A ring baffle sends incoming cold water down the downcomer, through the bottom, up through the fuel, and out the upper nozzles.
Large pressurizer and large water inventory buy time in some transients. Containment evolved by generation: V-230 used industrial buildings and accident-localization rooms; V-213 added a bubbler-condenser tower; VVER-1000 introduced a containment building with sprays and ECCS; VVER-1200 / TOI use double containment, a core catcher, and mixed active and passive trains.
THE CORE AND THE VESSEL
The reactor pressure vessel is a tall forged steel cylinder with an elliptical or hemispherical bottom and a bolted upper head. Internals include a core barrel, a protective tube unit, and the fuel.
Reactivity is controlled three ways: control rods / control assemblies, dissolved boric acid in the primary water, and burnable absorbers in the fuel. Refuelling is done shut down, head off, like other PWRs.
HISTORY
1950s: the USSR wanted both a graphite-channel line (RBMK) and a vessel PWR line. VVER work is usually dated from about 1955. Novovoronezh on the Don became the prototype farm.
1964 — VVER-210 (Novovoronezh-1), first commercial-scale VVER, about 210 MWe. Shut down 1988.
1970 — VVER-365 (Novovoronezh-2). Shut down 1990.
1971–73 — first VVER-440s (Novovoronezh-3 and 4, V-179). Serial first-generation model was V-230. Safety was thin by later standards. V-270 was a V-230 variant for seismic sites (Armenian plant).
1977 onward — VVER-440 V-213. Bubbler-condenser confinement, better emergency cooling. Loviisa in Finland wrapped V-213s in Western I&C and is licensed toward 2050.
1970s–80s — VVER-1000. Prototype V-187 at Novovoronezh-5, first criticality 1980. Workhorse serial plant is V-320 (Zaporizhzhia, 1984): four loops, one 1,000 MWe turbine, containment, ECCS. Design life 30 years; many modernized toward 50–60.
After Chernobyl (an RBMK accident, not a VVER accident) the USSR still finished VVER-1000s, but new-build slowed with the collapse.
1990s–2000s — export enhanced 1000s: Tianwan (V-428), Kudankulam (V-412), Bushehr, and others. Bridge to Generation III.
2016 onward — VVER-1200 (AES-2006), about 3,200 MWt / 1,200 MWe gross. Two layout schools: V-392M (Novovoronezh II) and V-491 (Leningrad II, Belarus Ostrovets). Double containment, core catcher, more passive cooling, 60-year design life. First unit: Novovoronezh II-1, grid 2016, commercial 2017. Export includes Turkey (Akkuyu) and Egypt (El Dabaa).
2018 onward — VVER-TOI (V-510 / “VVER-1300”). Typical, Optimized, Informatized. About 1,250–1,300 MWe gross, fewer vessel welds, new vessel steel, more passive safety, digital plant. Certified against European Utility Requirements in 2019 with reservations. First unit: Kursk II-1. Grid December 2025. Commercial 1 May 2026, about 1,252 MWe. Unit 2 aimed at 2027. Kursk II replaces aging RBMKs at the same site.
Shelved or side paths: VVER-1500 dropped in 2006. VVER-600 discussed for isolated grids. MIR-1200 and similar names were marketing wraps on 1200-class plants.
SAFETY IN PLAIN LANGUAGE
V-230 is Generation I and needed backfits wherever kept running. Several were shut. V-213 is Generation II with extra confinement. Serial V-320 is Generation II advanced, in the same broad class as French N4 or German Konvoi. Post-1990 export 1000s and all 1200 / TOI units are sold as Generation III or III+.
Traits that help: no bottom-head penetrations, horizontal generators with a lot of secondary water, large primary inventory, accumulators that can inject into the vessel.
Weaknesses of old plants: confinement that is not a full large dry containment (V-230 especially), original I&C and fire protection, steam-generator collector integrity, six-loop isolation valves on 440s.
WHERE THEY RUN
Countries that have or had them include Russia, Ukraine, Belarus, Armenia, Bulgaria, Czechia, Slovakia, Hungary, Finland, East Germany (shut), China, India, Iran, and new-build in Turkey and Egypt. Ukraine’s Zaporizhzhia six-pack of V-320s is the largest VVER site; its wartime status is a political and safety problem, not a design footnote.
Mid-2020s order of magnitude: a couple of dozen VVER-440s still running, VVER-1000 the most common type (about three dozen), a growing handful of VVER-1200s, and the first VVER-TOI in commercial service in 2026. Finland’s Loviisa shows a VVER can be run to Western rules for decades.
VVER VERSUS WESTERN PWR
Same: light-water moderator and coolant, PWR thermodynamics, UO2 fuel, boron plus rods, two-circuit plant, steam turbine.
Different: hexagonal assemblies vs square; Zr-Nb cladding vs Zircaloy; horizontal vs vertical steam generators; no vessel-bottom penetrations; 440-class used moving fuel/absorber assemblies; 1000+ uses clusters; 440-class used shrouded assemblies; wider plant layout; older units used localization compartments instead of full containment.
FUEL AND THE BACK END
TVEL supplies most Russian and many export cores. Some plants have used Westinghouse or other hexagonal fuel. Enrichment is typical LEU. Burn-up on modern 1000/1200 fuel is in the 40–60 GWd/t class, heading higher on TOI. Used fuel sits in a pool, then dry storage or reprocessing depending on the country. Russia still treats used VVER fuel as a resource more often than the U.S. once-through model.
STATUS IN 2026
VVER is Russia’s serial large reactor. Old 440s are shut, upgraded, or on extended licences. V-320s are mid-life workhorses. VVER-1200 is the current export standard. VVER-TOI is the new domestic standard, now making power at Kursk II. RBMKs are being replaced by VVERs where Russia still has graphite plants.
Limits are political as much as technical: sanctions, war risk, and customer willingness to take a Russian I&C and fuel lock-in. The physics and the mechanical layout are mature.
IN ONE SENTENCE
A VVER is a Russian pressurized-water reactor that puts hexagonal fuel in a vessel with no bottom holes, makes steam in horizontal generators, and has grown from 210 MWe prototypes at Novovoronezh in the 1960s into the 1,200–1,300 MWe Generation III+ plants Russia is building at home and abroad today.