VVER Russian reactors

VVER  Russian reactors

The Russian abbreviation VVER  stands for water cooled water moderated energy reactor. This describe the pressurized water reactor design. The earliest VVERs were  built before  1970. The VVER 44o model V230 is the most common design, delivering 440 MW of electric power. The V230 employees six primary coolant loops each with   a horizontal steam generator.

A modified version of VVER -440 model V213 was a product of first nuclear safety standards adopted by Soviet  designs. This model includes added emergency core cooling and auxiliary feed water systems as well as up graded accident localization systems. The largest VVER 1000 was developed after 1975 and is four loop system housed in a containment type structure with a spray steam suppression systems.VVER  reactor design have been elaborated to incorporate automatic control passive safety and containment system associated with western third generation nuclear reactors.

Reactor fuel rods are fully immersed in a water kept at 15 Mpa of pressure so that it does not boil at normal (220 to over 300 degree Celsius )operating temperature. Water in the reactor serves both as a coolant and moderator which is an important safety features . Should coolant circulation fail the neutron moderation effect of water diminishes reducing reaction intensity and compensatory for loss of cooling a condition known as negative void coefficient. The whole  reactor is encased in massive steel pressure shell. Fuel is enriched 2.4 per cent to 4.4.per cent of U-235or uranium dioxide (Uo2 ) or equivalent pressed into pallets and assembled into fuel rods.

Intensity of nuclear reaction is controlled by control rods that can be inserted into reactor from above. These rods are made from neutron absorbing material and depending on the depth of insertion hinder the chain reaction. If there is an emergency, a reactor shut down can be performed by  infill insertion of control rods.

Primary cooling circuit

As stated above, water in primary circuit is kept under constant pressure to avoid boiling. Since the water transfers all heat from the core and is irradiated, integrity of this circuit is more critical .in this circuit  four sub systems, can be  distinguished .

Reactor : water from through fuel rod assemblies and is heated by nuclear chain reaction.

Volume compensator :  to keep water under consistent but  under control pressure employing self regulation of saturated steam water interface and by mean of electrical heating and relief valves.

Steam generator  :  in the steam generator, heat from primary coolant water is used to boil water in secondary circuit.

Rump  ; The pump ensures proper circulation of water to ensure safety primary components  are redundant.

Secondary circuit

The secondary circuit also consists of different sub systems.

Steam generator  : secondary water is boiled taking heat from primary circuit . Before entering the turbine remaining water is separated from steam so that steam is dry.

Turbine :  the expanding steam drives turbines, which connects to electrical generator. The turbine is spit into high and low pressure sections. To prevent condensation (water droplet at high speed damage the turbine blades) . Steam is reheated between these sections. Reactor of VVER-1000 type deliver 1 GW of electric power.

Condenser   : the steam is cooled and allowed to condense, shedding waste heat into a cooling circuit.
Deaerater  ;  Removes gasses from coolant.

Pump   :  The circulation pumps are each driven by their own steam turbine.

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

Cooling circuit

The cooling circuit is open circuit diverting water from outside reservoir such as lake or river. Evaporating cooling tower, cooling basins or ponds exhaust heat from generation circuit, releasing it into environment. In addition to generating electricity most VVER s have capacity to supply heat for residential and industrial use.

Safety barriers

A typical design features of nuclear reactors is layered safety barriers preventing escape of radioactive material.VVER reactors have four layers.

Fuel pellets : Radioactive elements are retained within the crystal structure of fuel pellets.
Fuel rods     : The zirconium tubes provide a further barrier resistant to heat and high pressure.
Reactor shell  : A massive steel shell encase the whole fuel assembly hermetically.
Reactor building  : The concrete containment building that encases the whole first circuit is strong enough to resist pressure surge a breach the first circuit would cause.
Currently operating Russian VVERs are inherently safe design than RBMK reactor of Chernobyl disaster. The Soviet  union opted to construct graphite-moderated RBMK series nuclear reactors without containment structure on grounds of costs as well as relative ease of the re-fueling RBMK reactors. An RBMK reactor can be re fueled while still operational compared to VVER which needs to be shut down. Many levels of protection and containment have both been proposed and constructed for RBMK and  VVER reactors.

Operation life of VVER 1000
When first built design was intended to be operational for 35 years. Many VVER plants are now reaching and passing 35 years mark. More recent design studies have  allowed for an extension of life time up to 50 years with replacement of equipment. New VVER will be name plated with extended life time.
In 2010 the oldest VVER-1000 at Novo Voronezh was shut down for modernization  to extend life of additional 30 years . The first to undergo such an operating life extension. The works include the modernization of management protection and emergency systems and improvement of security and radiation safety systems.

VVER 1200
The VVER-1200 is an evolution of VVER 1000 being offered for domestic and export use. Specification include 1200 U.S. dollars per KW electrical capital cost, 54 month planned construction and expected 50 years life time at 90 per cent capacity factor.  The VVER 1200 will produce 1200MWe of power.

Safety features include containment building and missile shield. It will have fuel emergency systems, that include emergency core cooling systems, emergency back diesel power supply, advanced refueling machine, computerized reactor control systems back up feed water supply and reactor SCRAM system. The nuclear reactor and associated systems will be hosted in one single building and will another building for turbo generator. The main building will comprise the reactor ,refueling machine and diesel back up power supply ,steam generators and reactor control system.

If VVER 1200 experiences a loss of coolant accident or loss of power accident the turbo generator coast down for 30 seconds, during which time a shut down can be initiated using residential power  in the system. Further emergency power is available from back up set of diesel generators kept on stand by to maintain cooling flow reactor. This reactor design has been refined to optimize fuel efficiency