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The VVER-1000 models have been used as a design basis for the VVER-1200 reactors (their Russian model number is usually AES-2006), but the successful safety solutions of the VVER-600 model line developed simultaneously were also used. The clear purpose of the development of the units with an electrical capacity of 1,200 MW was to create a power plant type that is internationally competitive through the increase of output and efficiency and the optimisation of the manufacturing/construction costs. The VVER-1200 units were developed simultaneously in two renowned design offices of the Russian Atomenergoproject. The designs of the MIR-1200 (V-491) model (Modernized International Reactor) were made in Saint Petersburg, while those of the V-392M model in Moscow.

During their development, it was a primary consideration to enhance nuclear safety and reliability, which is essential in the case of the most recent Generation III+ reactor types. In addition to mechanical development, significant progress has also been made in the areas of fuel, architecture and instrumentation and control , on the basis of which it can be stated that the MIR-1200 meets the relevant international safety requirements and, with its technical solutions, well competes with other players in the market.

For the capacity maintenance in Paks, the MIR-1200 (AES-2006 V-491) model was selected. This model was designed in Saint Petersburg with a design life of 60 years.

Similarly to the VVER-440 reactors currently operational at Paks, the operation of the VVER-1200 units is based on the pressurised water technology. In the core of the reactor, cooling water under a pressure of 162 bar is warmed from 300° to 330°C. The primary cooling water transfers the generated heat through steam generators to the secondary loop under a pressure of 70 bar, where steam is generated and the turbines are driven by hot steam.

The inner diameter of the 330-tonne reactor pressure vessel is 4.25 metres, its height is more than 11 metres, and its typical wall thickness is 20 cm. The cold and hot leg nozzles are located above the core (Figure 1). 

  

Figure 1: The reactor pressure vessel and its main internal structural units (Source: rosatom.ru/en, page 14)

(1. in-core neutron detectors, 2. upper unit, 3. protective tube unit, 4. core barrel, 5. core basket, 6. steel samples for the surveillance programme, 7. core 8. reactor vessel)

In the core, there are 163 fuel assemblies. One fuel assembly contains 533 kg of UO2 fuel. The fuel assemblies have a hexagonal cross section and each fuel assembly contains 312 fuel rods. The fuel assembly is not surrounded by a closed fuel assembly wall (shroud).

The control rods are located within the fuel assemblies. Eighteen control rods move in guide tubes within the fuel assembly, in a cluster controlled by a joint drive unit. The cladding of the fuel assemblies is made of a zirconium alloy, which contains 1% niobium and has very good corrosion resistance.

The reactor is connected to four horizontal steam generators (Figure 2) with an inner diameter of 4.2 m via cold- and hot-leg primary pipes with a diameter of 850 mm.  

Figure 2: Main components of the primary circuit

(Source: iaea.org, page 24)

 (1. reactor vessel, 2. steam generator (4), 3. pressuriser, 4. main circulating pumps (4), 5. hydro-accumulator (4)

On the primary side of the steam generator, the coolant warmed up in the reactor flows in 10,978 horizontal tubes between two collectors. The cooling water fed to the secondary side is boiled in the horizontal vessel and exits towards the turbines via a steam collector; 1,602 tonnes of steam leaves each steam generator every hour. A 55 m3 pressure vessel, what is called pressuriser, is an important component of the primary circuit: it is an essential means for regulating the pressure in the primary circuit in pressurised water reactors. The coolant is circulated with one main circulating pump in each of the four primary loops. The operation of the main circulating pumps of MIR-1200 does not require any oil, their lubrication and cooling being provided by water.

In the new units, the reactor and the primary circuit are located within a double-walled protective building (containment). Emergency safety systems are also located there. An outer building with a diameter of 50 metres protects the equipment from external hazards. The internal containment is a cylindrical building with an inner diameter of 44 m, which is capped by a hemisphere. The cylindrical part is 44.6 m high. The wall is 1.2 m thick in the cylindrical part, while the upper hemispherical shell is 1 m thick. The walls made of prestressed concrete are covered by a 6 mm thick steel cladding from the inside, which prevents leakage.

 Figure 3: Cross-section of the containment of MIR-1200

(Source: Saint Petersburg Institute, page 36)

The inner containment hermetically separates the primary circuit containing radioactive materials from the environment. Air is continuously exhausted through filters from the air space between the inner and outer walls. The doors leading into the containment operate as air-locks and are closed hermetically.

Active safety systems

MIR-1200 has several active systems (requiring power supply) for handling accidents. The majority of these systems have four parallel, physically separated and independent legs; just one of them is sufficient for performing the given protective function. The most important active safety systems of MIR-1200 are as follows:

  • The high-pressure emergency cooling system feeds boric acid water into the primary circuit during loss of coolant accidents.
  • The low-pressure emergency cooling system starts to operate in the case of large diameter primary pipe breaks.
  • The emergency boron system conveys a coolant with high boric acid concentration into the pressuriser in the case of leakage between the primary and secondary circuit, and into the reactor to ensure a subcritical state if the safety protection system does not work.
  • The sprinkler system sprays cold water through atomisers into the free volume of the containment, thereby facilitating the condensation of steam in the containment, the cooling of the air space and the reduction of its pressure.
  • There is a water storage system with low and high boric acid concentrations, which provides boric acid supply in any operating condition of the reactor.
  • The residual heat removal system is connected to the primary circuit and, during events involving a shut-down, it prevents the overheating of the primary coolant.
  • The overpressure protection system of the primary circuit releases the steam from the pressuriser into the bubbling vessel if the primary pressure exceeds the limits for any reason.
  • The emergency gas removal system removes the mixture of steam and gas from the primary coolant (from the reactor, the pressuriser and the steam-generator collectors). In addition, in design and beyond design basis accidents, it also takes part in the depressurisation of the primary circuit.
  • The emergency feedwater system provides a supply of feedwater to the steam generators in design operating conditions where the normal and auxiliary feedwater systems are not available.
  • The depressurisation system of the secondary circuit protects against the excessive increase of pressure in the secondary circuit by releasing fresh steam from the steamlines.
  • The main steamline isolation system ensures the disconnection of the steam generators in accident situations where it is necessary to isolate the steam generators quickly and reliably on the secondary side.

In the case of accidents, the active safety systems are supplied with power by diesel generators; four such generators are available in one unit.

 

Passive safety systems

In addition to active safety systems, similarly to other Generation 3 nuclear power plant types, the MIR-1200 also contains a number of passive safety systems. Their common characteristics are that their operation does not require human intervention and an external energy source, the fulfilment of their functions being ensured by simple physical processes. In the case of an accident, the cooling of the reactor and the primary circuit is ensured for a long time without intervention by operators. In addition to active emergency cooling systems, the cooling of the core is ensured with four hydro-accumulators. A high pressure nitrogen cushion found in the gas space above the water level conveys cooling water into the reactor from these vessels.

Passive heat removal systems ensuring the long-term cooling of the primary circuit and the containment in the case of an accident are additional passive safety systems. Their detailed description can be read in the following section.

 

Severe accident management

Generation 3 reactors are designed in such a way that appropriate means are available in them for handling severe accidents.

Two passive systems that have prominent functions in the case of severe accidents are available for removing residual heat. One of them removes the heat from the steam generators, the other from the containment. Each of them consists of four parallel legs, in which flow is ensured by natural circulation. The passive cooling of the steam generators may be required if the active cooling systems become dysfunctional. If the active sprinkler system designed for cooling the atmosphere of the containment does not work, passive heat removal will ensure that the internal pressure of the protective building does not reach a value that would threaten the integrity of the building. The passive systems are able to prevent damage of the core even without any external intervention for 72 hours. Their appropriate operation is demonstrated by tests carried out on a variety of experimental equipment.

After any possible core damage, hydrogen generated as a result of a reaction between zirconium and steam may endanger the integrity of the containment. At the power plant to be constructed at Paks, passive autocatalytic recombiners placed in the upper part of the inner containment prevent the development of an explosive condition.

A so-called core catcher has been designed for mitigating the consequences of a core meltdown. This is a special vessel placed at the bottom of the reactor cavity, under the reactor vessel (Figure 4), and if the reactor vessel were damaged after a core meltdown, the corium is conveyed into that.

  Figure 4: Core catcher (Source: Saint Petersburg Institute, page 17)

 (1. base plate of containment, 2. reactor vessel, 3. concrete cavity, 4. concrete supporting structure, 5. coolant inlet, 6. coolant outlet, 7. annular chamber around core catcher, 8. core catcher, 9. protective plates, 10. heat insulation, 11. air-cooled ducts, 12. heat insulation, 13. lower supporting plate)

There are aluminium and ferric oxide-containing ceramics in the vessel, which are suitable for getting mixed with the core melt. As a result of the mixing the material properties of the corium change, the melted core becomes diluted and the residual heat generated in unit volume decreases. Some gadolinium is also fed to the ceramics, which absorbs neutrons, thereby increasing the subcriticality of the corium. The steel vessel of the core catcher is cooled with water from outside. With this, so-called dry catcher solution, the the molten core – concrete reaction with the base plate can be prevented. Using the core catcher, hydrogen generation and the release of radioactive fission products from the core debris can be reduced. 

Protection against external hazards 

During design, great emphasis was laid on providing protection against external events. Due to the methods applied, in addition to natural disasters, the reactor has appropriate protection against the most important hazards of human origin:

  • The power plant has been designed to resist earthquakes if their maximum free-surface horizontal acceleration does not exceed 0.25 g.
  • When the safety equipment of the nuclear power plant was designed, a shock wave following a possible external explosion was also taken into consideration.
  • The double-walled containment was designed to be able to resist even the crash of a large aeroplane on it.
  • It is able to resist snow pressure developing due to an up to several-metre thick wet snow cover.
  • The safety equipment resists extreme ambient temperatures, high winds and tornadoes.

 

(Source: Nukleon 2014. március VII. évf. (2014) 152, http://nuklearis.hu/sites/default/files/nukleon/Nukleon_7_1_152_Hozer.pdf)

 

 

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