Russian NPP-2006: a new generation of safety15 June 2017
Russia’s latest reactor design has recently started up at the Novovoronezh site. Rosatom introduces the reactor safety systems for the VVER-1200 unit.
In February 2017 the world’s first Generation III+ reactor, a VVER-1200 unit was connected to the grid at Novovoronezh NPP 2 in Russia. In addition to improved performance and cost efficiency, the unit fully complies with International Atomic Energy Agency (IAEA) post-Fukushima requirements. The Generation III+ design combines both innovative and proven nuclear power plant technologies with state-of-the- art safety systems.
Today, VVER-1200 projects are being implemented in Turkey, Bangladesh, Finland, Hungary and Belarus. All of these plants have the following characteristics:
- Design power output of 1200MW;
- Main equipment service life of 60 years;
- Improved fuel management, with state-of-the-art fuel cycles.
All Russia’s Generation III+ VVER-1200 power plants are designed to include protection against severe earthquakes (intensity 8 and higher on MSK-64), aircraft impacts, external shock waves, tornados and floods.
Protection against external events
An extra-strong containment structure consisting of two protection shells provides resistance to external events.
The outer shell is a reinforced-concrete structure composed of a cylinder connected to a hemispherical dome. It is designed to provide protection against external events including aircraft impacts.
The inner shell is made of pre-stressed reinforced concrete and has a reinforced- concrete base plate. The internal surface of the inner shell is clad with carbon steel to ensure tightness.
The containment design provides resistance to external man-induced and natural events and prevents direct impact or damage to internal structures and equipment.
Internal safety systems
Russian-designed power plants with Generation III+ VVER-1200 reactors include protection in case of emergencies that may cause damage to the containment, such as:
- a steam explosion in the reactor vessel;
- hydrogen detonation;
- recriticality of the reactor and corium;
- a steam explosion outside of the reactor vessel;
- direct heating of the containment;
- missiles; and
- the impact of corium on the core catcher walls and floor.
The safety systems are designed to prevent or mitigate damage to the reactor and confine fission products in case of an accident. Different systems have different functions, including protection, retention, support and management.
Overall, nuclear power plant safety is based on the “defence-in-depth” concept, which includes barriers to prevent ionising radiation and radioactive substances from escaping into the environment, as well as technical measures to maintain the barriers’ efficiency.
The Generation III+ VVER-1200 nuclear power plant design includes safety systems with the following functions:
- Reactor emergency shutdown and maintenance of subcritical conditions;
- Emergency heat removal from the reactor;
- Retention of radioactive substances within the set boundaries;
- Heat removal from the stored nuclear fuel. The reliability of the units, as well as their compliance with post-Fukushima safety standards, is provided by redundant active and passive safety systems, the latter of which require no power sources. The active safety systems can be operated using conventional power as well as emergency power produced by backup diesel generators. Passive systems that cover all critical safety functions have made it possible to improve the quality of nuclear power plant safety.
Protection against station blackout
The Fukushima accident happened due to the failure of the auxiliary power supply and because it was impossible to use the emergency power supply after seawater flooded the emergency diesel generator compartment. These events resulted in a total nuclear power plant blackout.
The key safety advantage of the latest generation VVER-1200 unit constructed at Novovoronezh NPP 2 is its capability to maintain safety without operator intervention virtually indefinitely in the case of a station blackout and to prevent fuel damage for at least 24 hours in the event of a blackout compounded by a loss of coolant accident.
Emergency core cooling system
In the case of an accident with primary coolant leakage under blackout conditions, the design ensures core cooling with several systems simultaneously.
The active part of the emergency core cooling system is designed to maintain the safe coolant level in the reactor core and to remove the residual heat in the case of the primary circuit leakage.
Passive core flooding system
The passive core flooding system (shown on accompanying wallchart) maintains the coolant level required for the reactor cooling to prevent core damage. The system is made up of hydraulic accumulators that can independently ensure core cooling for 24-hours in the case of a leakage of any size.
The primary accumulators start feeding boric acid solution to the core if the pressure drops below 5.9MPa. These accumulators ensure fast core flooding.
Secondary accumulators provide for long-term maintenance (at least 24 hours) of the primary coolant level required for reliable heat removal from the core. These accumulators start to feed boric acid solution if primary pressure drops below 1.5MPa.
In cases when there is no more water and the power supply has not been restored, the design includes additional technical means to feed the reactor and storage pool. These pump units are equipped with a mobile, air- cooled diesel generator (water coolant may be unavailable for the diesel generator) that uses a fan cooling tower to remove heat.
Steam generator passive heat removal system (SG PHRS)
Safety systems providing core cooling and reactor cool-down in an emergency also include the system for steam generator passive heat removal. This system removes heat through a closed circuit without feedwater in the case of beyond-design basis accidents.
The SG PHRS removes heat from the core by condensing steam from the steam generators in a heat exchanger, with condensate returned to the core. The heat exchanges are located 40m above ground, protected by structural steel, which prevents damage from floods or other environmental events (hurricanes, tornados and other natural disasters). The passive heat removal system uses air as the ultimate heat sink.
The emergency borating system
The emergency borating system is designed to bring the reactor into sub-criticality and reduce the primary circuit pressure in the case of leakage from the primary circuit to the secondary circuit.
Passive filtration system
The passive annulus filtration system is designed to provide controlled removal of steam and gas mixture from the annulus in the case of station blackout accidents.
The system eliminates any uncontrolled annulus leakage to the environment through the outer containment in any cases of the annulus ventilation failure. If the normal and emergency power supply fails, a mobile diesel generator can be used to power the annulus passive filtration system.
Hydrogen removal system
The hydrogen removal system, shown in Figure 1, plays a major role in accident mitigation. It prevents hydrogen explosion and any resulting damage to the reactor building. The hydrogen recombiners are installed inside the primary containment. These recombiners prevent hydrogen concentration rising and possible explosions in all emergency scenarios.
Overpressure protection system
The primary overpressure protection system is intended to protect the reactor piping and equipment in case of extreme pressure in the primary circuit. This system functions due to pulse safety devices installed in the bleed line from the primary side to the bubbler.
The secondary overpressure protection system is intended to prevent undue pressure in steam generators and fresh steam pipelines in the secondary circuit.
The sprinkler system supplies water to the annulus to prevent containment pressure rises and drops in case of an accident. In normal operation, the system removes heat from the spent fuel pool.
A core catcher protects against containment damage resulting from core meltdown. The core catcher, shown in Figure 2, is located under the reactor. It is designed to ensure containment integrity and prevent radioactive materials from entering the environment in the case of severe accidents. The core catcher weights about 750 tons.
This article gives an overview of the safety systems of Russia’s latest generation VVER-1200 reactor design, which has recently started operation at Novovoronezh NPP 2. It not a complete list of the systems intended to prevent any nuclear power plant failures. However, the Generation III+ design and safety systems exclude both external and internal events, and prevent radioactive substances from escaping into the atmosphere in an emergency.