On November, 30, Bangladesh saw the first concrete pouring into the reactor building foundation of its Rooppur Nuclear Power Plant. This marks the construction of Bangladesh’s first nuclear unit with Gen 3+ 1200 MW Russian VVER light-water reactors. Apart from improved performances and cost efficiency these units fully comply with the IAEA post-Fukushima requirements. Gen 3+ design combines both innovative and proven referent nuclear power plant (NPP) technologies with state-of-the-art safety systems.
Nowadays such projects are being implemented in Turkey, Finland, Hungary and Belarus. All of these plants have the following performances:
– Design power output: 1200MW;
– Main equipment service life: 60 years;
– Improved fuel management, up-to-date fuel cycles.
All Gen 3+ NPP designs include protection against severe earthquakes (intensity 8 and higher on MSK-64), aircraft crashes, external shock waves, tornados and floods.
Unit resistance to extreme external events is provided with an extra-strong containment structure consisting of two protection shells. The outer shell is a reinforced structure consisting of a cylinder connected to a hemispherical dome and designed to provide protection against external events including aircraft crashes. The inner shell is a cylindrical structure with a hemispherical dome made of prestressed reinforced concrete, and a base plate of reinforced concrete. To ensure tightness, the internal surface of the shell has welded cladding of carbon steel. This design ensures resistance to external man-induced and natural events and prevents direct impact on internal structures and equipment as well as their damage.
The containment is protected from the molten core by a core catcher which is a system designed to retain and cool the corium. The core catcher is arranged under the reactor (at the bottom of the reactor pit) to ensure the containment integrity and prevent radioactive wastes from entering the environment in the case of severe accidents. The core catcher weights about 750 tons.
Apart from protection in case of external unforeseen events the Russian design of NPP with Gen 3+ VVER-1200 includes protection in case of internal emergencies which may cause damage to the containment, such as steam explosion in the reactor vessel; hydrogen detonation; recriticality of the reactor and corium; steam explosions outside the reactor vessel; direct heating of the containment; missiles; impact of corium on the core catcher walls and floor.
Safety systems including protection, retention, support and management functions are designed to prevent or mitigate damage to the reactor and confine fission products in the case of the NPP accident.
The NPP safety is based on the in-depth protection concept including barriers preventing ionizing radiation and radioactive substances from escaping into the environment thus protecting the population; as well as technical measures to maintain the barriers efficiency.
According to the in-depth protection concept the design includes safety systems with main safety functions as follows:
– Reactor emergency shutdown and subcritical conditions maintenance;
– Emergency heat removal from the reactor;
– Retention of radioactive substances within the set boundaries; – Heat removal from the stored nuclear fuel.
The Fukushima Daiichi Nuclear Power Plant accident happened, first of all, because of the failure of the auxiliary power supply and because of the impossibility to use emergency power supply after seawater flooded the diesel generator compartment resulted in the NPP total blackout.
The key safety advantage of the NewGen unit under construction at Rooppur NPP is the capability of maintaining the NPP safety without operator intervention virtually unlimitedly in the case of a blackout and prevent fuel damage for at least 24 hours in the case of a blackout compounded by a coolant loss accident.
Utter reliability of Gen 3+ units as well as their compliance with post-Fukushima safety standards is provided with mutually redundant active and passive safety systems whose operation requires no power sources.
The active safety systems of Gen 3+ design are operated with normal power as well as emergency power produced by diesel-generators. Passive systems that have all critical safety functions made it possible to improve the quality of the NPP safety.
In case of an accident with primary leakage under blackout conditions the design ensures the core cooling with several systems simultaneously.
The system of passive core flooding maintains the core coolant level required for the reactor cooling. The system of passive heat removal from the secondary circuit designed to cool down the reactor under blackout conditions in the case of a coolant loss accident removes heat from the core due to the condensation of steam generated in the steam generator tubes and return of the condensate to the core. The heat exchanges of the system are arranged at the level of about 40 meters being protected by structural steel preventing their 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 NPP independence (no core damages) in the case of coolant leakages under blackout conditions is ensured with hydraulic accumulators of the passive core flooding system. Such accumulators ensure 24-hour independence in the case of a leakage of any size. Primary accumulators start feed boron solution if pressure drops below 5.9MPa. These accumulators ensure fast core flooding.
Secondary accumulators provide for long (at least 24 hours) maintenance of the level of the primary coolant required for reliable heat removal from the core. These accumulators start feed boron solution if primary pressure drops below 1.5 MPa.
In case when there is no more water but normal or emergency power supply is not restored the design includes extra technical means to feed the reactor and storage pool with pump units powered with a mobile diesel generator with air cooling (water coolant may be unavailable for the diesel generator); and a fan cooling tower to remove heat.
Safety systems providing core cooling and reactor cool down in emergency also include the system of steam generator emergency cooldown. This system removes heat through closed circuit without feed water in case of beyond-design accidents.
The filtering system for annulus leakages prevents radioactive substances from escaping into the atmosphere. If normal and emergency power supply of this system fails, the system is to be powered with a mobile diesel generator.
The hydrogen removal system plays an important role in accident mitigation. It prevents hydrogen explosion and reactor building damages. Passive hydrogen recombiners are installed inside the primary containment. These recombiners prevent hydrogen concentration rise and possible explosions in all emergency modes.
The primary overpressure protection is intended to protect the reactor piping and equipment in case of excessive 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 is intended to prevent excessive 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.
This is not a complete list of the systems intended to prevent any NPP failures. The design of Gen 3+ NPP safety systems totally excludes any severe external and internal impacts, and prevents radioactive substances from escaping into the atmosphere in emergency.