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Learn about the three thermodynamic cycles for electrical energy production in NEK.

The NEK operating diagram has been simplified and only the main components are shown.

How NEK works

How the nuclear power plant works?

NEK functions in a similar way to a conventional thermal power plant, except that heat is not produced by burning fossil fuels. Instead, it makes use of the heat released during the fission of uranium nuclei in a reactor. The reactor consists of a reactor vessel with fuel assemblies which form the core. Ordinary purified water and chemically treated water circulate through the reactor under pressure and carry the released heat into the steam generators, where it is turned into steam. The steam drives a turbine which in turn drives the electrical generator. All the equipment of the reactor and the primary coolant loop is housed in the reactor building, which in view of its function is also known as the containment.

The reactor vessel containing the fuel assemblies is sealed during operation. The primary coolant circulates through the reactor vessel. For scheduled refuelling, the power plant needs to be shut down. The period between two refuellings is known as the fuel cycle. At NEK, the fuel cycle lasts 18 months. At the end of every fuel cycle, the spent fuel elements are replaced with fresh ones.

The technical part of a nuclear power plant is divided into three basic thermodynamic sets of systems:

Since water circulates in all three sets of systems, which are separated from each other, we can also refer to them as loops. The first two systems are sealed while the third, which uses water from the Sava River to condense the steam, is connected to the outside environment.

Primary system

The heat released in the reactor core heats the water circulating in the primary system. The heat of the water is transferred to the water of the secondary system via the walls of the U‑tubes in the steam generators.

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Primary system

Reactor

It consists of the fuel assemblies, control rods, measuring instrumentation and guidance structures for directing primary coolant flow. These are installed in a cylindrical containment, in which a controlled nuclear chain reaction takes place and during nuclear fission energy is released.

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Primary system

Nuclear fuel

Nuclear fuel is in the form of sintered uranium dioxide (UO2) pellets (cylindrical shape of about 1 cm in diameter and 1 cm long). These are stacked into fuel rods, and 235 rods are arranged in a fuel assembly. The reactor core consists of 121 fuel assemblies with enriched uranium‑235 (to a maximum of 5%, the rest is uranium-238). The new fuel assembly weighs between 500 and 600 kg and it is approximately 4 m high.

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Primary system

Control rods

The power output of the reactor and the combustion of the fuel are regulated by lowering and lifting the control rods and with the adjustment of concentration of boric acid in the primary coolant. Control rods consist of silver, indium, and cadmium which are good neutron absorbers.

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Primary system

Water in the primary system

Ordinary purified water with the addition of boric acid circulates through the reactor under pressure and carries the released heat into the steam generator, where it is turned into steam. 

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Primary system

Reactor coolant pump

Water circulation in the two steam generators is facilitated by two reactor coolant pumps. The pumps maintain the pressure needed to overcome water resistance through the reactor core, pipelines, and steam generator.

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Primary system

Pressuriser

The pressuriser maintains the pressure in the primary system during normal operation and limits pressure changes during transients. The pressure is maintained by means of showers or electric heaters.

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Primary system

Steam generators

The steam generators are heat exchangers with U-tubes. The heat of the water is transferred to the water of the secondary system via the walls of the U-tubes in the steam generators.

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Secondary system

The steam produced on the secondary side of the steam generators drives a turbine which then drives the electric generator. The generator converts this energy into electric energy and sends it to the transmission grid. The used steam from the turbine goes to the condenser, where it turns back into water and then through several heat exchangers and feedwater pumps it is forced back into the steam generator.

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Secondary system

High pressure and low pressure steam turbines

Steam from the steam generator is directed onto the rotor blades of high-pressure and low‑pressure turbines, in which the steam energy is converted into mechanical energy. The steam drives a turbine rotor which in turn drives the electrical generator.

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Secondary system

Condenser

The condenser is a heat exchanger. Cold water from the Sava River flows through the condenser pipes and condenses the steam from the low-pressure turbines. In contact with the cooling pipes (Sava River) in the condenser it is turned back into water.

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Secondary system

Electric generator

The electric generator is a three-phase generator with rated power of 850 MVA and a voltage of 21 kV. Electric generator converts mechanical energy into electrical.

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Secondary system

Transformer

Transformer converts the generator’s voltage (21 kV) up to the voltages for long-distance transmission on the transmission grid (400 kV). Electrical energy is transmitted at high voltages to reduce energy loss in long-distance transmission.

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Tertiary system

Tertiary system is designed to dissipate the steam heat that cannot be usefully employed to produce electricity. The circulating water pumps force water from the Sava River into the condenser and back into the river. Water flows through the condenser at the rate of 25 m3/s.

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Tertiary system

Circulating water pumps

Three circulating water pumps force the filtered (through trash racks and travelling screens) water from the Sava River into the condenser and heat exchangers and back into the river.

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Tertiary system

Cooling towers

Cooling towers use ambient air to cool the heated water before being released into the Sava River. They are used in unfavourable meteorological conditions, high or low river flow and during periods when the river contains excessive sediment deposits.

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Tertiary system

Cooling tower pumps

Cooling towers pumps force the heated water from the Sava River through the cooling towers and enable water circulation between the cooling towers and the suction in the condenser.

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Tertiary system

Sava river dam

The dam provides sufficient water level at different flow rates or water levels of the Sava River for cooling the condenser and other power plant systems. In normal circumstances the water level of the Sava River is provided by the Brežice hydropower plant.

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The main control room

The main control room serves as a central space for monitoring and control, from which the operators monitor the technological process and maintain the desired power plant condition.

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Containment building

The reactor building enclosing a nuclear reactor with reactor coolant loops and safety systems consists of an inner steel shell and a strong reinforced outer concrete structure. The containment separates the primary system and radioactive substances from the environment and protects it from direct damage from external missiles.

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Low and intermediate level radioactive waste storage (LILW)

Low and intermediate level solid radioactive waste is stored in a temporary storage facility for low and intermediate level radioactive waste inside the power plant's fence.

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Spent fuel pool

Spent fuel assemblies are temporarily stored in the spent fuel pool in the Fuel Handling Building. The pool is filled with water that contains boric acid for neutron absorption to prevent nuclear chain reaction going. Water that circulates through the heat exchanger cools the spent fuel assemblies.

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Spent fuel dry storage

Spent fuel dry storage has a passive nature, as we do not need additional systems and energy to operate it. From a safety point of view, such storage is the best provisional technical solution in the world.

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Switchyard

Electrical energy from the generator is transmitted via two transformers to the 400-kV electric transmission system, from which transmission lines go to Maribor, two routes go to Ljubljana and two routes to Zagreb.

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Waste manipulation building

The new building maintains high standards and improved working conditions for equipment manipulation and radioactive waste shipments and for the management of low and intermediate level radioactive waste (LILW). However, the key point of this project is that we have gained storage room for the low and intermediate level waste, which will help us bridge the period until the final LILW disposal solution.

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Bunkered building 1 (BB1)

Bunkered building 1 is located at a site separated from the existing nuclear part of the power plant; its independence and redundancy represent an upgrade to the basic design safety functions of the power plant.

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Bunkered building 2 (BB2)

Bunkered building 2 is also located at a site separated from the existing nuclear part of the power plant and it contains safety systems for the mitigation of beyond design basis accidents.

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Decontamination Building

Contaminated equipment is taken to the Decontamination Building for controlled cleaning of radionuclides and for controlled storage. Its radioactivity decreases over time..

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