The possibility of using the energy stored in the nucleus is one of the most significant scientific discoveries of the 20th century.
Fission was discovered in 1939 by Hahn, Strassman and Meitner. It was found that, due to neutron irradiation, the nucleus of uranium atom separates into two medium sized nuclei. Later it was suspected that theoretically all the nuclei could be separated, but in practice, only a few uranium and plutonium isotopes can easily be split by fission (by using neutrons). These isotopes, in addition, are more energetically advantageous in the decaying process, so more energy is released than is needed for splitting.
This energy is used by the nuclear power plants.
When the uranium 235 isotope is split, an average of 2.47 neutrons are produced. If more than one neutron is slowed on average, the number of neutrons causing fission will increase steadily. This would result in more fissions occurring during the same time, the process would not be controllable and the equipment would be damaged. To avoid this, the number of neutrons must be kept at a constant level, need to be controlled.
The chain reaction is controlled in the nuclear reactors.
The number of neutrons can be controlled in two ways. Boron is dissolved in the reactor’s cooling water, as boron is a strong neutron absorber. Its quantity is adjusted so the average number of neutrons per fission is only slightly above one. The fine regulation is performed with the control rods inserting into the core. If you want to increase the number of neutrons, i.e. the power, you have the rods pulled out, thus the reactor reaches a new balanced state with an increased power level.
The nuclear power plant belongs to the group of thermal power plants. Thermal power plants produce heat, which is converted into kinetic energy at the first step, then turned into electrical energy. In nuclear power plants, heat generation is done in the reactor thanks to the controlled chain reaction.
Most of the currently operating nuclear power plants are of light water cooled, light water moderated reactor types, mainly (more than 60%) pressurized water reactors (PWR). The currently operating Paks NPP Units and planned Unit 5 and 6 also belong to this type. The second most common type is the boiling water reactor (BWR).
Boiling water reactor: In the BWR reactors, the water used as coolant in the reactor core is boiled and the steam generated drives the steam turbine. The kinetic energy generated by the turbine is converted into electricity by the generator. Exhaust steam leaving the turbine is condensed and returned to the reactor. For this reason, there is no need for a steam generator in BWR power plants, as they use a single-circuit closed and single-circuit open cooling system. Every part of the closed cooling system works in a contaminated medium.
Pressurized water reactor: In the PWR reactors, compared to the described above, pressurized water cools the fuel assemblies in the reactor core, the steam rotating the turbine is generated in a special heat exchanger, the steam generator. By using the steam generator, it is possible to prevent the radioactive medium cooling the core from coming into contact with the turbine. The PWR power plants therefore use two closed and an open circuit cooling system. Only one of the two closed cooling systems, the so-called primary circuit (and its equipment) work with contaminated medium.
Other reactor types:
Pressurized heavy-water reactor: Like the PWR, it has two closed coolant circuits, however the primary circuit water only transfers heat. It uses heavy water (hydrogen nuclei have a neutron next to the proton, this hydrogen isotope is called deuterium) as moderator, which enables the use of natural, not-enriched uranium as fuel.
Gas-cooled reactor: The two closed cooling looped reactor uses graphite as moderator and carbon-dioxide as coolant, therefore a much higher primary circuit temperature can be reached than with water coolant. High temperature carbon-dioxide vaporizes secondary circuit water through a heat exchanger.
RBMK reactor: A Soviet-designed reactor, besides electricity generation, it is capable to produce plutonium. It uses slightly enriched or natural uranium as fuel. Graphite moderates the neutrons, and like in BWRs, water boils in the reactor, then it reaches the turbine. A big disadvantage of this type is that once the coolant is lost, nuclear fission doesn’t stop, compared to water moderated reactor types. The tragic fated Chernobyl NPP belonged to this type. This reactor type is not built anymore, due to this safety defect.
The heat generating equipment of the pressurized water reactor is the nuclear reactor. The nuclear chain reaction is going on in the core which is inside the reactor. The generated heat is transferred to steam generators by flow of high-pressure, high-temperature water, which medium circulates in a closed system. This is the primary circuit in nuclear island.
The function of the pressurizer is to maintain and control the primary circuit pressure. The high pressure in the primary circuit ensures that the cooling water remains in liquid state of matter.
Each of the steam generators produces high-pressure steam, which rotate the axis of the turbines, therefore transforming thermal energy generated in the reactor into kinetic energy. This rotary motion of turbo-generator shaft and magnetic field induces electricity in generator, which is delivered to the national electricity grid through the Generator-Transformer and overhead-lines. Besides this, the steam of the turbines condensates, water is lead back to the steam generator. This is the secondary circuit in turbine island.
The Unit’s open circuit cooling system chills the condensing heat exchanger, ensuring that exhaust steam is converted into liquid matter of state again. This cooling system is connected to a cooling tower or a river with the sufficient amount of water runoff.
About the new NPP units
VVER-1200 belongs to the generation 3+, PWR reactor type. The planned gross electrical power is 1200 MWe per unit. The expected service lifetime of the units is minimum 60 years.
Visual of the new NPP units
In each new power Unit, the reactor and primary circuit are housed by a double-walled protective building (Containment), which civil structure is one of the three main physical defence barriers against internal and external hazards. The outer Containment is designed such a way that it withstand all severe external hazards, even in the case of a large aircraft crash impact, it guarantees safe shutdown of the NPP Units.
Protection against external hazards
The inner containment is a cylindrical building, covered by a hemisphere on top. The inner containment hermetically isolates the primary circuit and spent fuel pool which contain radioactive materials from the environment. Safety can be guaranteed furthermore by the automatic safety systems, which can operate without personnel in case of emergency situations. The high level of safety can also be guaranteed only by passive safety systems, which are driven by laws of physics (e.g. gravity), therefore they don’t require any external energy source.
The 330-ton reactor vessel’s inner diameter is 4,25 meters, its height is 11 meters, and mean wall thickness of high-quality steel alloy vessel is 20 centimetres. There are 163 fuel assemblies in the core, one assembly contains 533 kg of UO2 (uranium dioxide).
The cooling of turbine condensers is performed with fresh water, using around 132 m3 of Danube water per second. The harsh water is hermetically isolated and sealed from radioactive medium by the intermediate secondary circuit, so it is fully separated hermetically from primary circuit, therefore it can be safely let back into the river bed.