Hungarian Power Companies Ltd.

We provide energy to your everyday life

Skip Navigation LinksPaks II. Zrt. » Paks II. Ltd. » Nuclear Energy » Rudiments of Nuclear Energy Engineering

Skip Navigation LinksRudiments of Nuclear Energy Engineering

1. General history of nuclear energy
The possibility of utilising the energy stored in the nucleus of an atom belongs to the most important scientific discoveries of the 20th century. In accordance with Albert Einstein’s famous relativity theory, enormous energy potential is hidden in the building blocks of material.
The discovery of ionising radiation is linked to the name of Wilhelm Conrad Röntgen, who discovered a type of radiation with extremely high penetration during an experiment carried out at his home laboratory in 1895. For this discovery, Röntgen was awarded the very first Nobel Prize in Physics. Radioactivity was discovered by Henri Becquerel in 1896 accidentally, because that photographic plates used for taking X-rays, which he stored next to some uranium ore, turned black.
Jointly with Frederick Soddy, Ernest Rutherford formulated the theory of radioactive decay in 1902, and deduced the existence of nuclei in 1910, then that of protons in 1918 on the basis of experiments.
British physicist Sir James Chadwick discovered another particle of the nucleus unknown before, the neutron, in 1932. The discovery is highly important, since due to their neutral charge, neutrons are able to penetrate the negatively charged electron cloud of atoms and to penetrate the nucleus comprising positively charged protons.
In 1939 Otto Hahn, Fritz Strassmann and Lise Meitner discovered a reaction unknown before, nuclear fission, during the irradiation of uranium nuclei with neutrons. During nuclear fission, the heavy-weight nucleus of uranium split into two medium-sized nuclei and additional neutrons, thereby creating a possibility for chain reaction. The fission was accompanied by the release of a considerable amount of energy. The majority of the energy is released in the form of the kinetic energy of the fission fragments flying apart, i.e. heat.
As result of the outbreak of World War 2, research on, and the use of, nuclear energy were aimed at the achievement of the goals of the war industry. On 2 December 1942, headed by the Italian Enrico Fermi (with the support of Leó Szilárd), researchers managed to start a controlled chain reaction for the first time in the world beneath the grandstand at the University of Chicago stadium (this was a nuclear pile), for the purpose of weapons production for the time being.
Most originate the utilisation of nuclear energy for peaceful purposes only from 8 December 1953, since it was on that day that US President Eisenhower delivered his ‘Atoms for Peace’ speech, in which he recommended the establishment of the International Atomic Energy Agency, with the main responsibility of providing a possibility for the use of nuclear energy for peaceful purposes for humanity (electricity generation, medical science and agriculture).
Further information: atomeromu.hu


2. Working principle of the utilisation of nuclear energy
At thermal power plants, heat is generated, which is transformed into kinetic energy, from which electricity is produced. In addition to conventional types, nuclear power plants are also counted among thermal power plants.


2.1 Fossil thermal power plants
In the furnaces of conventional thermal power plants, by burning  fossil fuels (e.g. coal, petroleum and natural gas), their chemical binding energy is utilised (Figure 1), which results in the emission of greenhouse gases, including a significant amount of carbon dioxide. The heat released is typically used for heating or evaporating water. The high temperature and high pressure steam so produced drives a turbine. After this, the waste steam is condensed into water with the coolant fed to a condenser beneath the turbine; then it is returned to the boiler.

​Figure 1. Operation in principle of conventional (fossil) power plants
View enlarged picture​
(Source: wikipedia.org)


A generator is connected to the shaft of the turbine, which converts kinetic energy into electrical energy. The electricity produced in the generator is transformed to a higher voltage level with a transformer and is supplied to the electricity system.


2.2 Nuclear power plants

Nuclear power plants essentially differ from conventional thermal power plants by utilising the binding energy holding the nuclei of the fuel together (see ‘Nuclear Fission and Chain Reaction’) in the reactor for the purpose of heat generation. This process does not result in greenhouse gas emissions. The working medium is usually steam here, too. Depending on the nuclear power plant type, the heat generated in the reactor may heat the water indirectly or directly, such as in the case of the Boiling Water Reactor types, where the steam driving the turbine is produced directly by boiling and evaporating the cooling water of the reactor.

However, the most common type is the Pressurised Water Reactor type, which includes the four Paks units currently operating in Hungary as well as the new units to be established. In the case of Pressurised Water Reactors, the steam driving the turbine is produced in  steam generators (Figure 2). The steam generator is heated by water heated in the reactor, which does not reach its boiling point because it is held under high pressure. Therefore, the water coming into contact with the core of the reactor containing fissile material is circulated in a closed loop (primary circuit), and it transfers only its heat to what is called the secondary circuit coolant driving the turbine.

​Figure 2. Operation in principle of PWR nuclear power plants
View enlarged picture​
(Source: 45nuclearplants.com)

Further information: atomeromu.hu


2.3 Nuclear fission, chain reaction and reactor core

In nuclear reactors, water is heated with energy produced from a nuclear chain reaction. The word ‘nuclear’ refers to the nucleus, derived from the Latin word for core. Nuclei consist of protons and neutrons.

Protons and neutrons are elementary particles with a positive and neutral charge (without any charge), respectively. There are nuclei (typically those of larger atoms, such as uranium) that, upon being impacted on by a free neutron , can easily become unstable and may split into two fission fragments, while a huge amount of energy is released (Figure 3). The kinetic energy of the fission fragments flying apart at high speed is transferred to the other particles of the fuel via collisions, which appears as heat at the macroscopic level. The coolant removes this heat.

Figure 3. Nuclear fission
View enlarged picture​
(Source: reak.bme.hu)

This is nuclear fission, during which about two or three new neutrons are also released and fly apart at high speed. These so-called fast neutrons become slow neutrons after being slowed down to an appropriate speed, after which they may encounter other uranium nuclei, thereby triggering other fission. This is the chain reaction (Figure 4), which cannot really take place without slowing down the neutrons.

​Figure 4. Controlled nuclear chain reaction
View enlarged picture​
(Source: npp.hu)


Slowdown is carried out with what is called a moderator (decelerating medium); its role is played by the same water that is used for removing the heat released in the reactor. Consequently, water plays two roles: it is a coolant for the reactor and a moderator for the neutrons.

In the core of the reactor, materials suitable for absorbing neutrons must also be used besides the fuel containing fissile materials and the moderator in order to keep the nuclear chain reaction under controlled conditions. Such a material, for example, is boron, which is used in the form of boric acid mixed into water or in the form of boron steel control rods inserted into the reactor (Figure 5)

​Figure 5. Nuclear reactor
View enlarged picture​
(Source: atomeromu.hu)

In the core, the fissile material is arranged into fuel assemblies (Figure 6). The assemblies comprise fuel rods. These are zirconium cladding tubes, in which the cylindrical UO2 ceramic pellets are placed. Cooling water flows through the fuel assemblies, between the fuel rods, and is heated by the rods. At the end of the campaign, some of the fuel assemblies are removed. The remaining fuel assemblies are rearranged into new positions and fresh fuel assemblies are also placed into the core. The removed spent fuel assemblies, which are no longer able to continue working, are placed into a spent fuel pool next to the reactor vessel. They spend additional years there, while the most active isotopes, which generate the most heat, decay in them. 

​Figure 6. Various types of fuel assemblies
View enlarged picture​
(Source: tvel2012.ru/en)

The most commonly used fissile material in reactors is uranium. Naturally occurring uranium practically consists of two isotopes: these are uranium-235 and uranium-238. The nuclei of both isotopes comprise 92 protons; in addition, uranium-238 contains 146 neutrons, while uranium-235 has 143 neutrons. In thermal nuclear reactors currently used for electricity generation, uranium-235 can be used for fission. However, it amounts to less than 1% of natural uranium, the remaining 99% is uranium-238; therefore, the 235 isotope has to be enriched in the uranium for producing fuel that can be used in reactors.
(Figure 7)

​Figure 7. Proportion of the various isotopes of uranium before and after enrichment
View enlarged picture​
(Source: wikipedia.org)

Uranium enriched up to a uranium-235 content of 5% is arranged in pellets in the form of uranium dioxide (UO2). The pellets are placed into metal-clad rods, which are arranged in fuel assemblies. These fuel assemblies comprise the active zone of the reactor.

Further information: atomeromu.hu

3. Glossary of nuclear energy terms

(Based on a compilation by István Vinnay)

The following compilation provides definitions of the technical terms most often found in nuclear energy engineering and in related news.

Core: The reactor core that contains fuel and in which chain reaction occurs.

Alpha radiation: Highly ionized radiation with a very short effective range (travelling a few cm in the air). Actually, it is a flow of helium nuclei flying at high speed.

Nuclear power plant: A plant generating electricity by means of one or more nuclear reactors. Certain nuclear power plants also generate and sell heat besides electricity (e.g. for heating homes or supplying heat to industrial plants).

Refuelling machine: A high precision computer-controlled piece of equipment with which the fuel assemblies can be moved from one location to another without manual contact.

Becquerel, Bq (pronounced: 'be-kə-rel): The activity of radioactive materials is measured by the number of atoms decaying in it per second. Its unit is becquerel. The activity of a quantity of material in which one decay occurs per second is 1 Bq.

Beta decay: The transformation of certain nucleus types during which one neutron becomes a proton or one proton becomes a neutron in the nucleus and, simultaneously, one electron or one positron (a particle of the same weight as an electron, but having a positive charge) is produced, which exits at high speed. The former is called negative beta decay, the latter positive beta decay. Beta radiation is produced from a series of such decay occurring in a material. Beta decay is the typical form of decay of nuclei in which there is an excess of neutrons or protons. Fission products are just like this, therefore, they undergo beta decay. This is the origin of the very high radiation of spent (used) fuel assemblies.

Beta radiation: Radiation with a rather short effective range but longer than that of alpha radiation, comprising high-speed flying electrons. It results from the beta decay of nuclei.

Full-scope simulator: A computer-controlled device, which is used for simulating the behaviour of nuclear power plant units over time. It has a very important role in the training and in-service training of operating personnel.

Enrichment: A complex and energy-intensive process, in which the proportion of the uranium isotope with mass number 235, which is present in natural uranium in a very low proportion (0.7%), is increased. Most reactor types can operate only with enriched uranium.

Whole body counter: A radiation detector used for the measurement of all gamma and X-ray radiation emitted by the human body, which is well shielded against ambient natural radiation. It is used for the detection of radioactivity of incorporated material. At the nuclear power plant, employees potentially at risk are regularly checked with it.

Electron: A particle with a negative electric charge, which is about 2,000 times lighter than a proton or neutron. In normal state, there is no electron in the nucleus; it is produced only in the process of beta decay, but immediately exits the nucleus (a beam of a lot of such exiting electrons is beta radiation).

Half-life: The period during which the quantity of a radioactive isotope, and so its activity, is reduced to half as a result of the radioactive decay process. This is a natural constant for a specific radioactive isotope (a given nuclide type); e.g. in the case of radium, it is 1,620 years. The half-life of the various radioactive isotopes may extend from a very small fraction of a second to up to billions of years. 

Occupational exposure: The additional dose received by employees as a result of their work.

Liquid radioactive wastes: Radioactive liquids produced as a by-product of the utilisation of nuclear energy and other activities performed with radioactive materials (e.g. radiochemical laboratory works), which cannot be utilised.

Fusion: See nuclear fusion.

Fuel assembly: Uranium pellet-containing fuel rods are assembled in a common assembly, so the fuel can be handled (moved) as such units.

Gamma radiation: Electromagnetic radiation, such as light or heat radiation, but much ‘harder’ than those and with a shorter wavelength. While visible light or X-ray radiation is the result of processes occurring in the electron shell of atoms, gamma radiation results from processes occurring in the nucleus, which therefore has more energy. A gamma ray is emitted as a result of the transition of a nuclide from an excited state to a lower energy state. Therefore, gamma decay does not entail qualitative nuclear transmutation. (No other type of nuclide is produced. Nuclei produced as a result of alpha decay or beta decay will be different from the initial nucleus.)

Genetic radiation effects: Radiation effects that may appear not in the individual exposed to radiation, but to progeny born later.

‘Weakened’ uranium: A term without sense, invented by the media. In fact it is called ‘Depleted uranium’ (see there). 

Fission: See nuclear fission.

Fission products: Usually two medium-size nuclei  produced from a splitting heavy nuclei, their decay products as well as neutrons and other particles released upon fission.

Fissile materials: Material types the nuclei of which are fissionable.

Ion: If atoms that are electrically neutral in their ground state lose one or more of their electrons (or absorb additional electrons), positive (or negative) ions are produced. The (e.g. collision) process leading to this is ionisation.

Ionising radiation: High energy radiation, which is able to create ions penetrating a material. Its most important types are alpha, beta, gamma, X-ray and neutron radiation. (Neutron radiation ionises materials only indirectly; visible light and ultraviolet radiation do not belong here.) 

Isotopes: Variants of a particular chemical element (this clearly determines the number of protons), which differ only in the number of neutrons in the nucleus (and thereby in its weight) and, consequently, only in a few physical properties. Elements usually consist of a mixture of isotopes in their natural occurrence. 

Iodine prophylaxis: In the case of a reactor accident, a high amount of radioactive iodine may be released into the environment, which, after entering the human body, accumulates in a small part of it, the thyroid gland, thus it poses a risk of high radiation exposure in a small spot. Therefore, in the case of an accident, large amounts of stable (not radioactive) iodine are supplied to the population at risk in the form of tablets in order to saturate their bodies with iodine and thus to reduce radioiodine absorption by the thyroid gland.

Campaign: The operating cycle of a reactor unit, which lasts from start-up with freshly loaded fuel to the next reloading/maintenance. Most common are the 12- and 18-month campaign length.

Burn-out: A process where the uranium isotope with mass number 235 is being depleted as a result of a high number of fission reactions. It does not mean chemical combustion. 

Containment: A pressure-resistant, hermetically designed and built structure containing the nuclear reactor and its directly connected parts and system elements, the function of which is to prevent or limit the release of radioactive materials into the environment in the case of normal operation, anticipated operational occurrences and design accidents. There are containments with various design concepts. There are reinforced concrete, pre-stressed reinforced concrete, steel, and single- or double-walled containments with full or reduced pressure. The containment of the Paks Nuclear Power Plant is a reinforced concrete box structure.

Light water, heavy water: The former is common water made up of the most common variant of hydrogen, which contains a single proton in its nucleus, while in the latter, hydrogen appears in the form of what is called heavy hydrogen, in which there is one or two neutrons in addition to the proton (the former is called deuterium, the latter tritium). Heavy water is much more expensive, but absorbs neutrons to a smaller extent than common (‘light’) water; therefore, certain reactor types operate with heavy water (that contains deuterium). (There is no heavy water in Paks NPP.) 

Criticality: (Attention! Misleading terminology!) A normal operating condition of the reactor where exactly one neutron creates new fission on statistical average out of every two or three neutrons resulting from fission. In this case, the number of fission reactions and, with that, the quantity of energy produced, too, are constant over time. Throughout its continuous energy production, the reactor is in a ‘critical’ state.
Slow neutron, fast neutron: Fast neutrons are produced during the fission process. In order to enable them to generate new fission with higher efficiency they need to be slowed down. This deceleration is carried out by means of collisions in a moderator. (In the Paks reactors, the moderator is common light water.) The tasks of neutron deceleration and neutron absorption should not be confused. The latter is performed by boron in the form of boron steel or a boron solution. The moderator is needed because only slowed-down neutrons can maintain the chain reaction. 

Localization tower: The pressure-reducing system of the hermetic compartment operating passively, part of the containment. It contains water in large pools, which condenses the released steam in the case of a primary break, thereby preventing the development of a higher-than-allowable pressure.

Nuclear transmutation: The transformation of one nuclide into another nuclide. 

Nuclear energy: The energy holding the components of the nucleus (nucleons) together. Some of this energy can be released in nuclear reactions or nuclear transmutations.

Nuclear fusion (fusion): One of the possible methods of energy production, during which light nuclei are aggregated into heavier nuclei, while energy is released. Such a process also provides the energy of the Sun and the hydrogen bomb. Under terrestrial conditions, so far no controlled energy-producing fusion chain reaction could be maintained for an extended period. Nuclear fission, but not fusion, takes place in implemented nuclear reactors.

Nuclear fission: The separation of a heavy nucleus into two smaller nuclei. This process is usually accompanied by neutron radiation, gamma radiation and more seldom by the emission of charged nuclear fragments. Nuclear fission is usually triggered by neutrons penetrating the nuclei, but it may also occur spontaneously with a very low probability.

Moderator: A material in nuclear reactors used for slowing down neutrons produced from fission. See ‘Slow neutron, fast neutron’.

Monitor: A device aimed at measuring ionising radiation or the quantity of radioactive materials and possibly giving a warning if it becomes higher than a certain preset value. 

International Nuclear Event Scale (INES): A seven-stage scale introduced by the International Atomic Energy Agency, which aims to allow clear information to be provided to the media and the general public in the case of incidents or accidents occurring at nuclear power plants.
The scale distinguishes between three incident and four accident levels.

Level 1 event: It is not yet an incident, only an anomaly; a certain deficiency occurs in the protection of safety, but it does not pose a risk either to personnel or the population.

Level 2 event: An incident that may already have safety consequences, but not even the personnel can receive any radiation exposure over the dose limit.

Level 3 event: A serious incident, during which the radiation exposure of the personnel may exceed the dose limit, but the amount of radioactive materials released into the environment is very small.

Level 4 event: An accident with impacts primarily within the facility, the result of a partial core meltdown. Persons at the highest risk may receive a few mSv of radiation exposure. Acute health impacts may appear in a small number of personnel, but not in nearby residents.

Level 5 event: An accident also involving off-site risks; radioactive materials released due to a significant meltdown of the reactor core may also endanger the population. It is necessary to partially implement planned accident management counter-measures.

Level 6 event: A severe accident during which radioactive materials are released to such a great extent that it is necessary to fully implement planned local accident management counter-measures in order to prevent serious health impacts.

Level 7 event: A very serious accident, during which the majority of the radioactive materials in the reactor vessel are released into the environment, there is a risk of early (deterministic) radiation injury at the nuclear power plant and in its immediate vicinity, and late health impacts may also appear beyond national boundaries. (The Chernobyl accident was included in this category.)

Neutron: A particle without an electric charge, which has nearly the same mass as the proton (roughly one thousandth larger), the other component of the nucleus besides the proton.

Nuclear safety protection: Measures taken to protect humans and assets against the adverse effects of ionising radiation and radioactive pollution.

Nuclear chain reaction: A series of reactions in which the individual reactions create conditions for subsequent reactions. Fission chain reaction has decisive importance in nuclear energy production, in which neutrons produced in fission create further fissions.

Nuclear fuel: The material suitable for producing fission chain reaction (usually uranium), which is used as fuel in nuclear reactors through appropriate technical design.

Nuclide: The name of a nucleus characterised by a specific number of protons and neutrons. It is the nucleus of a specific isotope of a chemical element.

Pressurised Water Reactor (PWR): A reactor the primary coolant of which is under such pressure that, despite the high temperature (almost 300oC at Paks), the water does not reach its boiling point in it.

Self-sustaining nuclear chain reaction: A nuclear chain reaction in which the number of new reactions triggered by a reaction is equal to one on average. Thus, the process sustains itself. From the point of view of nuclear energy engineering, the self-sustaining fission chain reaction has decisive importance.

Primary circuit: The name for the combination of the reactor and the cooling loops connected to it. The medium in it is usually highly radioactive; preventing its release is an essential technical task. 

Proton: An elementary particle, one of the components of the nucleus. Its electric charge is +1.60219x10-19 coulomb and its rest mass is 1.67265x10-27 kg. 

Radioactivity: The structure of the nuclei of the isotopes of certain elements is unstable, thus they decay while emitting ionising radiation (α, β and γ radiation) and are transformed into other nuclei. A particular chemical element (potassium, iron, etc.) usually occurs with several isotopes, i.e. with nuclei that have different mass numbers. They usually exist on Earth in stable and radioactive variants.

Radioactive materials: Materials that contain unstable nuclei; therefore, ones that continuously emit radiation. They can be natural or artificially produced radioactive materials. Their depletion and thus the reduction of their intensity are characterised by their half-life.

Radioactive decay: A spontaneously occurring nuclear transmutation during which particles and/or gamma radiation exit the nucleus.

Radioactive waste: The non-recyclable radioactive by-products of the utilisation of nuclear energy or other processes applying nuclear technology (e.g. nuclear medicine, research, industrial materials analyses, etc.).

Reactor excursion: An uncontrolled, exponential increase in reactor output (incident/accident condition). 

X-ray radiation: Ionising electromagnetic radiation with high penetration power, which comes from energy transitions occurring in the inner layers of the electron shell of heavy atoms, have a much shorter wavelength/higher frequency (i.e. higher energy) than visible light, which is also electromagnetic radiation, but is the product of minor electron energy transitions appearing in the outermost layer of the electron shell.

Multiplication factor (k): The number which shows how many times more (less) fission is created in the next generation by neutrons resulting from fission occurring at a given moment. If k=1, the number of fission reactions is constant over time and energy is produced evenly (criticality). If k<1, the number of fission reactions keeps decreasing, then the chain reaction stops (subcriticality). If k>1, the number of fission reactions and, along with it, reactor output, is increasing (supercriticality).

Radiation accident (radiological accident): An unusual incident relating to the use of radioactive materials or the application of ionising radiation sources, during which the operating personnel or other persons in the vicinity have been exposed to radiation over the dose limit or have been contaminated with radioactive materials to an extent resulting in an exceeded dose limit. 

Radiation disease: A disease resulting from excessive (higher than a specific threshold) irradiation affecting the whole body or the majority of it, causing symptoms that can be well described.

Radiation infection: A term completely without sense, invented by the media. In the case of the effects of radiation, infection does not play any role. Instead, according to the factual situation, we can assert: ‘they have been exposed to radiation contamination or (radioactive) radiation’ or ‘have received a high radiation dose’ or ‘have been contaminated with a radioactive material’. 

Radiation protection, radiological protection: Measures related to limiting the adverse effects of ionising radiation on humans. For example, the limitation of radiation exposure affecting humans and the incorporation (entry into the body) of radionuclides, and the preventive limitation of physical damage arising from any of the forgoing.

Control rod: A rod (in the case of the existing Paks reactors, an assembly) containing a neutron-absorbing material, usually boron; by changing the vertical position to which they are pushed into the active zone of the reactor, the number of neutrons, and thereby the number of fission reactions, thus the quantity of energy produced by the reactor, can be changed.

Depleted uranium: When natural uranium is enriched, depleted uranium, which contains the uranium isotope with mass number 235 in a lower proportion than natural uranium, is also produced in addition to enriched uranium, which contains the isotope with mass number 235 in a higher proportion than natural uranium. Since the radioactivity of the uranium-235 isotope is higher than the one with mass number 238, the radioactivity of depleted uranium is about 40% lower (!) than that of natural uranium. It is not used for the purposes of nuclear energy production. However, depleted uranium is used in civilian life due to the high density of uranium (it is heavier than lead). Due to its high density, it is a material providing good radiation shielding. Thus, for example, it is efficiently used in medical X-ray devices or high level radiation sources (in hospitals) for radiation protection and radiation shielding purposes. Likewise, uranium-lined containers can be well used for storing or transporting radiating isotopes. Due to its high density, it also occurs in ships as ballast. In addition, it has been, for example, one of the components of the front armour on tanks for a long period. Furthermore, as a result of its high density, projectiles with very high penetration force can be made of it. The Americans are presumed to have used such projectiles in the Yugoslav war. This projectile does not make use of any nuclear property of uranium, only its greater mass (within a given volume).

Solidification: The conversion of radioactive wastes in liquid state through evaporation until materials in dry, stable, solid state are obtained, then embedding them into a solid material.

Natural background radiation: Ionising radiation present everywhere in our environment, which is independent of human activities. Its principal sources are the Earth’s crust and space.

Natural radioactivity: The radioactivity of radionuclides that also occur in nature.

Natural uranium: Uranium with an isotope composition occurring in nature. Its vast majority is uranium with mass number 238, and it contains only 0.7% of the uranium isotope with mass number 235, which has a decisive importance for nuclear energy production.

Refuelling: The replacement of spent fuel by new fuel, as well as the repositioning of partially spent fuel assemblies in a reactor.


Source: atomeromu.hu