Learn more about the technical terms used in the industry and necessary to understand how energy is generated in a nuclear power plant.

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Absorbed dose
Energy deposited by ionising radiation per unit mass of medium. It is a physical value whose unit is Gray (Gy).

Unforeseen event that causes damage to a facility or disrupts its normal operation, and which can result in one or more people receiving a dose higher than the established limits. (See "Incident".)

Accumulation factor
Factor that takes into account the strengthening of the theoretical flow of the direct radiation beam when it goes through a shielding material; it is originated by beam scattering caused by the multiple collisions undergone that are originated by the thickness of the material being considered.

Mechanism through which a non-radioactive atom becomes a radioactive one by being subjected to an irradiation process with neutrons or charged particles. Since only high-energy gamma rays cause activation, substances that are subjected to gamma radiation usually do not become radioactive.

Activation product
Radioactive nuclide that results when irradiating a stable material with particles owing to a nuclear capture reaction.

Physical magnitude that measures the number of spontaneous transformations (see "Radioactivity") which occur in a substance per unit of time. The unit is the becquerel (Bq).

Acronym for "As Low As Reasonably Achievable". It is one of the criteria of radiology and aims to achieve that exposures to ionizing radiations are as low as reasonably possible.

Alpha particle
A Helium-4 nucleus emitted during nuclear decay. By extension, any group of two protons and two neutrons. It is represented by the symbol α.

Annual dose limits
Maximum values, referred always to the last 12 months, of the doses that can be received by professionally exposed workers and members of the public. These values are set legally and do not take into account the doses resulting from the naturally occurring radioactive materials or exposure undergone as a result of medical examinations or treatments. The dose limits are applied to the sum of the doses received by external exposure during the period being considered, and of the integrated internal dose resulting from radionuclide intake that may have taken place during the same period.

Annual intake limit (AIL)
Activity which, upon entering the organism of a given individual, causes an integrated internal dose equal to the established annual dose limit.

Area under surveillance
Area where 1/10 of the annual dose limits can possibly be exceeded, and where it is very unlikely that 3/10 of those values will be reached, and therefore access to that area is regulated.

Area under the operator's control
According to regulations, an area established around a major nuclear or radioactive site which is under the control of the facility's operator.


Unit of activity; it is the activity of a certain amount of radioactive material that undergoes spontaneous atomic decay each second.

Beta particle
Positive or negative electron emitted during the decay of an atomic nucleus. It is represented by the symbol β.

Biological effect
Changes caused by ionising radiation when it affects a live organism, for example, the human body. There are two kinds of effects: Deterministic (or non-stochastic). Those where a causal relationship exists between the dose and the effect. They are only evident when the dose reaches or exceeds a given value (called threshold level). Therefore, their severity depends on the dose received. Probabilistic (or stochastic). Those where the relationship between the dose and the effect is probabilistic. They lack a threshold dose and their severity does not depend linearly on the dose received.

Packing with radioactive content as presented for transport or storage. This is a very broad term which does not specify whether it refers to drums, containers, etc., and is therefore widely used as a general term.


Capture, nuclear reaction
Nuclear reaction where the atom incorporates an external subatomic particle.

Operations Coordination Centre set up by the government offices in autonomous communities in Spain; it coordinates all the actions envisaged in the Nuclear Emergency Plans.

Chemical element
Substance formed by atoms with the same number of protons, although they can have a different number of neutrons. Its chemical behaviour is always the same, irrespective of its isotopic composition. (See "Enrichment".)

A fuel element that hermetically encloses the nuclear fuel preventing the fission products from escaping and guaranteeing mechanical strength to ensure the fuel's integrity.

Collective dose
The effective dose a certain group of people have been exposed to. It is measured in person-sieverts (p-Sv).

Committed collective dose
Committed dose that will be received by a certain group of individuals during a 50-year period. It is measured in person-sieverts (p-Sv).

Committed dose
The effective dose that a person will receive during the next 50 years (70 years for children) as a result of the amount of radioactive material intake into their organism. It is measured in sieverts (Sv).

A container designed to hold spent fuel or radioactive material in order to facilitate its transport or storage.

Structure used to house nuclear or radioactive facilities in order to reduce the possibility of environmental contamination. In nuclear power plants, the containment structure consists of a steel plate cladding and a 90 cm thick concrete wall, and it houses the reactor and the primary circuit.

Control rod
A rod-shaped element used to control the reactivity of a nuclear reactor. Inserting the control rods into the reactor's core leads to the cessation of the nuclear chain reaction.

Controlled area
According to legislation, an area where, owing to the existence of a source of ionising radiation, it is possible to receive more than 3/10 of the annual dose limits established for professionally exposed workers. Established within the controlled area are the limited presence area, when there is a risk of exceeding the annual dose limits, and the prohibited access area, when there is a risk of exceeding in one single exposure the annual dose limits.

Reduction in the activity of a radioactive material by means of decay.


Nuclide that is generated during radioactive decay. (See "Parent".)

Nuclear phenomenon where the mass and/or level of energy of a radioactive atom is reduced spontaneously, resulting in the emission of ionising radiation. The most usual forms of decay are the emission of alpha particles, beta particles, electron capture and spontaneous fission.

Decay chain
Series of radionuclides in which each member turns into the next one by means of radioactive decay, until a stable nucleus is finally reached.

Degree of burn-up
Amount of energy extracted from one unit of mass of a nuclear fuel from the moment it is inserted into the reactor. It is usually expressed in megawatts hour (or day) per ton of fuel. In a nuclear fuel and for a given fissionable nuclide, ratio between the number of atoms split and those present initially. It is usually expressed as a percentage.

Material in which the isotopic abundance of the nuclide considered of interest is less than the natural one.

Derived air concentration limit (DACL)
Average annual activity concentration in inhaled air which, in the event of it being breathed by the standard man for a working year of 2,000 hours, results in an intake equal to the annual intake limit. It is expressed in becquerel per cubic metre (Bq/m3).

Unless otherwise stated, the term dose is used to express more simply the term of effective dose (formerly effective equivalent dose). In nuclear medicine practice, as happens with commonly used terminology, the term dose is used incorrectly to refer to: - The amount of emitting substance (expressed as weight or activity) in the case of a non-encapsulated source administered to a patient (similarly to a dose of medication). - The energy radiated by a source or transported by a beam; in the case of the individual energy of photons or particles, it is expressed in MeV.

Instrument or device that enables an absorbed dose, an exposure or any other radiological amount to be measured or assessed. Dosimeters use different procedures for measuring the dose: radiation-sensitive photographic emulsions (photographic dosimeter), materials that absorb the radiation energy and then release it as light (thermoluminiscent dosimeters), chemical substances that change in the presence of radiation (chemical dosimeters, Fricke dosimeters), an electrical condenser (condenser dosimeter), etc. There are also different types of dosimeters based on their use: pocket, lapel, handheld, pen, etc.

System for measuring and recording absorbed doses.


Effective biological lifetime
Arithmetic mean of the lifetime of a radioactive species in a living organism.

Effective biological period
Time interval needed for the amount of a radionuclide incorporated into a living organism to be reduced by half. Its calculation takes into account the fact that this material disappears through two simultaneous channels: radioactive decay and elimination via natural biological means (sweat, excretion, etc.).

Effective dose (effective dose equivalent)
The equivalent dose weighted (corrected proportionally to) by the varying sensitivity of the different organs and tissues of the human body. The correction factors are called tissue weighting factors. It is measured in sieverts (Sv): 1 Sv = 1 J/kg. The old unit was the rem: 1 Sv = 100 rem. Until quite recently, this term was known as "effective dose equivalent", but the latest recommendations from the ICRP have simplified the name.

Elastic collision
Encounter between elementary particles in which there is no loss of kinetic energy. It can be compared to the collision between two snooker balls.

Stable elementary particle that is part of the most external region (crust) of atoms and has the smallest known negative charge. This charge, which is taken in physics as a unit, equals 1.602 E-19 coulomb. Its rest mass is 1/1836.2 that of the proton.

Electron capture
Radioactive transformation in which the nucleus captures one of its core electrons.

Electron-volt (eV)
Kinetic energy acquired by an electron when it goes through an electric potential difference of one volt in a vacuum. It equals 1.602 E-19 joule.

Encapsulated/non-encapsulated source
An encapsulated source consists of radioactive substances firmly incorporated into solid and effectively inactive matter or contained within an inactive enclosure that displays sufficient resistance to avoid any scattering of those radioactive substances, under normal conditions of use. When a source contains radioactive material in the absence of these restrictions, i.e., when the presentation and conditions of use do not enable any scattering of the radioactive substance it contains to be prevented, it is known as a non-encapsulated source.

Operation that involves increasing the isotopic abundance of a nuclide in a substance. In nuclear materials, enrichment refers to fissionable materials.

Equivalent dose
The absorbed dose weighted (corrected in proportion to) by the varying biological effectiveness of the different kinds of radiation on the live medium being considered. The correction factors are called "radiation weighting factors" (formerly radiation quality factors). It is measured in sieverts (Sv): 1 Sv = 1 J/kg. The old unit was the rem: 1 Sv = 100 rem.

1. Physical magnitude that characterises the ionisation that a radiation causes in the air. It is measured in roentgen (R): 1 R = 2.58 E-4 coulomb/kilogramme. 2. Effect of subjecting or being subjected to ionising radiation.

External cooling circuit
Circuit employing water taken from a natural source (river, dam, lake, sea) and used to condense the water steam once it has moved the turbine, similarly to those used in any other coal, fuel oil or gas power station. The water from the external cooling circuit is returned in a controlled manner to the river, dam, lake or sea at a temperature slightly higher than that at which it was taken. This water does not come into direct contact with the nuclear fuel.


Fission product
Stable or unstable nuclide which is a member of a fission chain. If can be a fission fragment or one of its descendants.

Free access area
In a nuclear or radioactive facility, an area where it is very unlikely that 1/10 of the annual dose limits established for exposed workers will be exceeded and, therefore, is not subjected to regulations for radiological protection reasons.

Fuel element
Arrangement consisting of various fuel bars (rods) located in an armature with frames and ready to be inserted directly into the core of a nuclear reactor.

Fuel pellet
Small, cylindrical portion of ceramic nuclear fuel. It is usually 1 centimetre high. It is obtained by pressing and treating oxide dust of fissionable material at high temperature.

Fuel rod
Nuclear fuel in the form of a rod consisting of fuel pellets contained in a metal tube.


Gamma rays
Electromagnetic radiation caused in the phenomenon of radioactive decay. Their wavelength is smaller than that of X-rays, and it is therefore an extremely penetrating radiation.

Genetic dose
The dose related to the genetic load of a population. It is calculated as the product of the genetically significant annual dose and the mean procreation age, which is estimated at 30 years.

Genetically significant dose
For a population, it is equal to the measure of the individual doses. Each individual dose is calculated by weighting the dose received by the gonads, taking into account the likely number of children that can be bred after exposure to radiation.

Global/partial exposure
Global exposure is considered to affect the entire body homogeneously. Partial exposure is limited essentially to one part of the organism, or to one or more organs or tissues, or is considered to be non-homogeneous exposure of the entire body.

Unit used in the International System of Units to measure the absorbed dose; it equals one joule per kilogramme (J/kg).


Head of the Radiological Protection Service
Person who has been recognised by the Spanish Nuclear Safety Council as an expert in radiological protection and has the relevant certificate issued by the NSC.

Heavy water
Water in which hydrogen is replaced or enriched in the deuterium isotope (whose nucleus contains a neutron in addition to the proton of light hydrogen).


Unforeseen event that has few implications, if any, outside the site. (See "Accident".)

Industrial radiography
Technique that makes it possible to obtain images of an object when it is traversed by X-rays. By extension, it also applies irrespective of the type of ionising radiation used.

The International Nuclear Event Scale (INES) is a work tool that enables the incidents and accidents that occur in nuclear sites to be catalogued in a homogeneous manner. It is divided into seven levels, in ascending order of severity, which correspond to specific characteristics. In addition, the scale is used to inform the public quickly and consistently about the meaning of the events that take place in nuclear sites in terms of safety.

Phenomenon whereby a radioactive substance existing in the environment enters the human body by inhalation, ingestion, through the skin, etc.

Internal/external exposure
The human body's exposure to external or internal sources.

Atom or group of atoms which, through losing or gaining one or more electrons, has acquired an electric charge.

Ionising radiations
Generic name used to denote corpuscular or electromagnetic radiation which either directly or indirectly produces ions in its interaction with matter.

Subjecting a material or living creature to the action of radiation.

Isomeric transition
For a nucleus, going from an excited state to a lower level one, by emitting the excess energy as gamma radiation.

Each of the different nuclides which have the same atomic number (number of protons) and, therefore, belong to the same chemical element, but which differ from each other as regards the number of neutrons.


Lower detectable activity level (LDAL)
Minimum amount of activity that can be measured with the analytical method used.


Mean lifetime
Arithmetic mean of the lifetime of an atomic species in a given state.

Member of the public
Irrespective of their employment relationship, and for the purpose of their radiological protection exclusively, Spanish legislation considers members of the public to be all the individuals in the population except for professionally exposed workers and students and trainees during their usual working hours.

Reduction in the kinetic energy of neutrons.

Substance that reduces the velocity of fast fission neutrons without significant neutron capture.


Naturally occurring radioactive materials
Collection of ionising radiation that exists in natural form in the environment, resulting from cosmic or terrestrial radioactive sources.

Elementary particle with no electric charge that is part of the nucleus of atoms; its mass is 1.00136 times that of the proton.

Nuclear chain reaction
Series of similar nuclear reactions that take place because one of the agents causing the reaction (usually a neutron) is the product of another of these reactions.

Nuclear fission
Nuclear reaction where a heavy nucleus splits, usually into two fragments whose size has the same order of magnitude, and in which neutrons are emitted and a large amount of energy is released.

Nuclear fuel cycle
Series of industrial operations that fissionable materials are subjected to so that they can be used in a nuclear reactor. It includes from the mining stage to irradiated fuel management.

Nuclear fusion
Reaction between nuclei of light atoms that leads to the formation of a nucleus heavier than the initial ones, accompanied by the emission of elementary particles and energy.

Nuclear medicine
Use of radioisotopes, such as encapsulated and non-encapsulated sources, with medical diagnostic or therapy purposes. They are used "in vivo" (nuclear medicine services) or "in vitro" (radioimmune analysis laboratories).

Nuclear power plant
Electric power production plant that uses fissile material to obtain heat through nuclear reactions.

Nuclear reaction
Reaction between particles where the intervening forces are of nuclear origin.

Nuclear reactor
Facility where a nuclear chain reaction can be initiated, maintained and controlled. The pressurised water (nuclear) reactor is cooled with natural water at a pressure higher than saturation pressure in order to prevent it from boiling. The boiling water reactor is cooled with natural water, which is left to boil in the core in considerable quantities.

Nuclear risk / undue risk
Nuclear damage contingency. If this risk is higher than the admissible level it is called undue risk.

Nuclear safety
Collection of rules and practices used for the location, project, control and operation of nuclear or radioactive facilities with no undue risk.

Nuclear substance
In Spanish legislation, name applied to nuclear fuel, except for natural uranium and depleted uranium, and to radioactive products and waste.

Atomic species (collection of identical atoms) characterised by the number of protons and neutrons it has.


Ongoing exposure
Extended external exposure whose rate, however, can vary over time, or the internal exposure resulting from a permanent intake whose intensity varies over time.

Person authorised by the Spanish Nuclear Safety Council to handle the control devices of a nuclear or radioactive site or to handle the radioactive devices or materials.

Situation where the dose limits are exceeded; when a dosimetric reading higher than the dose limits is obtained, i.e., in the event of a possible overdose, the Spanish Nuclear Safety Council requires the carrying out of special medical checks and conducts an investigation to establish whether the dose was actually received by the person using the dosimeter or only by the device. If the results of the investigation are positive, they are reflected in the case and dosimetry histories and on the worker's radiological card.

Owner/responsible operator
In the Nuclear and Radioactive Site Regulations, an individual or legal entity responsible for the site's operation, and who appears as the owner in the relevant authorisations.


In a decay process, a nuclide that decays; in a decay chain, the first nuclide in the chain. (See "Daughter".)

Particle accelerator
Electrical radiation generator in which the speed of a stream of charged particles (electrons, protons, etc.) is increased. Depending on the type of acceleration, they are divided into circular (betatrons, cyclotrons, etc.) or linear (linacs) accelerators. In radiotherapy, charged particles can be used directly (electrotherapy, proton therapy, etc.) or through a target which, on receiving the particle beam, emits high-energy X-rays (over 5 MeV) or neutrons (neutrotherapy).

Particle flow or fluence
Number of particles that enter a small sphere every unit of time, divided by the area of the sphere's maximum circle.

Population at large
For the purpose of radiological protection, the entire population including the members of the public and the professionally exposed workers and trainees and students during their usual working hours.

Antiparticle of the electron; it has the same mass and the same electric charge, but in the positron this charge is positive instead of negative, as happens with the electron.

In a decay chain, the radionuclide preceding the one being considered.

Primary coolant
Fluid used to extract the heat produced in the core of a nuclear reactor.

Primary cooling circuit
Closed system through which a nuclear reactor's coolant flows (in most reactors, water), in order to extract the heat formed in the core by the nuclear reaction. It is therefore the circuit that contains the fluid which comes into direct contact with the fuel elements.

Professionally exposed worker
Person who works in the presence of ionising radiation and may receive more than one tenth of the dose limits established for workers. If it is possible that the worker may receive more than 1/10, but it is very unlikely that he or she may receive more than 3/10 of the dose limits (more than 5 and less than 15 mSv), the worker is classified as category B. If it is possible that the worker may receive more than 3/10 of the established limits (15 mSv), he or she is classified as category A and will be required to use an individual dosimetry system.

Protected area
An area established around the area under the operator's control where means are provided to protect the population in the event of an accident at the site.

Elementary particle whose electric charge is positive and equal to that of the electron, and whose mass is 1.67 E-27 g. It is found in the nucleus of atoms.


An obsolete unit of absorbed radiation dose: 1 rad (rd) = 0.01 J/kg. The unit used today in the International System of Units is the gray: 1 gray = 100 rad.

Radiation weighting factor
Factor that the absorbed dose needs to be multiplied by in order to take into account the different effects caused by the same absorbed doses of different types of radiation. The result is the equivalent dose.

Radioactive contamination
Undesired presence of radioactive substances in living creatures, objects or the environment. There is surface contamination (when it affects the surface of objects), external contamination (the skin of human beings) or internal contamination (internal organs of human beings).

Radioactive discharge
Liquid or gaseous radioactive materials that, once they have been treated and measured, are discharged in a controlled manner into the environment at a nuclear or radioactive site.

Radioactive effluent
Radioactive fluid (liquid or gaseous) from a nuclear or radioactive site which is subjected to control and, where necessary, to treatment, prior to its discharge into the environment.

Radioactive equilibrium
Condition where the relationship between the activities of two or more radionuclides in a decay chain remains constant.

Radioactive equipment
Device that emits ionising radiation, either because it contains a radioactive material or because this radiation occurs during normal operation.

Radioactive material
According to Spanish legislation, any material containing substances that emit ionising radiation. According to this definition, any substance, including the human being, is radioactive material, since all existing substances contain radioactive isotopes. This does not mean that the existence of this radioactivity requires the adoption of some type of radiological protection measures. When it is necessary to state that a radioactive material contains radioactivity in a proportion that may call for the implementation of some kind of precaution, the term used is "radioactive substance".

Radioactive series
Chain of radionuclides in which each member turns into the next one by means of radioactive decay, until a stable nuclide is finally reached.

Radioactive standard
Material or equipment which for a long period of time can be considered invariable and serves as the ultimate term for comparison in measurements of a given magnitude.

Radioactive substance
Any material that contains one or more radionuclides whose activity should be taken into account for radiological protection purposes. (See "Radioactive material".)

Radioactive waste
Any waste material or product that displays traces of radioactivity and for which no use is planned. Contaminated residual liquids and gases are included. - High-activity waste: 1. Highly radioactive liquids containing most of the fission products and some actinides, which are separated in the first extraction cycle with solvents during chemical reprocessing of the irradiated fuel, as well as the effluents related to that process. 2. The irradiated nuclear fuel, if it has been declared waste. 3. Any other activity waste comparable to the above ones. - Medium-activity radioactive waste: Lower level of activity and heat generation than high-activity waste, but they still require shielding during transport and handling. The term is used generally to refer to all waste not defined as high or low level. - Low-activity radioactive waste: Waste which owing to its low radioactive content requires no shielding during handling and transport.

Property of some chemical elements that enables them to emit electromagnetic waves or particles. This property is due to the existence of an unbalance between the number of neutrons and protons in the atom's nucleus, which causes instability and a release of the energy accumulated in the form of particles or waves. Natural radioactivity is due to elements that emit radiation spontaneously, as in the case of uranium, thorium, radon, etc.

Radioactive isotope.

Radiological protection
Collection of rules and practices used to prevent the risks of receiving radiation doses and, if applicable, to mitigate the effects. (See "Nuclear safety".)

Property that enables certain substances to absorb the energy from ionising radiation, and subsequently to emit part of that energy as light.

Radioactive nuclide.

Toxicity caused by the intake into the organism of a radionuclide emitting ionising radiation and by its resulting products. Radiotoxicity depends not only on the radioactive characteristics of the radionuclide, but also on its physical and chemical state, and on the metabolism of that element in the organism.

Gaseous chemical element whose isotopes, all of them radioactive, belong to the natural series of uranium and thorium. The radon isotope with the longest semi-decay period is Radon 222, a descendant of Uranium-238, which is the main cause of air pollution due to natural radioisotopes.

Rate (of exposure, dose, etc.)
For each magnitude, the ratio between the increase it undergoes and the time interval during which such an increase has taken place. The decay rate is measured in decays per second (dps); the exposure rate in roentgen per hour (R/h); the absorbed dose rate in gray per hour; the equivalent and effective dose rate in sievert per hour (Sv/h) or sievert per year (Sv/y); etc.

Reactor core
Part of a nuclear reactor which houses the fuel and where the fission nuclear reaction and heat release take place.

Reactor vessel
Container that holds the nuclear reactor's core, the reflector, part of the coolant and other components.

Reference groups
Groups of people whose exposure is reasonably homogeneous and representative of that of the most exposed individuals of the population owing to a given practice.

Reference level
Value, below the discharge limit, which is taken as the basis for determining whether the operation of the corresponding systems is appropriate.

Material placed around a nuclear reactor in order to return by diffusion part of the neutrons that would otherwise escape.

Obsolete unit of equivalent and effective radiation dose. 1 rem = 0.01 J/kg. In the International System of Units it has been replaced by the sievert: 1 Sv = 100 rem.

Process undergone by irradiated nuclear fuel to recover the fissionable, residual or newly-formed atoms contained in it.

Residual heat
Heat associated with the radioactivity of irradiated nuclear fuel, once the reactor has been shut down or the fuel element has been removed from the reactor core.


Safeguards (International)
Term used to include the measures established to prevent failure to comply with a law or agreement that bans the use of nuclear materials or facilities with non-peaceful aims. It is usually used in the plural.

Safety barrier
A set of components, systems or administrative regulations used at nuclear or radioactive sites to prevent accidents. At a nuclear power plant there are usually three 3 safety barriers: the fuel rod cladding, the reactor vessel (the primary circuit) and the reactor containment. The fuel pellet itself is sometimes added as a first barrier, since under normal operating conditions its treatment enables it to retain a large part of the generated fission products.

Emergency room of the Spanish Nuclear Safety Council.

1. Fluorescent screen: Surface covered with a fluorescent substance that emits light when it receives an ionising radiation. 2. Reinforcement screen: Sheet which under the action of X-rays emits a secondary radiation that contributes to the printing of the plate. 3. Radiation-absorbing device that blocks its path in order to intercept radiation totally or partially.

Semi-decay period
Time interval needed for the number of atoms in a radioactive nuclide to be reduced by half through spontaneous decay.

Semi-reduction thickness
Thickness of a given substance that reduces by half the intensity of a certain type of radiation, when it is interposed in its path.

Shield (biological)
Material placed between a source of radiation and personnel in order to minimise the number of particles and radiation, and protect personnel from the effects of such radiation.

Shielding factor
Correction factor owing to radiation attenuation.

Sievert (Sv)
Unit of equivalent and effective radiation dose in the International System of Units: 1 Sv = 1 J/kg. The old unit is the REM. 1Sv = 100 REM.

Single exposure
Brief external exposure or internal exposure resulting from radionuclide intake during a short period of time.

Source (of radiation)
Device or substance capable of emitting ionising radiation.

In reactor technology, synonymous with shutdown. When following a nuclear reactor's stop the coolant temperature remains high, this is known as a hot shutdown. If the coolant temperature falls, the term used is "cold shutdown".

Person authorised by the Spanish Nuclear Safety Council and in charge of directing the operation of a nuclear or radioactive facility and the work of the operators and, where applicable, of replacing the operators.


Technological safeguard
Each of the systems used at a nuclear or radioactive site to prevent accidents or mitigate their consequences.

Temporary/final waste storage
Action involving keeping radioactive waste at sites designed specifically for that purpose. There are temporary storage sites, i.e., locations that enable storage for the period of time needed for the activity of the radioactive waste they contain to fall to pre-established values; temporary storage sites are designed to allow surveillance of the location and recovery of the waste they contain. There are also final storage sites, locations which are determined to be secure enough to hold radioactive waste which is not intended to be recovered.

Thermal neutron
A neutron which is approximately in a situation of thermal equilibrium with the medium in which it moves. This usually corresponds to an energy of 0.025 electron-volt for a temperature of 25 °C.

Thermal power
In a nuclear reactor, the power generated as a result of the fission reactions that take place within and released directly or indirectly as heat.

Luminiscence that takes place when a substance is heated moderately.

Tissue weighting factor
Factor that the equivalent dose received by the various organs and tissues of the organism needs to be multiplied by in order to obtain the effective dose for the entire human body.

Total exposure
The sum of internal and external exposures.

Emergency reactor shutdown imposed by the operation of one of its safety systems.


Chemical element with atomic number 92 and symbol U. The isotope formation of the uranium found in nature is as follows: U-235 0.71%, U-238 99.28 %, and the rest is U-234. Because of the presence of fertile and fissile isotopes, uranium can be used in any type of reactor. It is normally used as uranium dioxide (UO2), because the use of uranium metal has multiple limitations, as happens with uranium alloys.


Electromagnetic radiation caused in the transitions of electrons from deeper levels. Their wavelength is smaller than that of ultraviolet rays, and greater than that of gamma rays.