# Glossary

Course subject(s)
0. Getting Started with Understanding Nuclear Energy

The glossary provides a list of terms used in this course with their definitions. The different terms can be opened by clicking on the term.

### Absorbed dose

The amount of energy absorbed per unit mass, expressed in gray (Gy)

### ALARA

As Low As Reasonably Achievable

### Alpha decay

A decay mode in which the nucleus emits an alpha particle

### Alpha particle

A particle consisting of two protons and two neutrons, which is emitted from a nucleus in alpha decay

### Atom

The smallest particle of a chemical element that can exist

### Atomic mass unit

A unit of mass used to express atomic and molecular weights, equal to 1/12 of the mass of a carbon-12 atom

### Atomic number

The number of protons in a certain nuclide

### Attenuation

The gradual loss in intensity of radiation as it passes through a medium

### Attenuation coefficient

A coefficient used to express how easily particles or radiation can penetrate a material

### Barn

A unit of area most commonly used to express nuclear cross sections

### Beta minus decay

A decay mode in which the nucleus emits an electron and an antineutrino after converting a neutron to a proton

### Beta plus decay

A decay mode in which the nucleus emits a positron and a neutrino after converting a proton to a neutron

### Boiling Water Reactor

A type of reactor in which steam is generated inside the primary loop to drive a turbine directly

### Bremsstrahlung

A type of electromagnetic radiation (photon) produced when a charged particle is decelerated by another charged particle

### Burnup

A measure of the amount of fissile material used in a reactor expressed in gigawatt-day/tonne (GWd/t)

### Charge

A physical property of matter that causes it to experience a force in an electromagnetic field

### Chart of nuclides

A graph containing all known nuclides

### Cladding

The layer of material separating the nuclear fuel from the coolant

### Compound nucleus

The temporary combination of an incident particle and the target nucleus after a collision

### Compton effect

The decrease of the energy of a photon, as a result of the inelastic scattering of this photon with an electron

### Control rod

Rods of neutron absorbing material that can be inserted into the reactor to control the fission rate

### Coolant

The material used to extract heat from the nuclear fuel

### Coulomb barrier

The energy barrier due to electrostatic interaction that two particles need to overcome so they can get close enough to undergo a nuclear reaction

### Covalent bond

A chemical bond, which involves the sharing of electron pairs between atoms

### Delayed neutron

A neutron emitted by one of the fission products after a nuclear fission event

### Deterministic health effects

Health effects for which a threshold is believed to exist and which vary in severity with the dose

### Dosimetry

The calculation and assessment of the radiation dose received by the human body

### Effective dose

The tissue-weighted sum of the equivalent doses for all tissues and organs in the human body

### Effective multiplication factor

The ratio between neutron production and neutron losses (absorption and leakage)

### Ejectile

A particle that is emitted during a reaction

### Electron

An elementary particle with a negative charge, which orbits the nucleus of an atom

### Electron capture decay

A decay mode in which the nucleus absorbs an electron from its shells and emits a neutrino after converting a proton to a neutron

### Element

A species of atoms having the same number of protons in their nuclei

### Endothermic reaction

A reaction in which the system absorbs energy from its surroundings

### Enrichment

The process used to increase the concentration of a fissile isotope

### Epithermal neutron

A neutron with a kinetic energy higher than 0.2 eV, but lower than 1 MeV

### Equivalent dose

The dose absorbed by a certain tissue, summed over all types of radiation, expressed in sievert (Sv)

### Exothermic reaction

A reaction in which the system releases energy to its surroundings

### Fast breeder reactor

A nuclear reactor that creates more fissile material than it consumes

### Fast fission factor

The ratio of the total number of fission neutrons and the number of fission neutrons from thermal fission alone

### Fast neutron

A neutron with a kinetic energy of 1 MeV or higher

### Fast non-leakage probability

The probability that a fast neutron will not leak out of the reactor

### Fission neutron

A neutron that is a product of a fission reaction

### Fission products

The nuclides that result from a fission reaction

### Fission reaction

A type of reaction in which a nucleus splits into smaller parts

### Fuel pellet

The smallest unit of nuclear fuel in a typical reactor

### Fuel rod

A long cylindrical tube of cladding material filled with fuel pellets

### Gamma ray

A photon emitted spontaneously by a nucleus

### Gray

The unit used to express the absorbed dose

### Half-life

The time required for half of an initial amount of radioactive material to decay

### Infinite multiplication factor

The effective multiplication factor of an infinitely large reactor (no leakage possible)

### Ionization density

The number of ionizations per unit path length caused by ionizing radiation

### Ionizing radiation

Radiation consisting of particles or electromagnetic waves with sufficient energy to cause ionization in the medium through which it travels

### Isobar

All nuclides with a certain mass number, but a different atomic number

### Isomeric transition

A decay mode in which a nucleus in an excited metastable state emits a gamma ray

### Isotone

All nuclides with a certain neutron number, but a different atomic number

### Isotope

All nuclides with a certain atomic number, but a different neutron number

### Light Water Reactor

A group of reactor types that use regular water as the coolant and neutron moderator

### Macroscopic cross section

The probability that a particle will undergo a reaction with another particle or nucleus in a certain volume

### Mass defect

The difference between the mass of a composite particle and the sum of the masses of its parts

### Mass number

The number of neutrons and protons in a certain nuclide

### Mean free path

The average distance travelled before a particle undergoes an interaction

### Microscopic cross section

The probability that a particle will undergo a reaction with a single other particle or nucleus

### Neutrino

An elementary particle that interacts only through the weak force and gravity

### Neutron

A subatomic particle with no charge and a mass of approximately 1 atomic mass unit

### Neutron capture

A nuclear reaction in which a nucleus and one or more neutrons collide and merge to form a heavier nucleus

### Neutron current density

The number of neutrons crossing some unit area in a certain direction per unit time

### Neutron density

The number of neutrons per unit volume

### Neutron flux density

The path length covered by neutrons per unit volume per unit time, calculated by multiplying the neutron density and the neutron velocity

### Neutron moderation

The process of slowing fast neutrons down to thermal energies

### Neutron number

The number of neutrons in a certain nuclide

### Neutron poison

A material (nuclide) with a large absorption cross section for neutrons

### Neutron reproduction factor

The ratio of the number of fast fission neutrons produced by thermal fission and the number of thermal neutrons absorbed in the fuel

### Nuclear binding energy

The energy required to disassemble the nucleus into its component parts

### Nuclear fission

A reaction in which a nucleus splits into two smaller parts

### Nuclear fusion

A reaction in which two or more nuclei merge to form one or more different nuclei.

### Nuclear reaction

A process in which two nuclei, or a nucleus and a subatomic particle collide to produce one or more nuclides that are different from the nuclide(s) that began the process

### Nucleon

One of the particles of a nucleus; either a neutron or a proton

### Nucleus

The dense region at the center of an atom, consisting of neutrons and protons

### One-group approximation

An approach to solving the neutron transport equation by assuming all neutrons have the same energy, and therefore velocity

### Pair production

The process in which a high energy photon is converted into an electron and a positron

### Particle density

The average number of particles per unit volume

### Photoelectric effect

The complete transfer of energy from a photon to an electron, which is more than the electron binding energy of the material

### Photon

An elementary particle, which is the quantum of electromagnetic radiation

### Positron

An elementary particle, which is the antiparticle of the electron

### Pressurized Water Reactor

A type of reactor in which the primary loop is under such high pressure that the water remains liquid

### Prompt neutron

A neutron emitted immediately by a nuclear fission event

### Proton

A subatomic particle with a positive charge and a mass of approximately 1 atomic mass unit

### Q-value

The amount of energy released by a certain reaction

### Radiation dosimeter

A device that measures exposure to ionizing radiation

### Radiation weighting factor

A dimensionless factor used to obtain the equivalent dose based on the type of radiation absorbed

### Radioactive decay

A process in which an unstable nucleus releases energy by emitting radiation

### Radiosensitivity

The relative susceptibility of cells, tissues, organs, and organisms to the harmful effect of ionizing radiation

### Reactivity

A measure of the deviation from a critical condition

### Reactor pressure vessel

The metal vessel containing the reactor core and coolant

### Resonance escape probability

The fraction of fission neutrons that manage to slow down from fast to thermal energies without being absorbed

### Sievert

The unit used to express the equivalent dose

### Slow neutron

A neutron with a kinetic energy similar to that of atoms in its environment (less than 0.2 eV)

### Spontaneous fission

A decay mode in which the nucleus splits into two parts

### Stochastic health effects

Health effects which occur by chance (e.g. cancer, genetic effects)

### Theoretical density

The density a material would have if no porosity was present

### Thermal non-leakage probability

The probability that a thermal neutron will not leak out of the reactor

### Thermal utilization factor

The probability that a neutron that gets absorbed does so in the fuel

### Tissue weighting factor

A relative measure of the risk of stochastic effects that might result from irradiation of a specific tissue

Understanding Nuclear Energy by TU Delft OpenCourseWare is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Based on a work at https://ocw.tudelft.nl/courses/understanding-nuclear-energy/.