A glossary of terms used in nuclear energy and nuclear physics.
Posted by Peter Havercan on 27 March 2025
This is a glossary of terms used in nuclear energy and nuclear physics.
Some astrophysics and thermodynamics terms loosely related to nuclear physics are also included.
Use a long press (press and hold) on images to see intellectual property rights.
actinide
Any element with an atomic number in the range 89 (actinium) to 102 (nobelium).
This includes all of the elements used as nuclear fuel.
The actinides other than thorium, uranium, and plutonium are
known as minor actinides.
Originally Thorium Ltd, and later The Radiochemical Centre Ltd, which was incorporated into
the United Kingdom Atomic Energy Authority.
The Centre installed the world’s first commercial cyclotron at Amersham, Buckinghamshire,
in 1966, for the production of medical radioisotopes.
It was the first public company to be privatized by the government of Margaret Thatcher in 1982,
and was nearly 25 times over-subscribed at its sale.
The company was renamed Amersham PLC in 1999, and was eventually acquired by the multinational
conglomerate General Electric, and its headquarters was moved to Chicago.
atom
Originally, the smallest particle of matter. John Dalton proposed the name from the Greek atomos (ἄτομος),
meaning “indivisible” or a particle that could not be further divided.
However, later atomic theory discovered that an atom could, indeed, be divided into smaller particles,
known as subatomic particles.
Atomausstieg
A German term meaning “nuclear phase out”. A political decision to close Germany’s fleet of nuclear reactors.
It was finally triggered by fear of the Fukushima Daiichi accident,
but followed decades of protest by anti-nuclear activists.
It is part of the Energiewende or “energy transition” policy which promotes the shutdown of
fossil fuel and nuclear in favour of renewable energy. But Fossilerausstieg remains elusive.
atomic energy
An old name for nuclear energy.
The term fell out of use as it was realized that such energy was derived only from the atomic nucleus.
atomic mass
The mass of an atom, measured in units called daltons (Da).
A speech at the UN General Assembly
on 8 December 1953 given by President Dwight D Eisenhower, encouraging the peaceful use of nuclear energy.
Dwight D Eisenhower giving his ‘Atoms for Peace’ speech at the UN.
B²FH
An astrophysics paper describing nucleosynthesis, entitled
Synthesis of the Elements in Stars.
It is identified by the initials of its four authors:
Margaret Burbidge, Geoffrey Burbidge, William A Fowler, and Fred Hoyle.
It suggested that most elements were formed in supernovae, but that theory
has been superseded by the merging neutron stars theory.
banana equivalent dose
A non-serious measurement of the dose of ionizing radiation
emitted by a single banana.
Because bananas are rich in potassium, they contain radioactivepotassium-40,
so they produce a radiation dose of about 0.1 microsievert.
becquerel
A unit of radioactivity, defined as one atomic decay per second.
Since this is an extremely tiny quantity, measurements frequently use a gigabecquerel
or petabecquerel instead.
beta particle
The name for an electron or positron when it is involved in a nuclear reaction.
beta radiation
A beam of beta particles emitted from a radiation source.
Emitted electrons are known as β- radiation,
and emitted positrons are known as β+ radiation.
At a quark level, β- radiation occurs when a
virtual W- boson is emitted from a neutron,
which transforms one of its “down” quarks into an “up” quark.
The neutron is changed into a proton and the emitted virtual W- boson
rapidly decays to an electron and an electron antineutrino.
The net result is that the neutron emits an electron and turns into a proton,
thereby increasing the atomic number of the nucleus by one.
Beta radiation can also refer to the much rarer β+ positron emission,
when a proton emits a positron and converts to a neutron,
thereby decreasing the atomic number of the nucleus by one.
Positron emission is used in the medical imaging technique called positron emission tomography,
which uses radioactivecarbon-11 as a positron source.
The difference in mass between an atomic nucleus and the masses of the nucleons
from which it is composed, expressed as an energy.
Except for hydrogen-1, the mass of a nucleus is always less than
the mass of the nucleons that compose it, and the difference, called the mass deficit, is compensated for
by the binding energy, due to E=mc².
When lighter nuclei are fused together, the binding energy in the nucleus is reduced,
and the excess energy is emitted as a nuclear fusion reaction.
For heavier nuclei, the opposite is true. The binding energy is reduced when the nucleus breaks apart,
and the excess energy is emitted as a nuclear fission reaction.
A light water reactor in which the reactor coolant is allowed to boil.
This produces steam to drive the turbine directly. The cooling system is still under pressure,
at about 75 atmospheres, so the water boils at a temperature of about 285°C.
A nuclear reactor which produces more fissile material than it consumes.
It does this by converting fertile nuclear material into fissile material
by neutron capture.
Also known as a fast breeder reactor.
Such reactors use fast neutrons and do not use a moderator.
The fast neutrons are absorbed by the surrounding fertile material.
Typically the surrounding material is uranium-238 and it is transmuted
to plutonium-239, but uranium-233 could also be bred from thorium-232.
The site of the first commercial nuclear reactor in the world, based at
Sellafield, Cumbria, known at the time as Windscale and Calder.
The reactor was a gas-cooled Magnox design.
It was commissioned, owned, and operated by
the United Kingdom Atomic Energy Authority.
It was opened on 17 October 1956 by Queen Elizabeth II, and closed on 31 March 2003.
Calder Hall nuclear power station, shortly after opening in 1956.
Carnot’s principle
A theorem of thermodynamics which states that the maximum efficiency
of a heat engine depends only on the working temperatures of the hot and cold sides of the machine.
Expressed algebraically:
ηmax = 1 ‑ Tcold/Thot,
where Thot and Tcold are
the absolute temperatures of the hot and cold sides of the machine.
This is a theoretical maximum efficiency for a hypothetically perfect machine.
Efficiencies of real machines will always be less than this theoretical maximum.
The efficiency is always higher when Thot is higher.
The efficiency of the hypothetical machine is only 1 if Tcold is absolute zero.
CANDU
CANada Deuterium Uranium. A Canadian-manufactured nuclear reactor,
which uses heavy water as a moderator and originally used natural
(unenriched) uranium as a fuel.
carbon-11
An isotope of carbon whose nucleus contains 11 nucleons,
of which 6 are protons and 5 are neutrons.
It is produced artificially in a cyclotron by bombarding nitrogen with protons.
It is radioactive, and decays to boron-11 by positron emission,
with a half-life of 20 minutes.
It is used as a source of positrons in positron emission tomography.
carbon-12
An isotope of carbon whose nucleus contains 12 nucleons, of which 6 are protons and 6 are neutrons.
It is the most abundant stable form of carbon.
It is used as the basis of the definition of the dalton unit of atomic mass.
carbon-14
An isotope of carbon whose nucleus contains 14 nucleons, of which 6 are protons and 8 are neutrons.
It is produced continually in the upper atmosphere, where cosmic rays produce
thermal neutrons, which interact with nitrogen-14 atoms to produce carbon-14
atoms and hydrogen-1 (protons).
The carbon-14 is oxidized to carbon dioxide, absorbed by plants which are consumed by animals,
and so enters the human food chain.
It is radioactive, producing beta emissions,
with a half-life of about 5700 years.
It is the second largest source of radioactivity in the human body
(after potassium-40), at a rate of about 3000 becquerel.
As the atmospheric carbon-14 is no longer replaced when an organism dies, the carbon-14 decays,
and the ratio of carbon-14 to carbon-12 can be used to estimate the date of death.
This is used in archaeology to date organic material by the process of so-called “carbon dating”.
CERN
Originally Conseil Européen pour la Recherche Nucléaire, and now the European Organization for Nuclear Research.
The international research establishment near Geneva, dedicated to high energy physics research.
It is best known as the location of the Large Hadron Collider and for the discovery of the Higgs boson.
The World Wide Web was also invented here by Tim Berners-Lee.
chain reaction
A process that occurs when a single nuclear reaction triggers one or more subsequent reactions.
It usually refers to the emission of one or more neutrons from a nuclear fission
reaction, which then go on to cause fission in other nearby nuclei.
In a nuclear reactor, criticality is achieved when each fission
causes exactly one further fission event.
The rate of the reaction is constrained by control rods.
In a nuclear weapon, the chain reaction is supercritical, and each fission produces far more
extra fissions per fission event, and the chain reaction grows exponentially.
chemical symbol
A notation to represent atoms in a chemical or nuclear reaction.
The basic symbol is a one- or two-letter abbreviation assigned by
the International Union of Pure and Applied
Chemistry.
It can be further decorated with: a left superscript containing the mass number of an isotope,
a left subscript containing the atomic number of the element,
a right superscript containing the ionization level, a right subscript containing
the number of atoms (in a molecule).
Chernobyl
[Preferred Ukrainian spelling: Chornobyl (Чорнобиль).]
The site of “the world’s worst nuclear accident”.
An explosion and subsequent meltdown of one of the reactors at the Chornobyl
nuclear power plant near Prypiat (Припять), Ukraine (at the time in the Soviet Union),
on 26 April 1986.
The accident was caused by a test to simulate shutdown procedures, which went wrong.
Two engineers were killed immediately, and two others were severely burned.
Thirty workers died from radiation sickness.
An UNSCEAR report in 2018 into thyroid cancer stated that there were 6,848 cases
for those under the age of 18 in 1986, of which fifteen were fatal.
A 2022 report from UNSCEAR states that “the vast majority of the population need not
live in fear of serious health consequences due to the radiation from the Chornobyl accident.
For the most part, they were exposed to radiation levels comparable to or a few times higher
than annual levels of natural background, and future exposures continue to slowly diminish”.
Cockcroft-Walton multiplier
A device used to generate a high static electrical voltage from a low-voltage alternating current.
The first one was built in 1932 to power an electrostatic particle accelerator
used for nuclear bombardment, causing the transmutation of lithium into
helium (as alpha particles).
It was the first experimental confirmation of Einstein’s formula E=mc².
The original Cockcroft-Walton multiplier at the Cavendish Laboratory, Cambridge.
control rod
A rod of neutron-absorbing material that is inserted into a nuclear reactor
to control the rate of fission.
Potential materials include silver, cadmium, and boron carbide.
corium
The fused reactor material remaining after a meltdown. It may contain a mixture of nuclear fuel,
fission products, control rods, and other reactor construction material.
The state in which a nuclear chain reaction is self-sustaining.
This is the normal state of operation of a nuclear reactor.
Culham
Site of the UK national centre for fusion research.
Originally the Royal Naval Air Station HMS Hornbill, it was transferred to
the UKAEA in 1960.
It later became known as the Culham Science Centre, and more recently as the Culham Campus.
It now contains the Culham Centre for Fusion Energy (CCFE).
The CCFE was responsible for the Joint European Torus (JET) which operated from 1983 to 2023, and
the Mega Ampere Spherical Tokamak (MAST), which operated from 1999 to 2013,
The successor to MAST, known as MAST Upgrade, began operation in 2020.
curie
An obsolete measure of radioactivity, originally the activity emitted by one
gram of radium-226 in one second. Now defined as 3.7×1010becquerel.
cyclotron
A particle accelerator used to accelerate charged particles in a magnetic field.
The particles follow a spiral path between two hollow D-shaped electrodes (called “dees”) held within
a strong fixed magnetic field, and charged with a high-frequency oscillating voltage.
For high-energy physics research applications, cyclotrons have been replaced by synchrotrons,
but they are still used for producing medical radioisotopes.
Diagram of a cyclotron from a 1947 issue of Radio-Craft magazine.
dalton
The unit used to measure atomic mass.
It is defined as exactly 1/12 of the mass of a neutral carbon-12 atom.
Also known as the (unified) atomic mass unit, or amu.
It is equal to 1.66053906892×10−27 kilogram.
Daresbury Laboratory
[Pronounced Darsbury.] A scientific laboratory complex situated between Runcorn and Warrington in Cheshire.
It was originally the Daresbury Nuclear Physics Laboratory, opened by Harold Wilson in 1967.
It was seen as a northern counterweight to southern establishments like Harwell
and the Rutherford Laboratory in Oxfordshire.
The first major research machine was NINA (Northern Institutes’ Nuclear Accelerator),
an electron synchrotron, whose name reflected the collaboration of northern
universities (Glasgow, Lancaster, Liverpool, Manchester, and Sheffield).
When NINA closed in 1977, it was replaced by the Synchrotron Radiation Source.
Daresbury was also the site of the Nuclear Structure Facility, a vertical 30MV
tandem van de Graaff generator constructed in 1974 and operated
between 1983 and 1993. Its tower continues to dominate the local landscape.
The site has been expanded to contain the Cockcroft Institute and the Accelerator Science and Technology Centre (ASTeC)
and a number of other facilities.
ALICE (Accelerators and Lasers in Combined Experiments) at ASTeC was an Energy Recovery Linac
and infrared Free Electron Laser, presumably named with a nod to Alice in Wonderland and
Daresbury’s connection with Lewis Carroll, who was born in the parish vicarage.
(It is distinct from a similarly-named experiment at CERN.)
Darlington Nuclear Generating Station
A Canadian nuclear power station located on Lake Ontario at Clarington, Ontario.
It contains four CANDU reactors.
Darlington Unit 1 took the world nuclear operation record of 976 continuous days of generation in 2020,
and then 1106 continuous days in 2021.
decay
The change in a substance due to radioactivity.Actinium series decay chain If the
product of the decay is another unstable isotope, that will also decay, giving rise to
a “decay chain” of successive decay products, eventually ending in a stable element,
which is most frequently an isotope of lead.
There are four decay chains for heavy elements, known as the thorium, neptunium, uranium, and actinium series.
The neptunium series terminates with bismuth-209, which was thought to be stable but actually decays to
thallium-205 with a half-life of 2×1019 years, considerably longer than the age of the universe.
A decay mode known as electron capture occurs when the nucleus captures an electron from
an inner shell of the surrounding electron cloud and transforms one of its protons to a neutron,
with the emission of an electron neutrino.
deep geological repository
A location deep in the Earth’s crust where nuclear or other hazardous waste can be stored permanently.
The repository should be up to 1000m deep, geologically stable, and below natural water sources.
The Finnish government have been constructing the Onkalo repository near
the Olkiluoto Nuclear Power Plant since 2004.
It is over 400 metres deep and is expected to receive high-level nuclear waste in the mid 2020s,
although “the exact start date has not yet been set”.
The United States has been investigating a repository at Yucca Mountain, Nevada, since 1978.
It was designated as the sole repository in 1987, but after years of political wrangling it has not
yet been brought into production.
In the UK, the responsibility for the development of a repository lies with a subsidiary of the
Nuclear Decommissioning Authority known as
Radioactive Waste Management, which became part of Nuclear Waste Services in 2022.
The repository is known as the Geological Disposal Facility (GDF).
Candidates for its location are Theddlethorpe, Lincolnshire, as well as Mid Copeland, South Copeland,
and Allerdale, all in Cumbria.
deuterium
A familiar name for hydrogen-2.
When deuterium replaces the hydrogen in water, the result is heavy water.
Dounreay
Site of the Dounreay Fast Reactor and the Prototype Fast Reactor, in Caithness on the north coast of Scotland.
All the reactors at Dounreay were fast breeder reactors.
Generation ceased in 1994 and the site was handed over to
the Nuclear Decommissioning Authority in 2007.
Dounreay Fast Breeder Reactor.
dry cask storage
A process of storing high-level nuclear waste (spent nuclear fuel)
in concrete containers.
The waste is first cooled off for several years in a spent fuel pool, to allow the most
active radiation to decay. Then the spent fuel is moved into steel containers filled
with inert gas, which are later enclosed within concrete casks.
The concrete provides shielding so that no radiation can leak outside of the casks.
A publicly-owned electricity generation company, owned by the French government.
More widely known as EDF.
It owns 56 active nuclear reactors in France at over 18 sites.
The most recently opened reactor was Flamanville Unit 3.
Its British subsidiary, EDF Energy, now owns all the nuclear power stations in Britain
and is responsible for building the new ones at Hinkley Point C
and Sizewell C.
A subatomic particle that carries a negative electrical charge.
In a neutral atom, a cloud of electrons surround the positively charged nucleus,
cancelling out the positive charge.
All of the chemical and electrical properties of elements are due to their electron cloud.
The electron cloud is structured into shells and orbitals.
Each shell consists of one or more subshells, and each subshell consists of one or more orbitals.
The chemistry of the atom is mostly determined by the configuration of the electrons in the outermost shell.
The name comes from the ancient Greek electron (ἤλεκτρον), meaning “amber”,
because of the charge obtained by rubbing it.
electron volt
The amount of energy transferred when an electron falls through
a potential difference of one volt.
It is extremely small, but useful when describing atomic interactions.
It is 1.602176634×10−19joules.
A famous equation reputedly due to Albert Einstein. It defines the equivalence of mass and energy, and gives the
energy E equivalent to a mass m multiplied by c2, where c is the speed of light.
It is commonly believed that the equation forms part of Einstein’s
Special Theory of Relativity,
but it actually occurred later, in a different form, in
Does the inertia of a body depend upon its
energy-content?, which states:
“If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²”.
energy
The capacity of a system to perform work. This is a somewhat circular definition,
as work is defined as energy transferred by the application of force.
This is a consequence of the first law of thermodynamics.
Energy is measured in joules, which is the work done when a force of
one newton displaces an object by one metre.
At the atomic scale, energy can be measured in electron-volts.
In the obsolete centimetre-gram-second (CGS) system, heat energy was measured in calories,
equivalent to 4.184 joules. Dieticians adopted the calorie to measure the energy produced by food.
But the Calorie used by dieticians is actually a kilocalorie, or 1000 calories, equivalent to 4184 joules.
Another inexact unit for energy used in the popular media is “enough energy to power a home for a year”.
Based on OFGEM figures,
this is about 2.7 megawatt-hours of electricity (9.7×109 joules)
for a medium sized home. OFGEM also quotes an additional average annual usage for gas
of 11.5 megawatt-hours (41.4×109 joules).
The process of increasing the proportion of uranium-235 in a quantity of natural uranium.
The natural uranium is converted chemically to gaseous uranium hexafluoride.
This is then processed in a gas centrifuge to separate the 235U compound,
which is less dense than the 238U compound.
The separation is only partial, and needs to be repeated
thousands of times to achieve the desired level of enrichment.
Low-enriched uranium (LEU, with 3% to 5% of 235U)
is suitable for use in light-water reactors,
high-assay low-enriched uranium (HALEU, up to 20% 235U) is required for small modular reactors,
but high-enriched uranium (HEU, or weapons-grade) requires up to 85% 235U.
entropy
A measure of the energy in a closed system that is no longer available to perform work.
It is also a measure of the disorder of the system.
If the temperature of a system remains fixed, the change in entropy (ΔS)
for a change in heat (ΔQ) is given by ΔS = ΔQ/T,
where T is the absolute temperature.
The second law of thermodynamics states that the entropy of
a closed system can never decrease, so ΔS≥0.
An organization created under the Euratom Treaty on 25 March 1957,
establishing the European Atomic Energy Community.
Parties to Euratom are EU Member States.
The United Kingdom ceased to be a full member of the organization on 31 January 2020,
but participates as an associated state.
European Pressurised Reactor
A generation III pressurized water reactor
designed in Europe by Framatome, EDF, and Siemens.
It was later known as the Evolutionary Power Reactor, and then simply as EPR.
Two EPR reactors are being installed at Hinkley Point C in the UK,
and a further two are being planned for Sizewell C.
In nuclear reprocessing technology, a device for reducing the volume of the raffinate
liquid produced by the PUREX process. The evaporation process concentrates the liquid into
Highly Active Storage, which is then mixed with molten glass to produce vitrified material suitable
for long term storage.
exergy
The maximum amount of usefulwork that can be obtained from a system.
It is measured in the same units as energy (joules),
but is a measure of the quality of that energy.
Unlike energy, exergy is not conserved, as it decreases due to irreversibilities in the system,
such as heat leakage, steam leakage, or friction.
The relationship between exergy and entropy is given by the Gouy-Stodola equation,
which states (roughly) that, at a fixed absolute temperature T0, the
destruction of exergy is proportional to the creation of entropy: B = T0·S.
Capable of sustaining a nuclear chain reaction.
Practically, only three isotopes are fissile:
uranium-233, uranium-235, and plutonium-239.
Of these, only uranium-235 is available naturally by mining ore.
Uranium-233 has to be bred from thorium-232, and plutonium-239 has to be bred from uranium-238.
Isotopes of some other elements are also fissile, but they are rare and exotic,
like curium-244 and californium-249.
fissionable
Capable of nuclear fission. An isotope can be fissionable but not fissile if its nuclei can be split,
but without sustaining a chain reaction.
fission products
The elements produced when nuclear fuel atoms are split. Each larger fissionable nucleus splits
into two smaller nuclei. The total binding energy of the fission product
nuclei is lower than that of the original nucleus, and the difference is output as fission energy:
as gamma rays, and as the kinetic energy of the fission products and additional neutrons.
Typical fission products include caesium, technetium, zirconium, strontium, and iodine.
Flamanville
The site of a French nuclear power station in the Manche département
on the Cotentin Peninsula. It contains three nuclear reactors owned by EDF.
Units 1 and 2 are of PWR design, rated at 1330 MWe each.
The most recent reactor, Unit 3, opened on 21 December 2024.
It is an EPR design with a nameplate capacity of 1630 MWe.
It is claimed to be the most powerful nuclear reactor connected to the grid.
fuel rod
A container for nuclear fuel that can be inserted into a reactor.
It consists of an outer tube of cladding material, containing pellets of fuel,
usually uranium dioxide or MOX fuel.
In a PWR reactor, the cladding is zirconium alloy,
which is transparent to neutrons.
In an AGR reactor, the cladding is stainless steel.
In earlier Magnox reactors, the cladding was a magnesium aluminium alloy
and the fuel was metallic uranium.
Fukushima Daiichi
Site of the nuclear accident caused by the Tōhoku earthquake and tsunami in 2011.
Daiichi means “number one” in Japanese, so the name just refers to “Fukushima plant 1”.
The site contained six boiling water reactors, of which three
were damaged by the tsunami.
Three of the reactors suffered a partial meltdown.
However, the UNSCEAR committee reported that
“No adverse health effects among Fukushima residents have been documented that are
directly attributable to radiation exposure from the Fukushima Daiichi nuclear plant accident”.
The insurers of the Tokyo Electric Power Company paid compensation for one death due to lung cancer,
but that disease is not typically caused by radiation.
The site achieved a “cold shutdown” condition, when the cooling water is below 100°C, by December 2011.
The tritiated water produced by cooling the residual fuel was controversially
retained on site for twelve years, before a gradual release of the water was begun in 2023.
Daini means “number two”, so the nearby Fukushima Daini is “Fukushima plant 2”.
It was not so severely affected by the earthquake, and achieved cold shutdown within two days.
gamma ray
A beam of high energy electromagnetic radiation (photons) emitted in radioactive decay.
Gamma rays have frequencies of 30 exahertz (3×1019 Hz) and above,
and wavelengths less than 10 picometers (1×10-11 m).
A gamma ray is not emitted in isolation: it usually follows an alpha
or beta emission, or a nuclear fission,
nuclear fusion, or neutron capture event,
any of which leave the nucleus in an excited state.
A rule that states that, for alpha radiation, short-lived isotopes emit more
energetic alpha particles then long-lived ones.
This is a consequence of the tunnelling effect by which alpha particles are emitted.
It can be expressed mathematically as
log10 t½ = A(Z) / √Qα + B(Z)
where t½ is the half life, Qα is the decay energy,
and A and B are empirical constants that depend on the atomic numberZ.
The law was derived theoretically by George Gamow in 1928.
Generation IV International Forum
An international consortium of countries promoting the development of
generation IV nuclear reactors.
generation IV reactor
One of a range of proposed reactor designs to replace the current generation II and III designs.
They include:
Graphite Low Energy Experimental Pile.
The first nuclear reactor in Western Europe, developed in 1947 at the
Atomic Energy Research Establishment at Harwell.
It was an early simplified version of the BEPO (British Experimental Pile-0).
The central pile consisted of 505 tonnes of graphite, 12 tonnes
of natural uranium metal sprayed with aluminium, and 20 tonnes of uranium dioxide
encased in aluminium tubing.
It went critical on 15 August 1947.
Diagram of GLEEP, from the September 1977 issue of ATOM
(the UKAEA bulletin) 30th Anniversary edition.
A unit of ionizing radiation dose, defined as the absorption of one joule of
radiation energy per kilogram of absorbing matter.
Great British Nuclear
A nuclear energy company owned by the UK Government.
It is a reincarnation of British Nuclear Fuels Ltd (BNFL) under a new
trading name since 20 March 2024, but using the
same registration
at Companies House.
It intends to deliver the government’s long-term nuclear programme,
initially based on small modular reactor technology.
In the 2024 Budget, it was announced that GBN would buy the sites at Wylfa (Anglesey) and
Oldbury (Gloucestershire) for future nuclear development, but the plans were placed under doubt
by the new Labour government later in 2024.
Gigawatt (electrical). A measurement of the electrical power output
of a power station, in gigawatts. This is used to distinguish the electrical
output from other forms of energy, such as heat.
HALEU
High-assay low-enriched uranium.
Enriched uranium that contains 10% to 20% of uranium-235.
It is suitable for use in SMRs.
half-life
The time taken for a radioactive substance to decay to half of its original mass.
According to the Geiger Nuttall law, the higher the energy of an isotope’s alpha decay
the shorter its half-life.
Hartlepool
Site of a British nuclear power station in County Durham.
It contains two AGR reactors rated at 590 MWe.
It began operation in 1983 and will close in 2027.
Harwell
Now the site of one of Britain’s premier science parks, in Oxfordshire.
It was the original site of the Atomic Energy Research Establishment in 1946.
Europe’s first fission reactor, GLEEP, was developed there in 1947.
It was taken over by the UKAEA in 1954 and developed
the experimental fusion reactor ZETA.
It was expanded as the Harwell Science and Innovation Campus in 2006, where the
Rutherford Appleton Laboratory and the European
Centre for Space Applications and Telecommunications (ECSAT) are also also co-located.
Harwell Campus, featuring the Diamond Light Source and the RAL Space building.
The European Space Agency ECSAT building is in the foreground.
The most abundant isotope of helium.
Its nucleus is composed of two protons and two neutrons.
Helium-4 is the second-most abundant element in the universe, making up 25% of its total mass.
Most of this was formed in the first twenty minutes after the origin of the universe,
by “Big Bang nucleosynthesis”. Helium was formed preferentially because it has a
higher binding energy per nucleon than other light nuclei.
The preponderance of helium in stars was discovered by Cecilia Payne in 1925.
The helium on Earth is mostly produced by alpha emission
from thorium and uranium.
HEU
Highly-enriched uranium.
It is enriched uranium that contains more than 20% of uranium-235.
When the proportion of uranium-235 is 80% or more, it is also known
as “weapons-grade” uranium.
Heysham
[Pronounced Heesham.] Site of two British nuclear power stations
near Morecambe, Lancashire:
Heysham 1
contains two AGR reactors with a combined capacity of 1150 MWe.
It began operation in 1983 and will close in 2027.
Heysham 2
contains two AGR reactors with a combined capacity of 1250 MWe.
It began operation in 1988 and will close in 2030.
The Heysham 2 plant held the record for the longest continually operating
commercial power reactor (940 days) in 2016, until it was overtaken
by Darlington Unit 1.
Hinkley Point C
A nuclear power station currently under construction in Somerset.
It will contain two EPR reactors, each of 1630 MWe for a total capacity of 3260 MWe.
Construction began in 2017.
The concrete pour for the “common raft” foundation was completed in June 2019.
Three massive steel containment rings for each reactor were installed between December 2020 and October 2024.
The 47m containment dome for Unit 1 was installed in December 2023.
The reactor pressure vessel for Unit 1 was installed in December 2024.
Hinkley Point Polar Crane rotating the reactor pressure
vessel inside the Reactor Building, December 2024.
hormesis
A model of radiation dose response which asserts that small levels of ionizing radiation
can be beneficial to an organism, because they trigger a natural repair mechanism.
Compare with linear no-threshold.
hydrogen-1
An isotope of hydrogen whose nucleus contains only a single proton,
and no neutrons. It is sometimes known as protium.
It is the most abundant element in the universe.
An intergovernmental organization, based in Vienna, that promotes the peaceful use of nuclear energy.
It was created on 29 July 1957 as a response to Eisenhower’s “Atoms for Peace” speech.
International Nuclear and Radiological Event Scale
An internationally recognized seven-level scale for rating the severity of nuclear-related events,
created by the International Atomic Energy Agency.
The seven levels are:
Anomaly
Incident
Serious incident
Accident with local consequences
Accident with wider consequences
Serious accident
Major accident.
INES rating chart
In the history of nuclear power generation, only two events, at Chernobyl
and Fukushima, have been been given a rating of 7 (major accident).
Damage at these locations was caused by chemical explosions, not by nuclear explosions.
ion
An atom or molecule with a net electrical charge, caused by an excess or deficit of electrons
over the number of protons. An ion with excess electrons is negatively charged,
and an ion with fewer electrons than protons is positively charged.
ionization
Transformation of a neutral atom or molecule into an ion by the addition or removal
of one or more electrons.
ionizing radiation
Nuclear or electromagnetic radiation that is powerful enough to ionize the atoms
of a substance by detaching electrons from them.
This is primarily of concern when the ionized substance is within a living being,
when the radiation might cause biological damage.
Although radiation from nuclear energy sources is rightly regarded as ionizing,
ultraviolet light (UVB, wavelength 280-315 nm) found in sunlight, is also ionizing radiation,
and can cause sunburn and skin cancer.
The more energetic UVC, wavelengths less than 280nm, is also ionizing, but it is absorbed by the ozone layer
in the stratosphere and poses no risk at the Earth’s surface.
X-rays, with wavelengths from 10 pm to 10 nm, are an ionizing radiation used safely for
medical diagnosis under controlled conditions.
Cathode-ray tubes in early colour televisions also emitted small amounts of X-rays.
Travellers in high-flying aircraft are also exposed to ionizing radiation from cosmic rays.
iron-56
An isotope of iron whose nucleus contains 56 nucleons, of which 26 are protons and 30 are neutrons.
It has the lowest mass per nucleon, and so has the highest binding energy.
It represents a point in stellar evolution at which no further energy
can be produced by nuclear fusion.
isotope
One of the various forms of an element that are formed from nuclei of different masses.
Nuclei containing the same number of protons are the same element,
but nuclei of the same element with different numbers of
neutrons - and therefore of different masses - are different isotopes.
See also nuclide.
ITER
Originally, the International Thermonuclear Experimental Reactor, but the acronym is no longer used.
It is an international research project into nuclear fusion,
using a tokamak reactor, based in Cadarache, France.
Its construction is expected to complete in about 2033, with operation beginning in 2036-2039.
ITER is not designed to generate electricity, and will never be connected to the grid.
After Brexit, the UK announced in September 2023 that it will no longer pursue an association
agreement with Euratom, but that it will seek to continue and
enhance its international partnerships, including with ITER.
JET
The Joint European Torus. It was a European research project into nuclear fusion,
using a tokamak reactor.
Based at Culham, Oxfordshire, it began operation in 1983 and closed down in December 2023.
joule
A unit of work, or energy.
It is the work done when a force of one newton displaces an object by one metre.
laws of thermodynamics
The scientific laws controlling energy systems:
zeroth law of thermodynamics
If two systems are in thermal equilibrium with another system,
they must be in thermal equilibrium with each other.
(This law was invented after the others to provide an
axiomatic definition of temperature.
“In thermal equilibrium” means “at the same temperature”.)
first law of thermodynamics
Energy within a closed system cannot be created or destroyed,
but can only be transformed from one form to another.
(This law had to be extended to incorporate the equivalence of mass
and energy implicit in Einstein’s E=mc² rule.)
second law of thermodynamics
In a closed system, heat cannot flow spontaneously from a colder
region to a hotter one.
An alternative definition is that the entropy of a closed
system can never decrease.
third law of thermodynamics
The entropy of a system approaches a constant value as its
temperature is reduced to absolute zero.
An alternative definition is that it is impossible to reduce the temperature
of a system to absolute zero in a finite number of reversible steps.
lead-cooled fast reactor
A fast neutron reactor cooled by molten lead, or lead-bismuth eutectic alloy.
It is proposed as a future generation IV reactor.
The benefits of lead over sodium as a coolant are its higher boiling point (1670-1737°C),
its relative chemical stability, and better shielding against gamma rays.
LEU
Low-enriched uranium.
Enriched uranium that contains 3% to 5% of uranium-235.
It is suitable for use in light-water reactors.
light water
Plain water, whose hydrogen content is mostly hydrogen-1 (¹H₂O).
It is so named to distinguish it from heavy water.
A particle accelerator, also known as a linac, which accelerates
charged particles in a straight line.
It does this by applying an oscillating electrical charge to the stream of particles using electrodes
at differing phases of a radio-frequency alternating voltage (10 MHz to 1 GHz).
In high-energy physics, the linac is used to inject particles into a synchrotron.
In medical applications the linac is used to generate high energy X-rays for use in radiology.
linear no-threshold
A model of radiation dose response which asserts that any intensity
of ionizing radiation, however small, has a deleterious effect.
The health effect is presumed to depend linearly on the intensity of the radiation.
Compare with hormesis.
liquid metal reactor
A nuclear reactor in which the coolant is a liquid metal, typically sodium or lead.
It operates at a higher temperature and lower pressure than a water-cooled reactor.
A gas-cooled nuclear reactor designed to run on natural uranium,
cooled by carbon dioxide with a graphitemoderator.
The name arose from the “magnesium non-oxidizing” alloy used to clad the fuel rods.
The world’s first commercial nuclear power station at Calder Hall was a Magnox reactor.
Other Magnox reactors included Hunterston A, Hinkley Point A, Trawsfynydd, Dungeness A, Sizewell A,
Oldbury (Gloucestershire), and Wylfa (Anglesey), which was the last to close in 2015.
Manhattan Project
A research and development program in the USA during World War II to produce the first nuclear weapons.
The project also developed the first operational nuclear reactor, the Chicago Pile-1.
A quantity of energy equivalent to that produced by
a power source of one megawatt running for one hour
(3.6×109joules).
meltdown
Melting of the central core of a nuclear reactor.
It occurs when the heat generated by the reactor exceeds the heat removed by the
cooling system, causing a fuel element to exceed its melting point.
The resulting fused material is known as corium.
microreactor
A very small nuclear reactor with a capacity less than 10 MWe.
Such reactors would be useful for off-grid or microgrid applications.
minor actinides
Actinide elements other then thorium, uranium, or plutonium.
They are produced by neutron capture in reactors, so occur in the spent nuclear fuel.
The most significant such elements are americium, curium, and neptunium.
A nuclear reactor in which a molten salt is mixed with a fissile fuel
to provide a reactive working fluid.
The most frequently suggested molten salt is FLiBe, a mixture of lithium fluoride (LiF)
and beryllium fluoride (BeF2),
combined with uranium tetrafluoride (UF4) as a fuel.
In a liquid fluoride thorium reactor (LFTR), the fuel is thorium tetrafluoride (ThF4),
but the thorium is converted to fissile uranium-233 in the thorium fuel cycle.
A prototype reactor was build at Oak Ridge National Laboratories in the 1960s,
but there has been no commercial development since then.
Amid renewed interest, the technology is regarded as one of
the generation IV reactor candidates.
MOX fuel
Mixed oxide fuel. It is a blend of the oxides of multiple nuclear fuels,
especially a mixture of uranium and plutonium oxides.
About 10% of the fuel used in French nuclear reactors is MOX, but it is not used in British reactors.
Megawatt (electrical). A measurement of the electrical power output
of a power station, in megawatts. This is used to distinguish the
electrical output from other forms of energy, such as heat.
National Ignition Facility
An American nuclear fusion research device
at the Lawrence Livermore National Laboratory, California.
It uses inertial confinement powered by lasers to compress the fuel sufficiently to trigger fusion.
National Nuclear Laboratory
The UK’s leading civil national laboratory for nuclear fission, established in 2008.
It is owned by the Department for Energy Security and Net Zero.
In December 2024, it was announced that the Laboratory would be renamed
the United Kingdom National Nuclear Laboratory,
and would use the Government’s Royal Coat of Arms.
Its stated mission is to deliver nuclear outcomes for the government,
and to support growth of the UK nuclear sector.
natural fission reactor
A nuclear fission reactor formed naturally, without human intervention.
There are several instances of such a reactors, all at Oklo, Gabon.
They were formed about 1.7 billion years ago (before the evolution of multi-cellular life).
At that time the proportion of uranium-235 in natural uranium was over 3%,
comparable to low-enriched uranium used in modern reactors.
Natural groundwater acted as a moderator.
A subatomic particle that carries no electrical charge.
It is a component of every atomic nucleus except hydrogen-1.
It is composed of one “up” quark and two “down” quarks.
neutron capture
A process in which a neutron is absorbed by an atomic nucleus.
It is frequently followed by a beta emission, which causes
elemental transmutation.
This is the major mechanism by which elements heavier than iron-56 are produced in
stellar nucleosynthesis.
Neutron capture can be “rapid” (r-process), when the nucleus captures many neutrons before it decays
by β- emission; or “slow” (s-process), when the nucleus decays before capturing
further neutrons. The r-process occurs when the neutron flux is very high,
as in neutron star mergers.
A neutron-absorbing substance in a nuclear reactor that slows down
the chain reaction. Sometimes they are introduced intentionally. and sometimes they are
an unavoidable side-effect of the nuclear reaction. In a field already blighted by negative connotations,
the word poison is unfortunate.
neutron star
The residual object left over after a supernova.
It is composed of degenerate matter in which all the atoms have collapsed into neutrons.
The entire mass is condensed into a few kilometres, but the star’s original angular momentum is conserved
so the neutron star rotates very rapidly, typically hundreds of rotations per second.
Beams of electromagnetic radiation (light, radio, X-ray, gamma-ray) can emerge from
the magnetic poles of the star, which are not necessarily aligned with the axis of rotation.
They sweep through space like a lighthouse, and are observed as pulsars.
neutron star merger
A collision between two neutron stars, which results in gravitational waves,
an intense magnetic field, short gamma ray bursts,
and an intense flux of fast neutrons.
The merger may also cause ultra-high-energy cosmic rays.
The event is also known as a kilonova.
As singleton neutron stars are unlikely to meet and collide, a merger can only occur when
the original pre-supernova stars are in a stellar binary system.
The high neutron flux triggers the nucleosynthesis of most of the heavy elements,
including thorium, uranium, and plutonium, by neutron capture.
newton
The SI unit of force. It is the force required to accelerate a mass of one kilogram at a rate of
one metre per second per second.
A nuclear-related event rated 4 to 7 on the International Nuclear and Radiological Event
Scale (INES).
Accidents in nuclear power stations can never result in a nuclear explosion, because
the enrichment of the fuel is insufficient to permit a supercritical chain reaction,
which can only be produced under very precise millisecond timing conditions that are not present in a reactor.
A non-departmental public body responsible for decommissioning of Britain’s closed-down
nuclear sites. Their mission statement is:
“We’re charged, on behalf of government, with the mission to clean-up the UK’s
earliest nuclear sites safely, securely and cost effectively”.
nuclear energy
Energy derived from the fission or fusion of atomic nuclei.
The energy is generated by the conversion of some of the mass of nucleons by
releasing their binding energy.
The energy released is determined by the mass-energy equivalence rule (E=mc²).
A process in which heavy atomic nuclei are split apart to release
their binding energy.
nuclear fusion
A process in which light atomic nuclei are merged to release
their binding energy.
The colliding nuclei must have enough energy to overcome the electrostatic forces that
normally make them repel one another, but if they get “close enough”,
quantum tunnelling allows the fusion to proceed.
An establishment which generates electrical power from nuclear energy
The station contains: a nuclear reactor, which generates steam from nuclear energy;
a steam turbine, which converts the steam energy to rotational energy; and an electrical generator,
which converts the rotational energy to electrical power.
nuclear power stations in the UK
The following nuclear power stations previously or currently operated in the UK,
or are planned for the future.
They are all on mainland Great Britain or Anglesey, as there have never been any
nuclear stations in Northern Ireland (or in the Republic of Ireland).
Radioactive material left over after being used in a nuclear environment.
There are usually considered to be three levels of waste.
The following descriptions incorporate definitions from
the Nuclear Decommissioning Authority.
low-level waste
Protective clothing, packaging material, etc, that has been contaminated
with radioactive substances. It is defined as having a radioactive content not
exceeding 4 gigabecquerels per tonne
of alpha activity or 12 GBq/t
of beta or gamma activity.
This waste is disposed at near-surface facilities at the
Low Level Waste Repository (LLWR) at Drigg, Cumbria.
intermediate-level waste
Contaminated material with a higher radioactive content than low-level waste,
but which does not generate enough decay heat to require special handling.
It is treated by packaging in suitable containers, and stabilized with cement.
The fission device is often known as an atomic bomb. It explodes when the fissile material
within it achieves a supercritical chain reaction.
The chain reaction is initiated by a neutron generator of beryllium and polonium.
The fuel must be extremely pure weapons grade material, and it must be brought to a critical mass
by conventional explosives within a sub-second interval, otherwise the fuel just melts,
and does not explode.
The fusion device is also known as a thermonuclear weapon or hydrogen bomb.
The nuclear fuel originates as lithium deuteride (6Li2H),
which is decomposed to helium, tritium, and deuterium
by an intense neutron flux from an inbuilt fission bomb.
The deuterium and tritium then fuse to release their binding energy within a sub-microsecond interval.
The weapons reactions can only occur under carefully engineered conditions using weapons grade fuel.
Nuclear explosions cannot occur inside nuclear reactors.
The process within stars by which new elements are produced. The seminal B²FH paper described
how this could occur by nuclear fusion in stars and by neutron capture
in supernova explosions.
More recent theories postulate that most of the heavier elements are produced by
colliding or merging neutron stars.
This includes the radioactive nuclear fuels, thorium and uranium.
Plutonium could also be produced, but because of its relatively short half-life
(up to 80 million years for plutonium-244,
compared to 4.5 billion years for uranium-238),
any primordial plutonium produced in the solar system will have decayed away.
See also stellar evolution.
Periodic Table of the elements colour-coded to show probable element origins.
A category of substance characterised by the nuclear state of its atomic nuclei.
It has a similar meaning to isotope, but emphasises the nuclear properties
of the substance over its chemical properties.
(This distinction is not observed in this glossary.)
Oklo
A region near Franceville in Gabon, Central Africa.
It is the site of several natural fission reactors.
It is also the name of a California-based nuclear technology startup company specializing
in nuclear microreactors, which develops the Aurora Powerhouse,
a fast neutron reactor using HALEU fuel.
particle accelerator
A device used to accelerate charged particles such as electrons or ions to a high speed,
and therefore to a high energy.
An electrostatic accelerator uses a fixed high-voltage electrode to attract or repel the particles being accelerated.
Designs include the Cockcroft–Walton multiplier and
the van de Graaff generator.
A tandem generator changes the polarity of the accelerated particles in mid-flight,
so that they are first attracted and then repelled.
An electromagnetic particle accelerator uses a magnetic field to accelerate the particles.
A linear accelerator accelerates the particles in a straight line.
A cyclotron accelerates the particles in a spiral.
A synchrotron accelerates the particles in a circular path.
periodic table
A list of all the chemical elements arranged in order of atomic number.
Elements with similar chemical properties occur periodically, so can be arranged in columns to produce a grid.
See the chart under nucleosynthesis.
An obsolete name for a nuclear reactor.
The first self-sustaining chain reaction was initiated in the Chicago Pile-1 reactor
on 2 December 1942, as part of the Manhattan Project.
The first reactor in Western Europe was named the Graphite Low Energy Experimental Pile (GLEEP),
built at Harwell in 1947.
pitchblende
An old name for uraninite, the major ore of uranium.
Madame Curie famously processed tons of this ore to extract a decigram of radium chloride.
plasma
A fourth state of matter consisting of highly charged particles such as ions and electrons.
Plasmas are the natural state of matter inside stars, and occur on Earth as lightning and aurorae.
A high temperature plasma is used to generate conditions required for nuclear fusion.
An isotope of plutonium whose nucleus contains 238 nucleons,
of which 94 are protons and 144 are neutrons.
It is radioactive, and decays to uranium-234 with a half-life of 87.7 years.
The heat generated by this decay enables it to be used as a long-lasting heat source for a
radioisotope thermoelectric generator.
It is produced by the neutron irradiation of neptunium-237 recovered from spent nuclear fuel.
plutonium-239
An isotope of plutonium whose nucleus contains 239 nucleons,
of which 94 are protons and 145 are neutrons.
It is radioactive, and decays to uranium-235 with a half-life of 24,110 years.
It is also fissile, so can be used in a nuclear reactor.
Plutonium-239 is manufactured by bombarding uranium-238 with neutrons, which converts it to
uranium-239, which decays to plutonium-239 by two beta emissions.
The plutonium is chemically separated from the uranium by the PUREX process.
plutonium-244
The isotope of plutonium that has the longest half-life of 80 million years.
Because of this, it is feasible that a small amount of primordial plutonium-244
might still exist on Earth, but attempts to discover it are inconclusive.
positron
An antimatter version of an electron. Whereas an electron has a negative charge, a positron
has a positive charge. Positrons can be emitted from a nucleus as beta radiation,
labelled as β+ radiation.
potassium-40
A naturally occurring primordialisotope of potassium,
whose nucleus contains 40 nucleons, of which 19 are protons and 21 are neutrons.
Potassium-40 is radioactive, with a half-life of 1.25 billion years.
It is the largest source of radioactivity in the human body,
at a rate of about 4000 becquerel.
Bananas are high in potassium, so contain potassium-40. This has led to radiation exposure being humorously
measured in units of “banana equivalent dose” equal to 0.1 microsievert.
Potassium-40 decays into calcium-40 (β- decay)
or argon-40 (β+ decay or electron capture).
Nearly all of the argon in Earth’s atmosphere has been produced from the electron capture decay.
It is possible to determine the ages of potassium-containing minerals by the ratio of argon-40 to potassium-40.
The heat caused by the decay of potassium-40 contributes to the radiogenic heat
in the Earth’s crust.
power
The rate at which energy is transferred. Since energy is conserved, it can only be converted
from one form to another, and power is a measure of rate at which this transference occurs.
Power is measured in watts, defined as joules per second.
pressurized water reactor
A light water reactor in which the reactor coolant is
not allowed to boil, but remains under high pressure.
The high-temperature high-pressure water passes through a heat exchanger or steam generator
to produce steam in a different water circuit.
Existing prior to the formation of the solar system.
For hydrogen and helium, existing immediately after the Big Bang, and prior to the formation of stars.
proton
A subatomic particle that carries a positive electrical charge.
It is a component of every atomic nucleus.
It is composed of two “up” quarks and one “down” quark.
From the Greek proton (πρῶτον), meaning “first”.
In 1923, it was proposed than the proton should be named the prouton in honour of William Prout,
who noted in 1815 that the atomic weight of each element is an
integer multiple of that of hydrogen, so the hydrogen atom was a fundamental unit
[Prout’s hypothesis]. (The proposal was never enacted.)
PUREX
Plutonium uranium reduction extraction.
A chemical method of recovering plutonium and uranium from spent nuclear fuel.
The spent fuel is dissolved in nitric acid, and the plutonium and uranium salts are extracted
using tributyl phosphate dissolved in kerosene.
The plutonium is then separated from the uranium using reducing chemicals such as hydrazine or hydroxylamine.
The solution remaining after the extraction of plutonium and uranium is known as
a raffinate, whose volume is reduced in an evaporator
prior to being stabilized in a vitrification process that incorporates it into glass.
A quantum mechanical effect in which an object passes through a potential energy barrier
that should not be passable according to classical mechanics because the object has insufficient energy.
Because of the probabilistic nature of quantum mechanics, there is a small but non-zero probability
of the object being on the other side of the barrier, which then sometimes occurs.
quark
A fundamental particle from which most matter is composed.
Nucleons in the atomic nucleus are composed of three quarks.
Quarks have a quantum property known as isospin, which has the state of “up” or “down”.
An “up” quark has an electrical charge of +⅔,
and a “down” quark has an electrical charge of -⅓.
A neutron contains one “up” quark and two “down” quarks, so is electrically neutral.
A proton contains two “up” quarks and one “down” quark, so has a net electrical charge of +1.
(The name is derived from a line in Finnegans Wake: “Three quarks for Muster Mark!”.)
rad
Roentgen absorbed dose. An obsolete unit of ionizing radiation dose,
originally defined as 100 ergs per gram of absorbing material. It is now defined as 0.01 gray.
radioactivity
The spontaneous decay of a substance by the emission of subatomic particles
(alpha or beta radiation)
or electromagnetic radiation (gamma rays.
The heat is responsible for the melting of magma in the crust, and for natural hot springs,
geysers, and fumaroles.
It can be exploited as geothermal energy.
radioisotope thermoelectric generator
A device in which the decay heat of a radioactive source is used to
create electricity directly, using a thermocouple.
Such devices are used to power deep-space vehicles, including Voyager 1, Voyager 2, Cassini, and New Horizons.
They are also used on Mars lander vehicles such as Curiosity and Perseverance.
A red hot pellet of plutonium-238 dioxide ready for use in a radioisotope thermoelectric generator.
A radioactive gas that is produced as an indirect decay product of uranium-238.
Radon has 39 different isotopes, but only radon-222 has a significant half-life (3.8 days).
It is especially prevalent in areas that are rich in granite rock,
which can contain 5 to 20 parts per million of uranium.
In the United Kingdom these are mainly in Cornwall and Wales.
Radon is claimed to be a major cause of lung cancer, but Cancer Research UK states that only 5% of lung cancer
cases are related to radon, and that such cases are caused by the combination of radon and smoking.
raffinate
After a chemical separation, the product remaining after valuable components have been removed.
In nuclear reprocessing, the substances other than uranium and plutonium that remain
after the PUREX extraction.
rem
Roentgen equivalent man.
An obsolete unit measuring the “effective dose” of ionizing radiation.
It is now defined as 0.01 sievert.
Rutherford Appleton Laboratory
Originally the Rutherford High Energy Laboratory, which was established near Harwell in 1957 to operate
a 50MeV proton linear accelerator.
In 1975, it absorbed the Atlas Computer Laboratory, home of one of the Ferranti Atlas computers,
which were the first computers to implement virtual storage.
In 1979, Rutherford Atlas Laboratory absorbed the Appleton Laboratory – at the time the centre of
British space and RADAR research – to become the Rutherford Appleton Laboratory.
The laboratory now houses the ISIS Neutron and Muon Source, the Central Laser Facility, and
the Diamond Light Source synchrotron.
scram
The rapid emergency shutdown of a nuclear reactor.
Allegedly coined by Enrico Fermi as “Safety Cut Rope Axe Man” for an
axeman who would chop a rope suspending the cadmium control rod above
Chicago Pile-1, allowing it to drop rapidly and quench the chain reaction.
But it is more likely to be a backronym.
Sellafield
Location of the multi-purpose British nuclear site, part of which was once known as Windscale.
The settlement had been known as Sellafield since ancient times, and Sellafield railway station was opened in 1850.
The Royal Ordnance Factory Sellafield was established there in 1942.
Parts of the area were known as Windscale from the 1950s while nuclear facilities were being developed
by UKAEA, but the name reverted to Sellafield in 1981.
The Thermal Oxide Reprocessing Plant was constructed at Sellafield
between 1981 and 1988.
A unit measuring the “effective dose” or health risk of ionizing radiation.
It is meant to measure the biological effect of radiation, but it is problematic
as it incorporates the disputed LNT model of radiation tissue damage.
It also uses somewhat arbitrary weighting factors for allocating the dose,
depending on the type of radiation
(e.g. alpha, beta, or gamma radiation)
and the affected organ.
Sizewell B
A British nuclear power station owned by EDF, at Leiston, Suffolk.
It contains Britain’s only pressurized water reactor,
with a nameplate capacity of 1250 MWe.
It began generating in 1995 and is expected to close in 2035.
The site is surrounded by Sizewell Marshes, a Site of Special Scientific Interest,
and the Suffolk Coast and Heaths Area of Outstanding Natural Beauty.
Sizewell C
A British nuclear power station being planned at Leiston, Suffolk,
adjacent to Sizewell B.
It will consist of two EPR reactors of 1630 MWe each.
A nuclear site licence for the project was granted in May 2024, and funding from the
UK Government was committed in the 2024 Autumn Budget.
The design of Sizewell C is very similar to that of
Hinkley Point C, which should lower the costs of developing it.
SL-1
Stationary Low-Power Reactor Number One.
An American military nuclear reactor near Idaho Falls, Idaho,
which sustained an accident on 3 January 1961.
The accident was caused by the inadvertent withdrawal of a control rod
causing a meltdown and explosion. Three military personnel were killed.
The accident led to changes in reactor design so that removal of a single control rod
could not produce a meltdown.
small modular reactor
A small nuclear reactor made from factory-built parts on a production line, rather than constructed
on site using traditional building materials.
The proposed output power for such reactors is up to 300 MWe.
A prototype sodium-cooled reactor (PFR) operated at Dounreay between 1975 and 1994.
stellar evolution
The process by which a star changes its composition during its lifetime.
The process is mostly driven by nuclear fusion.
Gravitational pressure forces lighter nuclei to fuse together,
Once the star has fused to mainly iron-56, fusion cannot produce further energy.
If the star’s mass exceeds about eight times that of the Sun, the iron-rich star explodes
as a supernova, in which new elements heavier than iron are produced.
(Smaller stars ultimately collapse into a white dwarf.)
After the supernova, the remains of the star form a neutron star.
If the exploding star is part of a binary system, resulting in a double supernova, the two
resulting neutron stars will eventually collide in a neutron star merger,
which causes a high neutron-flux event in which most of the heaviest elements are formed
by neutron capture.
See also nucleosynthesis.
subatomic particle
One of the three types of particle that make up an atom.
These are protons, neutrons, and electrons.
Modern nuclear theory proposes that protons and neutrons are themselves composite particles,
made up of quarks.
An alpha particle might also be considered to behave as a subatomic particle,
as it is emitted in its entirety during alpha radiation.
supercritical water cooled reactor
A proposed generation IV nuclear reactor in which the working fluid
is supercritical water, which is a phase of water in which liquid and vapour co-exist,
at a temperature above 374°C and a pressure above 218 atmospheres (22 MPa).
The efficiency of such a reactor could be as high as 44%.
supernova
A massive explosion of a star after it is unable to sustain nuclear fusion reactions.
After the explosion, the remaining stellar material condenses into a neutron star.
A high-definition image from the James Webb Space Telescope’s
Near-Infrared Camera, showing details of supernova remnant Cassiopeia A.
synchrotron
A particle accelerator in which charged particles are accelerated within a
varying magnetic field that is synchronized with the kinetic energy of the particles being accelerated.
The synchrotron evolved from the cyclotron, in which the magnetic field remains constant.
The largest synchrotron is the Large Hadron Collider at CERN, which has a diameter of 8.6km.
The largest synchrotron in the United States was the Tevatron at Fermilab, which had a diameter of 2km.
It closed in 2011.
The Diamond Light Source, at the Rutherford Appleton Laboratory,
is a 179m diameter synchrotron designed to produce synchrotron radiation.
thermal neutron
A low speed neutron whose effective temperature is near to “room temperature”,
or about 20°C. Distinguished from a fast neutron.
Thermal neutrons are more easily absorbed by fissile material, allowing the use
of lower enriched fuel.
Thermal Oxide Reprocessing Plant
A nuclear fuel reprocessing plant constructed between 1981 and 1994 at Sellafield.
Reprocessing ceased in 2018, but it is still being used to store used nuclear fuel.
Storage pond inside the Thermal Oxide Reprocessing Plant, used to store
used nuclear fuel from the UK’s remaining operational nuclear reactors.
A naturally occurring primordialradioactive element.
Its most abundant isotope is thorium-232, whose nucleus contains 232 nucleons,
of which 90 are protons and 142 are neutrons.
This is radioactive, and decays by alpha decay to radium-228 with a half-life
of 14 billion years.
It is fertile rather than fissile, as it can be converted by neutron capture
to thorium-233, which subsequently undergoes two successive beta decays to form uranium-233, which is fissile.
Through most of the era of domestic gas lighting, the brightness of the light was enhanced with
a mantle made of thorium dioxide.
thorium fuel cycle
The process of obtaining nuclear energy from thorium.
Because natural thorium-232 is not fissile, it first has to be converted into fissile uranium-233.
The process involves neutron capture to produce thorium-233,
which then decays through two beta emissions to produce uranium-233.
The site of two pressurized water reactors
located near Harrisburg, Pennsylvania, USA.
It is most famous because the Unit 2 reactor suffered a partial meltdown on 28 March 1979.
Although some radioactive material was released, official reports concluded that no
adverse health effects or deaths were caused by the accident.
Unit 1 continued to operate until its closure on 20 September 2019.
In September 2024, plans were announced to reopen the plant,
with all of its output to be consumed by the Microsoft Corporation.
A device used to contain plasma, using strong magnetic fields,
as part of a nuclear fusion reaction.
Inside the reaction chamber of the DIII-D, an experimental
tokamak fusion reactor operated by General Atomics in San Diego.
Torness
A British nuclear power station owned by EDF, at Dunbar, East Lothian.
The site contains two AGR reactors cooled by carbon dioxide,
with a combined capacity of 1290 MWe.
It began generating in 1988 and is expected to close in 2030.
transmutation
The transformation of a substance from one element into another one.
This involves a change in the atomic number of the element by changing the
number of protons in its nucleus.
Medieval alchemists tried to transmute elements using chemistry, but this affects only
the electrons in the atoms of the substance, so was doomed to failure.
Some definitions of transmutation include the transformation from one isotope
to another of the same element, by changing only the number of neutrons.
The 1951 Nobel Prize for Physics was awarded to Sir John Cockcroft and Ernest Walton
“for their pioneer work on the transmutation of atomic nuclei by artificially
accelerated atomic particles”.
Tristructural isotropic nuclear fuel. It is a fuel consisting of tiny manufactured pellets,
each containing a uranium-bearing kernel encapsulated in a ceramic shell, which prevents the
release of radioactive fission products.
The diameter of each pellet is about 0.8mm, described in the literature as “poppy-seed” sized.
The kernel consists of a mixture of uranium carbide and uranium oxide.
The pellets can resist temperatures up to 1800°C.
TRISO particles rendered image.
tritium
A familiar name for hydrogen-3.
It is slightly radioactive, decaying to stable helium-3 by the emission of
a beta particle (electron), with a half-life of about twelve years.
But there is little evidence that this low level of radioactivity causes
any biological effect whatsoever.
It is produced in small quantities by most nuclear reactors, where it is oxidized
to tritiated water, and enters the cooling water.
Water in which some of the hydrogen-1 atoms have been
replaced by tritium.
The most common form only contains one atom of tritium per molecule of water (¹H³HO).
Although the radiological half-life of the tritium in the water is twelve years,
the biological half-life of the tritium in the water, if it is ingested, is seven to fourteen days,
due to excretion.
The contaminated cooling water from the Fukushima nuclear accident contained dilute
tritiated water after all the other radioactive contaminants were removed.
Since tritium is chemically identical to hydrogen-1, it is almost impossible
to separate the tritium from the remaining water.
After processing, the radioactivity due to tritium was reduced
to 148,900 becquerel per litre, which was then diluted
to 1,500 Bq/L before being released into the ocean over a thirty year period.
This is considerably less than the tritium limit for drinking water (10,000 Bq/L)
recommended by the World Health Organization.
Originally the authority responsible for the United Kingdom’s entire nuclear programme, both civil and military.
It was established on 19 July 1954.
In 1971, fuel manufacturing was divested into British Nuclear Fuels Ltd.
Medical isotope production was moved to The Radiochemical Centre Ltd,
(which later became Amersham International, and was then absorbed into GE HealthCare).
In 1973, the Atomic Weapons Research Establishment arm of UKAEA was moved to the Ministry of Defence.
In 1995, parts of the remaining Authority were privatized into AEA Technology plc,
which later became Ricardo Energy & Environment after divesting all its nuclear-related business.
UKAEA persists, but is now only responsible for the development of fusion energy.
It owns the Culham Science Centre, and was responsible for JET until
its closure in 2023.
United Nations Scientific Committee on the Effects of Atomic Radiation
A committee established by the United Nations General Assembly in 1955.
Its mandate is to assess and report levels and effects of exposure
to ionizing radiation.
They hold annual meetings, and produce major reports on the effect of radiation.
They have produced reports on the effects of the Chernobyl
and Fukushima nuclear accidents.
The major ore of uranium, also known as pitchblende.
The uranium is present as uranium dioxide (UO2).
It is usually converted on-site at the mine into yellowcake.
Major deposits of uraninite are found in Australia, Kazakhstan, Canada, and Russia.
Uranium is named after the planet Uranus, discovered in 1781, but it was also the
name of a primitive planetarium constructed in 1758 by the astronomer Roger Long,
at Pembroke College, Cambridge.
uranium-233
An isotope of uranium whose nucleus contains 233 nucleons,
of which 92 are protons and 141 are neutrons.
It does not occur naturally, but is bred from thorium-232 in the thorium fuel cycle.
It is fissile.
It is radioactive, and decays to thorium-229 with
a half-life of 160,000 years.
uranium-235
An isotope of uranium whose nucleus contains 235 nucleons,
of which 92 are protons and 143 are neutrons.
It is fissile.
It is radioactive, and decays to thorium-231 with
a half-life of 704 million years.
uranium-238
An isotope of uranium whose nucleus contains 238 nucleons,
of which 92 are protons and 146 are neutrons.
It is fertile, as it can be converted to fissile plutonium-239.
It is radioactive, and decays to thorium-234 with
a half-life of 4.5 billion years.
uranium hexafluoride
A compound of uranium and fluorine (UF6), used in the enrichment
of uranium, and known in the industry as hex.
It is manufactured by treating yellowcake with fluorine gas.
It is a volatile solid at room temperature, and sublimes to a gas at about 56.5°C.
The gaseous form is spun in high speed centrifuges to separate the uranium-235 compound from
the uranium-238 compound.
Van de Graaff generator
A mechanical device for generating a high static electrical voltage.
An electrical change is carried on an insulated belt to the inside of a central domed electrode.
The charge accumulates on the outside of the electrode due to the Faraday cage effect (Gauss’s law).
In industrial applications, the insulated belt may be replaced by a peletron or laddertron,
a chain of metal pellets or “rungs”.
The high voltage can be used to drive an electrostatic particle accelerator.
very high temperature reactor
A gas-cooled high temperature nuclear reactor, working at temperatures above 850°C.
It is proposed as a future generation IV reactor.
It will be a helium-cooled SMR, with a graphitemoderator,
and will use TRISO particles as fuel.
The basic unit of power.
It is a measure of the rate of flow of energy,
and is defined a one joule per second.
It was originally defined as an electrical unit, as the power produced when a current of one
ampere flows across a potential difference of one volt.
Now that definition is reversed, and the volt is defined as the electrical potential required
to produce one watt of power from a current of one ampere.
A quality of an enriched nuclear fuel that is suitable for use in
a nuclear weapon.
In practice, this means uranium enriched to 85% uranium-235 (HEU),
or plutonium enriched to 93% of plutonium-239.
Wigner effect
A disruption of the crystal structure of a solid due to neutron radiation.
It can lead to a build-up of energy within a solid neutron moderator
such as graphite.
Windscale
Location of the multi-purpose site of the first British nuclear establishments.
It was originally the site of the Royal Ordnance Factory Sellafield.
The nuclear site on the seaward side of the River Calder, comprising the 1950s Windscale Piles,
the 1962 Windscale Advanced Gas-cooled Reactor (WAGR),
and the early reprocessing plants and ponds, was named after Windscale Nook on the bank of the River Calder.
Calder Hall, built on the inland side of the river, was never regarded as part of Windscale,
so the total site became known as “Windscale and Calder”.
When BNFL was split from UKAEA
in 1971, all the plants managed by BNFL, including Calder Hall, were designated as Sellafield.
The Piles and the WAGR, still owned by UKAEA, remained in Windscale.
Windscale Piles
Two graphite-moderated air-cooled nuclear reactors at Windscale.
(A pile was the old name for a nuclear reactor.)
Unlike Calder Hall, they were used exclusively for the production of plutonium.
On 10 October 1957, the graphite moderator on Unit 1 caught fire due to excessive heat created by the
Wigner effect. Radioactive iodine-131 and polonium-210 were released.
Milk contaminated with iodine-131 in surrounding dairy farms had to be destroyed.
Winfrith
A site in Dorset set up in 1957 by
the United Kingdom Atomic Energy Authority
to develop a range of experimental nuclear reactors.
Designs developed there included: the Dragon Reactor, the first demonstration
high-temperature gas-cooled reactor, which operated between 1964 and 1976;
and the Winfrith Reactor, a prototype steam-generating heavy water reactor,
which operated between 1967 and 1990.
The last operational reactor at Winfrith closed in 1995.
World Association of Nuclear Operators
An international non-profit organization of nuclear power station operators.
It was set up in 1989 in response to the Chernobyl nuclear accident.
Its objective is to accomplish the highest levels of operational safety and reliability.
work
In thermodynamics, the energy transferred to or from an object
when it is displaced by a force.
Strictly, it is the scalar product of the force and displacement vectors.
yellowcake
A powdered uranium concentrate, obtained by the acid leaching of uranium ore).
Its major component is triuranium octoxide (238U₃O₈).
Clumps of yellowcake reprocessed uranium ore.
ZETA
Zero Energy Thermonuclear Assembly. A prototype nuclear fusion reactor that was developed
at Harwell in 1957, but, in spite of early optimism, never actually achieved fusion.
The left side of the ZETA reactor as seen prior to its public demonstration in January 1958.
