What is Uranium? How Does it Work?
- Uranium is a very heavy metal which can be used as
an abundant source of concentrated
- Uranium occurs in most rocks in
concentrations of 2 to 4 parts per million and is as common in the
Earth's crust as tin, tungsten and molybdenum. Uranium occurs in
seawater, and can be recovered from the oceans.
- Uranium was discovered in 1789 by
Martin Klaproth, a German chemist, in the mineral called
pitchblende. It was named after the planet Uranus, which had been
discovered eight years earlier.
- Uranium was apparently formed in
supernova about 6.6 billion years ago. While it is
not common in the solar system, today its slow radioactive decay
provides the main source of heat inside the
Earth, causing convection and continental
- The high density of uranium means
that it also finds uses in the keels of yachts and as
counterweights for aircraft control surfaces, as well as for
- Uranium has a melting point is 1132°C.
The chemical symbol for uranium is
The Uranium Atom
On a scale arranged according to the increasing mass of their
nuclei, uranium is one of the heaviest of all the
naturally-occurring elements (Hydrogen is the lightest). Uranium is
18.7 times as dense as water.
Like other elements, uranium occurs in several slightly
differing forms known as 'isotopes'. These isotopes differ from
each other in the number of uncharged particles (neutrons) in the
nucleus. Natural uranium as found in the Earth's crust is a mixture
largely of two isotopes: uranium-238 (U-238), accounting for 99.3%
and uranium-235 (U-235) about 0.7%.
The isotope U-235 is important because under certain conditions
it can readily be split, yielding a lot of energy. It is therefore
said to be 'fissile' and we use the expression 'nuclear
Meanwhile, like all radioactive isotopes, they decay. U-238
decays very slowly, its half-life being about the same as the age
of the Earth (4500 million years). This means that it is barely
radioactive, less so than many other isotopes in rocks and sand.
Nevertheless it generates 0.1 watts/tonne as decay heat and this is
enough to warm the Earth's core. U-235 decays slightly faster.
Energy from the uranium atom
The nucleus of the U-235 atom comprises 92 protons and 143
neutrons (92 + 143 = 235). When the nucleus of a U-235 atom
captures a moving neutron it splits in two (fissions) and releases
some energy in the form of heat, also two or three additional
neutrons are thrown off. If enough of these expelled neutrons cause
the nuclei of other U-235 atoms to split, releasing further
neutrons, a fission 'chain reaction' can be achieved. When this
happens over and over again, many millions of times, a very large
amount of heat is produced from a relatively small amount of
It is this
process, in effect "burning" uranium, which occurs in a nuclear
reactor. The heat is used to make steam to produce electricity.
Inside the reactor
Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas.In a nuclear reactor the uranium fuel is assembled in such a way
that a controlled fission chain reaction can be achieved. The heat
created by splitting the U-235 atoms is then used to make steam
which spins a turbine to drive a generator, producing
The chain reaction that takes place in the core of a nuclear
reactor is controlled by rods which absorb neutrons and which can
be inserted or withdrawn to set the reactor at the required power
The fuel elements are surrounded by a substance called a
moderator to slow the speed of the emitted neutrons and thus enable
the chain reaction to continue. Water, graphite and heavy water are
used as moderators in different types of reactors.
Because of the kind of fuel used (ie the concentration of U-235,
see below), if there is a major uncorrected malfunction in a
reactor the fuel may overheat and melt, but it cannot explode like
A typical 1000 megawatt (MWe) reactor can provide enough
electricity for a modern city of up to one million people.
Uranium and Plutonium
Whereas the U-235 nucleus is 'fissile', that of U-238 is said to
be 'fertile'. This means that it can capture one of the neutrons
which are flying about in the core of the reactor and become
(indirectly) plutonium-239, which is fissile. Pu-239 is very much
like U-235, in that it fissions when hit by a neutron and this also
yields a lot of energy.
Because there is so much U-238 in a reactor core (most of the
fuel), these reactions occur frequently, and in fact about one
third of the fuel's energy yield comes from "burning" Pu-239.
But sometimes a Pu-239 atom simply captures a neutron without
splitting, and it becomes Pu-240. Because the Pu-239 is either
progressively "burned" or becomes Pu-240, the longer the fuel stays
in the reactor the more Pu-240 is in it. (The significance of
this is that when the spent fuel is removed after about three
years, the plutonium in it is not suitable for making weapons but
can be recycled as fuel.)
From uranium ore to reactor fuel
Uranium ore can be mined by underground or open-cut methods,
depending on its depth. After mining, the ore is crushed and ground
up. Then it is treated with acid to dissolve the uranium, which is
recovered from solution.
Uranium may also be mined by in situ leaching (ISL), where it is
dissolved from a porous underground ore body in situ and pumped to
The end product of the mining and milling stages, or of ISL, is
uranium oxide concentrate (U3O8). This is the
form in which uranium is sold.
Before it can be used in a reactor for electricity generation,
however, it must undergo a series of processes to produce a useable
For most of the world's reactors, the next step in making the
fuel is to convert the uranium oxide into a gas, uranium
hexafluoride (UF6), which enables it to be enriched.
Enrichment increases the proportion of the uranium-235 isotope from
its natural level of 0.7% to 4 - 5%. This enables greater technical
efficiency in reactor design and operation, particularly in larger
reactors, and allows the use of ordinary water as a moderator.
After enrichment, the UF6 gas is converted to
uranium dioxide (UO2) which is formed into fuel pellets.
These fuel pellets are placed inside thin metal tubes which are
assembled in bundles to become the fuel elements or assemblies for
the core of the reactor.
For reactors which use natural uranium as their fuel (and hence
which require graphite or heavy water as a moderator) the
U3O8 concentrate simply needs to be
refined and converted directly to uranium dioxide.
When the uranium fuel has been in the reactor for about three
years, the used fuel is removed, stored, and then either
reprocessed or disposed of underground (see Nuclear Fuel
Cycle or Radioactive
Waste Management in this series).
Who uses nuclear power?
Over 13% of the world's electricity is generated from uranium in
nuclear reactors. This amounts to over 2500 billion kWh each year,
as much as from all sources of electricity worldwide in 1960.
It comes from some 440 nuclear reactors with a total output
capacity of about 377 000 megawatts (MWe) operating in 30
countries. Over 60 more reactors are under construction and another
150 are planned.
Belgium, Bulgaria, Czech Republic, Finland, France, Hungary,
Japan, South Korea, Slovakia, Slovenia, Sweden, Switzerland and
Ukraine all get 30% or more of their electricity from nuclear
reactors. The USA has over 100 reactors operating, supplying 20% of
its electricity. France gets three quarters of its electricity from
See also Table of the World's Nuclear Power Reactors
Who has and who mines uranium?
Uranium is widespread in many rocks, and even in seawater.
However, like other metals, it is seldom sufficiently concentrated
to be economically recoverable. Where it is, we speak of an
orebody. In defining what is ore, assumptions are made about the
cost of mining and the market price of the metal. Uranium reserves
are therefore calculated as tonnes recoverable up to a certain
Australia's reasonably assured resources and inferred resources
of uranium are 1,673,000 tonnes of uranium recoverable at up to
US$130/kg U (well under the market 'spot' price), Kazakhstan's are
651,000 tonnes of uranium and Canada's are 485,000 tU. Australia's
resources in this category are almost one third of the world's
total, Kazakhstan's are 12%, Canada's 9%.
Several countries have significant uranium resources. Apart from
the top three, they are in order: Russia, South Africa, Namibia,
Brazil, Niger, USA, China, Jordan, Uzbekistan, Ukraine and India.
Other countries have smaller deposits which could be mined if
Kazakhstan is the world's top uranium producer, followed by Canada
and then Australia as the main suppliers of uranium to world
markets - now over 50,000 tU per year.
Uranium is sold only to countries which are signatories of the
Nuclear Non-Proliferation Treaty (NPT), and which allow
international inspection to verify that it is used only for
Other uses of nuclear energy
Many people, when talking about nuclear energy, have only
nuclear reactors (or perhaps nuclear weapons) in mind. Few people
realise the extent to which the use of radioisotopes has changed
our lives over the last few decades.
Using relatively small special-purpose nuclear reactors it is
possible to make a wide range of radioactive materials
(radioisotopes) at low cost. For this reason the use of
artificially-produced radioisotopes has become widespread since the
early 1950s, and there are now over 200 "research" reactors in 56
countries producing them. These are essentially neutron
factories rather than sources of heat.
In our daily life we need food, water and good health. Today,
radioactive isotopes play an important part in the technologies
that provide us with all three. They are produced by bombarding
small amounts of particular elements with neutrons.
In medicine, radioisotopes are widely used
for diagnosis and research. Radioactive chemical tracers emit gamma
radiation which provides diagnostic information about a person's
anatomy and the functioning of specific organs. Radiotherapy also
employs radioisotopes in the treatment of some illnesses, such as
cancer. More powerful gamma sources are used to sterilise syringes,
bandages and other medical equipment. About one person in two in
the western world is likely to experience the benefits of nuclear
medicine in their lifetime, and gamma sterilisation of equipment is
In the preservation of food,
radioisotopes are used to inhibit the sprouting of root crops after
harvesting, to kill parasites and pests, and to control the
ripening of stored fruit and vegetables. Irradiated foodstuffs are
accepted by world and national health authorities for human
consumption in an increasing number of countries. They include
potatoes, onions, dried and fresh fruits, grain and grain products,
poultry and some fish. Some prepacked foods can also be
In the growing of crops and
breeding livestock, radioisotopes also play
an important role. They are used to produce high yielding,
disease-resistant and weather-resistant varieties of crops, to
study how fertilisers and insecticides work, and to improve the
productivity and health of domestic animals.
Industrially, and in mining, they are
used to examine welds, to detect leaks, to study the rate of wear
of metals, and for on-stream analysis of a wide range of minerals
There are many other uses. A radioisotope derived from the
plutonium formed in nuclear reactors is used in most
Radioisotopes are used to detect and analyse pollutants in the
environment, and to study the movement of surface water in streams
and also of groundwater.
There are also other uses for reactors. About 200 small nuclear
reactors power some 150 ships, mostly submarines, but ranging from
icebreakers to aircraft carriers. These can stay at sea for long
periods without having to make refuelling stops. In the Russian
Arctic where operating conditions are beyond the capability of
conventional icebreakers, very powerful nuclear-powered vessels
operate almost year-round, where previously only two months could
be used each year.
The heat produced by nuclear reactors can also be used directly
rather than for generating electricity. In Sweden and Russia, for
example, it is used to heat buildings and to provide heat for a
variety of industrial processes such as water desalination. Nuclear
desalination is likely to be a major growth area in the next
High-temperature heat from nuclear reactors is likely to be
employed in some industrial processes in future, especially for
Both uranium and plutonium were used to make bombs before they
became important for making electricity and radioisotopes. The type
of uranium and plutonium for bombs is different from that in a
nuclear power plant. Bomb-grade uranium is highly-enriched (>90%
U-235, instead of up to 5%); bomb-grade plutonium is fairly pure
Pu-239 (>90%, instead of about 60% in
reactor-grade) and is made in special reactors.
Since the 1990s, due to disarmament, a lot of military uranium
has become available for electricity production. The military
uranium is diluted about 25:1 with depleted uranium (mostly U-238)
from the enrichment process before being used in power generation.
Military plutonium is starting to be used similarly, mixed
with depleted uranium.