What is nuclear power?

What is nuclear power?
Media briefing
November 2001
Nuclear reactors and reprocessing plants were first designed and created to
produce plutonium - for nuclear weapons. Electricity was simply a by-product.
The first nuclear power station in Britain was built at Calder Hall in Cumbria, in
1953. And when this was connected to the national grid in 1956, it became the
first nuclear power station in the world to provide electricity.
Today there are 15 nuclear power stations operating in the UK, generating a
quarter of the nation’s electricity. Many still use aged Magnox reactors identical
to, and including, the one built at Calder Hall. They were only designed for a
maximum of 20 to 25 years’ use, and yet the oldest stations are now more than
40 years old.
The UK government is beginning to talk about nuclear power as the energy
source of the future. The argument is that nuclear power stations produce less
carbon dioxide (the main contributor to global warming) sulphur dioxide and
oxides of nitrogen (responsible for acid rain) than fossil-fuelled stations.
Weigh this up against the severe long-term dangers to human health, the
environment, and global security - caused by the production, transport, storage
and reprocessing of highly radioactive nuclear materials - and the stupidity of
the argument becomes clear.
The nuclear reaction
Nuclear power involves the liberation of energy from an atom, by the process of
fission - the splitting of its nucleus into two or more parts.
Fission is initiated by bombarding nuclei with neutrons, causing them to fly apart
into two large fragments and to simultaneously release several free neutrons of
their own. These neutrons then cause other nearby atoms to fission, producing
even more neutrons, setting off a chain reaction. The two large fragments of the
split nucleus become new chemical elements - mostly highly radioactive -
including isotopesi such as iodine-131, caesium-137 and strontium-90.
Only a few isotopes of heavy elements lend themselves easily to nuclear fission,
notably uranium-235 and plutonium-239.
Nuclear reactors
In nuclear weapons, energy release is uncontrolled. Commercial nuclear power
stations control the release of heat energy, raising steam to produce electricity.
A thermal nuclear reactor is a device for sustaining a fission chain reaction.
Fission takes place within the reactor core, which is contained within a pressure
vessel and biological shield. Within the core is a moderator, usually made of
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graphite or water. The moderator acts to slow down the neutrons so that an
efficient chain reaction occurs.
Fuel rods containing fissile material are placed within this core, and control rods
are interspersed between them. These control rods are made of a material that
absorbs neutrons, thereby controlling or stopping the reaction. The control rods
are withdrawn to start the reaction and reinserted to shut it down. A coolant,
such as water or gas, passes through the reactor and conveys heat away from
the reactor core. Heat is then converted into electricity via a system of heat
exchangers and turbines, in the same way as in a fossil-fuelled station.
There are three different reactor designs used in the UK:
• Magnox reactors
Seven of the nuclear power stations in the UK contain Magnox reactors.
These were designed to produce plutonium for nuclear weapons, not for the
safest and most economic production of energy for civilian purposes. They
were also designed to run for only 20 to 25 years. But all the UK’s Magnox
reactors are now over 30 years old. The oldest station – at Calder Hall,
Sellafield – has been operating for 45 years.
Magnox reactors use natural uranium metal as fuel, have a graphic
moderator and use pressurised CO2 as coolant.
• Advanced gas-cooled reactors (AGR)
These are the successors to the Magnox reactors, also developed in the UK.
Seven were built between 1976 and 1988.
AGRs use enriched uranium clad in stainless steel cans, a graphite moderator
and pressurised CO2 as coolant. This allows them to operate at a higher
temperature than Magnox reactors.
Neither Magnox nor AGR reactors have the same level of safety features as
more recent stations. They would not be approved by safety regulators
today.
• Pressurised water reactors (PWR)
These were developed in the US, and are the most common reactors
throughout the world. The only PWR in the UK is at Sizewell in Suffolk – a
1188MW reactor that first supplied electricity to the grid in February 1995.
The PWR reactor is contained in a steel pressure vessel. Pressurised water,
acting as both moderator and coolant, is pumped around the reactor and
through the boilers. The pressure vessel, the boilers and connecting pipe-
work form a sealed primary pressurised circuit, which is contained within a
steel-lined pre-stressed concrete containment building, which also acts as a
biological shield. Safety depends on the integrity of the pressure vessel,
which can become brittle over time. An accident involving loss of coolant
water can have catastrophic consequences.
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The nuclear fuel cycle
• Uranium ore is first mined, crushed and ground. It is then leached to dissolve
the uranium, which is separated out and precipitated as a concentrate
containing 90 per cent, or more, oxides of uranium. This ‘yellowcake’ is then
refined to pure uranium oxide, which can be used in Magnox reactors. For
use in AGR and PWR reactors, however, the uranium must be enriched in
order to increase the proportion of uranium-235 it contains.
• The uranium is then converted into ceramic fuel pellets. These are packed
into stainless steel tubes for AGRs, to form fuel pins, or zirconium alloy tubes
for PWRs, to form fuel rods. The pins or rods are assembled into fuel
elements, then stacked to form a fuel assembly, which is loaded into a
reactor. AGRs typically contain 300 to 330 fuel assemblies in one reactor. A
fuel assembly will remain in an AGR reactor for between four to eight years,
or in a PWR reactor for three to five years.
• Spent fuel from a nuclear reactor is highly radioactive. It contains
approximately 96 per cent uranium and one per cent plutonium, the rest
consisting of highly radioactive isotopes. Once removed from the reactor, it is
stored – to cool and to allow some of the short-lived radioactive isotopes to
decay. The choice is then between long-term storage or, after a minimum of
three years for AGR fuel and five years for PWR fuel, reprocessing.
• Reprocessing involves the separation of uranium, plutonium and other high-
level waste (HLW), by dissolving the fuel in nitric acid. The resulting materials
are then stored and the reprocessed uranium can be recycled into new AGR
and PWR elements. Commercial reprocessing facilities exist at La Hague, in
France, and at BNFL’s Sellafield site. The process began as a way of
producing plutonium for nuclear weapons and for ‘fast-breeder’ reactors.
These fast-breeder reactors were never successfully developed and
reprocessing has been responsible for some of the world’s worst radioactive
pollution problems.
• Plutonium can then be combined with fresh uranium, to form mixed oxide
reactor fuel (MOX) – an expensive and dangerous process.
The UK nuclear power industryii
There are 11 Magnox stations in the UK, seven of which are still operating. All
remain in the public sector and are run by British Nuclear Fuels Ltd (BNFL) - a
government owned company, whose main activity is reprocessing spent nuclear
fuel from UK and abroad, at Sellafield in Cumbria.
There are a further eight nuclear power stations in the UK - seven AGRs and one
PWR. These are run by British Energy (BE), which was privatised in 1996. An
unattractive, uneconomic option, they were sold for £1.5 billion – less than the
cost of building the PWR at Sizewell that was completed just 12 months earlier.
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i The nucleus of every atom contains protons (positively charged) and neutrons (with no electrical charge)
surrounded by a cloud of negatively charged electrons. All atoms of the same chemical element have the same
number of protons. This is the atomic number of the element. Atoms of the same element can however have
different numbers of neutrons, and these are called isotopes of the element. The number of an isotope (also
called the mass number) refers to the total number of protons and neutrons in the nucleus. Uranium, for
example, has two common isotopes: uranium-235 (92 protons and 143 neutrons) and uranium-238 (92 protons
and 146 neutrons).
ii
Station
commissioning date
no. of reactors
capacity (MW) operator
Magnox
Calder Hall
1956
4
192
BNFL
(Cumbria)
Chapelcross
1959
4
196
BNFL
(Dumfries & Galloway)
Berkeley*
1962
2
276
BNFL
(Gloucestershire)
Bradwell**
1961
2
240
BNFL
(Essex)
Hunterston A*
1964
2
320
BNFL
(Ayrshire)
Trawsfynydd*
1965
2
390
BNFL
(N. Wales)
Sizewell A
1965
2
430
BNFL
(Suffolk)
Dungeness A
1965
2
445
BNFL
(Kent)
Hinkley Point A*
1964
2
475
BNFL
(Somerset)
Oldbury on Severn
1967
2
430
BNFL
(Gloucestershire)
Wylfa
1971
2
1050
BNFL
(Anglesey)
*Undergoing decommissioning
**To close in 2002
AGRs
Hinkley Point B
1976
2
1300
BE
(Somerset)
Hunterston B
1976
2
1150
BE
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(Ayrshire)
Dungeness B
1985
2
1104
BE
(Kent)
Hartlepool
1983
2
1237
BE
(Cleveland)
Heysham I
1984
2
1148
BE
(Lancashire)
Heysham II
1988
2
1320
BE
(Lancashire)
Torness
1988
2
1250
BE
(East Lothian)
PWRs
Sizewell B
1995
1
1220
BE
(Suffolk)
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Document Outline

• What is nuclear power?
• Media briefing
• November 2001
• Nuclear reactors and reprocessing plants were first designed and created to produce plutonium - for nuclear weapons. Electricity was simply a by-product. The first nuclear power station in Britain was built at Calder Hall in Cumbria, in 1953. And when th
• The nuclear reaction
• Nuclear reactors
• Magnox reactors
• Pressurised water reactors (PWR)
• The nuclear fuel cycle
• The UK nuclear power industry

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