128 / Basics of Environmental Science
Steam-driven turbines generate the electrical power and nuclear reactors produce the heat to turn
water into steam. The core of the reactor comprises a structure with vertical holes or channels, some
containing rods of fuel, others containing rods of cadmium or boron, all the rods being embedded in
a substance called a moderator.
The fuel consists of an isotope of uranium, uranium-235 (often written as^235 U), that occurs as one
part in 140 in natural uranium. When a slow-moving neutron collides and merges with the nucleus of
an atom of^235 U that nucleus splits into two with the release of two or three neutrons. This is fission.
If these neutrons also strike^235 U nuclei, the number of neutrons and nuclear fissions proliferate
exponentially. This is a chain reaction and as the particles come to rest much of their energy is
converted into heat.
At least one neutron from each fission must merge with a^235 U nucleus in order to sustain a chain
reaction. Only slow-moving neutrons are able to cause fission; more energetic neutrons are not
absorbed. Fission releases neutrons at many speeds and so fast-moving ones must be slowed. This is
the purpose of the moderator. Different reactor designs use different moderator materials. Graphite,
deuterium oxide (heavy water), and ordinary (light) water are widely used, light water being the
most popular of all.
Cadmium and boron absorb neutrons, removing them from the reaction process. This means rods
made from these elements can be used to regulate the speed of the chain reaction. The rods can be
raised or lowered, accelerating or slowing the power output.
Surrounding the core, a coolant carries away the heat. In the most popular reactor design, the
pressurized water reactor (PWR), the coolant is water under pressure. There are also designs that use
boiling water. Magnox reactors, of which eight were built in Britain, the first in 1956 at Calder Hall
(now Sellafield), Cumbria, use carbon dioxide as a coolant. The name ‘Magnox’ refers to the
magnesium oxide alloy that is used to clad the uranium fuel rods. The advanced gas-cooled reactor
also uses carbon dioxide as a coolant. Molten sodium can be used as a coolant.
Heat from the coolant passes to the water that is turned into steam to drive the turbines. In the event
of a failure in the cooling system, not only would the turbines cease to function, the core would
overheat, perhaps dangerously. Backup cooling systems are fitted that come into operation should
this occur. These are just one of the many safety systems.
Nuclear fuel cycle
Uranium fuel for nuclear reactors is obtained from ores, in the same way as
other metals. The ore is mined and milled to extract the uranium from it.
Natural uranium contains 140 parts of^238 U to every part of^235 U. It is only^235 U
that can sustain the chain reaction necessary to generate heat. Consequently,
for most reactors (but not all) it is necessary to increase the proportion of^235 U
to about 3 per cent. This is called ‘enrichment’.
Enriched uranium is then made into fuel pellets. These are placed in metal
canisters. They are then fuel rods and ready to be used.
When the^235 U fuel is depleted, the rods must be removed from the reactor and
replaced. They are very hot and highly radioactive. They are stored under
water for a time, while their temperature and level of radioactivity fall.