Nuclear fission and fusion Flashcards
what’s nuclear fission ??
splitting of a large atomic nucleus into smaller nuclei.
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In a
nuclear reactor
a neutron is absorbed into a nucleus - . This causes the nucleus to become uranium-236, which is violently unstable.
what’s a
chain reaction
The entire nucleus splits into two large fragments called ‘daughter nuclei’. In addition to the ‘daughter’ products, two or three neutrons also explode out of the fission reaction and these can collide with other uranium nuclei to cause further fission reactions. This is known as a
chain reaction
.
how does energy pass to other components and why ?
The fast moving neutrons carry most of the energy from the reaction with them (99%) but before the neutrons can collide with fresh uranium nuclei, they need to be slowed down.
This is so that the energy can pass on to other components in the nuclear reactor, which is used to heat water to drive the
turbines
that turn the
generators
.
A fission reactor contains a number of different parts:
nuclear fuel
the fuel is held in rods so that the neutrons released will fly out and cause
nuclear fission
in other rods
graphite core
- graphite slows the neutrons down so that they are more likely to be absorbed into a nearby fuel rod
control rods
- these are raised and lowered to stop neutrons from travelling between fuel rods and therefore change the speed of the
chain reaction
coolant
- this is heated up by the energy released from the fission reactions and is used to boil water to drive
turbines
in the power station
concrete shield - the daughter products of the fission reaction are
radioactive
and can be a
hazard
Many of the features of the reactor are designed to
control the speed of the reaction and the temperature inside the shielding. An uncontrolled fission reaction is the basis of an
atomic bomb
.
What’s Nuclear fusion?
Nuclear fusion
is when two small, light
nuclei
join together to make one heavy nucleus. Fusion reactions occur in stars where two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium
isotope
.
There are a number of different nuclear fusion reactions happening in the Sun. The simplest is when four hydrogen nuclei become one helium nuclei.
masses and missing mass
The combined mass of four hydrogen nuclei is 6.693 × 10-27 kilograms (kg). The mass of one helium nucleus is 6.645 × 10-27 kg. This means that there is a missing amount of mass equalling 0.048 × 10-27 kg.
The missing mass is converted to energy, which
radiates
away. This is seen happening in the Sun. In all
nuclear reactions
a small amount of the mass changes to energy.
However, the issue with fusion is that
it requires the fusing of nuclei, which are positive particles. As two nuclei approach each other, they will repel because they have the same
charge. The fusion of the nuclei has to happen quickly so that the repulsion of the charges does not have time to stop it from happening.
A way that particles can travel that quickly is by being in a hot gas or in
plasma
, like in the Sun.
