This time, I bring you the introduction I wrote for a paper last semester. Halfway through the job I realised I was making the introduction way too long and detailed for the scope of the paper, and I had to redo the whole thing. Nevertheless, I saved my previous work because in it you can find some juicy tidbits of information.
You are very welcome to email me for the references and/or for the whole paper. :)
Nuclear
fission
Definition
Nuclear fission is a nuclear reaction in
which the nucleus of an atom splits in two (sometimes, but rarely, three) parts
of the same size.
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Fig. 1: binding energy
graph (European Nuclear
Society, 2003)
|
Nuclear
fission can occur both spontaneously
and artificially.
Theoretically, spontaneous fission should take place
with nuclei with a mass larger than only 130 atomic mass units (amu), as the
graph in Fig. 1
suggests. This graph shows that the binding energy of the heaviest atoms is
slightly less negative than that of most of the medium-sized nuclei
(Z > 5), which is around 8 MeV per nucleon. (The binding
energy of an atom nucleus is the amount of energy required to pull infinitely
apart particles that are bound by nuclear or electromagnetic forces. Thus, the
energy necessary for the actual nucleus to keep together is negative by
convention.)
Experimentally, though, this takes place
only with the heaviest atoms, those with mass numbers 230 or more. Also, the
reactions take place extremely slowly, with half-times of the order of 1015
or 1016. As an example, the half-life of 238U (also
referred to as “depleted uranium”) is 4,500 millions of years.
 |
Fig. 2: chain reaction
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However,
an artificial reaction takes much
less time. Induced fission consists of bombarding a heavy nucleus with a
thermal neutron, which splits the nucleus in two daughter nuclei of uneven
mass, releasing more neutrons and a great quantity of energy. These released
neutrons then hit other nuclei, which in turn split and yield yet more neutrons
and energy, and so on, developing a chain of reaction of an exponential rate.
This chain reaction is highly energetic and exothermic, and, if uncontrolled,
can lead to explosion. (This is the principle of the atomic bomb. Various uses
of the nuclear fission reaction will be discussed later.)
Fig. 2
illustrates the beginning of a chain reaction and shows the splitting of more
nuclei as a result of the neutrons emitted by the fission of the first nucleus.
This could well be the representation of an atom of 235U, which is
the only naturally occurring isotope in which fission can be induced.
Applications
 |
Fig. 3: comparison of
energy sources
(Nuclear Energy
Institute, 2012)
|
The amount of energy released per gramme of
235U is of an average of 80 million kJ. In comparison, a gramme of
natural gas yields only 50 kJ. This is one aspect of nuclear energy that
makes it attractive. Fig. 3 shows
a comparison of energy sources.
Among the applications of nuclear fission
energy, electric power generation is one of the most common. The graph in Fig. 4 shows
the percentage of nuclear electricity production on the total of generated
energy in the world in the past 30 years.
Another
application of nuclear fission reactions is in marine transportation. Over 220
small reactors are used for nuclear propulsion in Army and Navy ships (mainly
in the USA, Russia, and France), and 56 different countries operate a total of
280 reactors for research purposes (House of Representatives Industry and Resources Committee (Australian
Government), 2006).
 |
Fig. 4: percentage of
nuclear electricity generation in the past decades (World Nuclear Association, 2012)
|
Nuclear fission is of great importance in
many other areas, where the main use is the production of radioisotopes
(unstable or radioactive isotopes). These radioisotopes are used for such
diverse fields as the food industry (fertilisers, breeding, preservation);
investigation of water resources, both from the point of view of quality and
from the dynamics of various hydrologic accidents; in medicine, for diagnosis,
treatment, and also sterilisation of instruments; and on a wider industrial
field, from dating and tracing to non-destructive analysis of materials and
components.
Heat emission from the decay of
plutonium-238 is used to power navigation beacons,
satellites, and vehicles such as the Mars rover Curiosity, and much research is being conducted also in this field.
Nuclear weapons are
also an important application of nuclear fission. However, the environment
isn’t exactly a major concern in the use of such devices, so they will be
discussed no further.
Use of Uranium
In order to satisfy the demand for
electricity production, the main use of uranium, each year 77,000 tonnes of
uranium oxide concentrate are required, which would amount to 65,500 tonnes of
uranium (World Nuclear Association, 2010). Adding to this the
other less important applications, the total current world demand for uranium
ascends to 68,500 tonnes per year (World Nuclear Association, 2012).
The graph in Fig. 5 shows
the world uranium production and demand over the past decades.
 |
Fig. 5: world uranium
production and demand (World Nuclear Association, 2012)
|
It is seen that production from world
uranium mines supply only about 75% of the total demand. This primary
production is supplemented by ex-military material and other secondary supplies
(World Nuclear Association, 2010). Also, as can be
observed, demand is slightly increasing and uranium mining is doing so more
significantly.
Effects of
Radioactivity on People and the Environment
The impact of radiation on living creatures
is generally evaluated by the effective biological damage of radiation. The
unit used to quantify such measurements is the sievert (Sv).
For reference, the average person receives
3,1 mSv per year due to natural radiation (United States Nuclear Regulatory Comission, 2012). A chest CT scan
would result in 6,8 mSv; nuclear power workers are allowed a maximum of
50 mSv per year; and a dose of 10 Sv is usually fatal.
It is generally accepted that the release
of radioactive wastes to the environment is likely to increase the exposure of
wildlife to radiation (Linsley, 1997). Although radiation may cause cancers
at high doses and high dose rates, currently there are no data to establish
unequivocally the occurrence of cancer following exposure to low doses and dose
rates – below about 100 mSv (United States Nuclear
Regulatory Comission, 2012).
