1 May 2013

Introduction to the paper "RADIOACTIVE WASTE MANAGEMENT: An overview of existing technologies and future possibilities"

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


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.

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

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.



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).

This is where I left it. I'd also found a table which gave information on the health effects of different amounts of radioactivity. I hope you've enjoyed it! :)

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