Title of paper

4. Decay chain of Uranium-238

The natural decay chain of uranium-238 is as follows:

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  The uranium-238 decays, through alpha-emission, with a half-life of 4.5 billion years to thorium-234
  
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  which decays, through beta-emission, with a half-life of 24 days to protactinium-234
  
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  which decays, through beta-emission, with a half-life of 1.2 minutes to uranium-234
  
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  which decays, through alpha-emission, with a half-life of 240 thousand years to thorium-230
  
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  which decays, through alpha-emission, with a half-life of 77 thousand years to radium-226
  
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  which decays, through alpha-emission, with a half-life of 1.6 thousand years to radon-222
  
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  which decays, through alpha-emission, with a half-life of 3.8 days to polonium-218
  
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  which decays, through alpha-emission, with a half-life of 3.1 minutes to lead-214
  
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  which decays, through beta-emission, with a half-life of 27 minutes to bismuth-214
  
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  which decays, through beta-emission, with a half-life of 20 minutes to polonium-214
  
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  which decays, through alpha-emission, with a half-life of 160 microseconds to lead-210
  
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  which decays, through beta-emission, with a half-life of 22 years to bismuth-210
  
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  which decays, through beta-emission, with a half-life of 5 days to polonium-210
  
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  which decays, through alpha-emission, with a half-life of 140 days to lead-206, which is a stable nuclide.
  

When we talk about the half-life of an element we mean the time it takes for the element to lose half of its decay products. However we should remember that it will take far longer than two half-lives for the element to lose all its radio-activity. Instead it often takes between 10 to12 half-lives before the element is stable. This means that plutonium, for example, with a half-life of 24,400 years will actually take around 244,000 years before it stabilizes fully. In practical terms, this means that we need to be able to protect our environment and people from exposure to plutonium for just under a quarter-of-a-million years.

This raises the question of how able we are as a human species to manage this separation over such a huge length of time. To get some perspective on this question, humans (homo sapiens sapiens) are thought to have been culturally functional (for example able to make implements) for about 50,000 years. We have only had agriculture for around 12,000 years. We have only had cities for about 5,000 years. And yet we are supposed to be technologically able to guarantee that the plutonium we generate in nuclear reactors can, after removal, be insulated from human contact for 244,000 years!

5. The periodic table

The periodic table arranges all the elements in a logical sequence. The lightest element is hydrogen (H) with an atomic weight of 1. One of the heaviest of the naturally-occurring elements is uranium (U) with an atomic number of 92, equal to the number of protons as we have already seen.
All the elements with a higher atomic number than uranium (92) are not found in nature but manufactured in a laboratory or in other ways by human beings. Those coloured yellow are found in the form of a gas, while those in blue have a liquid form.

6. Isotopes

To understand better the link between uranium and the production of energy and weapons, we need to appreciate the concept of the different forms of uranium, or isotopes. When uranium is mined, it is mostly (99%) found in the form of uranium-238. A small amount (0.7%) of uranium-235 is brought to the surface, and even less (0.006%) of uranium-234. Of these three isotopes, it is easiest to get energy or weapons from uranium-235.
In order to get energy from reactors, the amount of uranium-235 needs to be raised from 0.7% to around 4%. To get weapons, it must be raised to around 90%. The technology needed to extract energy or weapons from the uranium-235 is called enrichment. Enrichment is a very complex, expensive, energy-intensive and technologically sophisticated process. Only a few countries have developed the know-how to do the enrichment. Once the process has been mastered, it can be used for manufacturing nuclear weapons.
The reason for raising the amount of uranium-235 is because uranium-235 is the only isotope of uranium that is fissile. This means that it is easiest to get energy out of the uranium at these higher concentrations of uranium-235. The way it works is that one atom of uranium-235 is bombarded by a neutron, causing it to split and release other neutrons and energy. The result of this is a chain reaction, capable of producing large quantities of energy. This can be controlled in a nuclear reactor. Or the energy can be concentrated in the form of a very powerful weapon. When the weapon is detonated it is capable of destroying entire cities, as was seen in August 1945 over Hiroshima and Nagasaki. These Japanese cities were destroyed and hundreds of thousands of people perished.
Splitting the atom:

Step1: A neutron bombards an uranium-235 atom, which becomes unstable, splits, releasing more neutrons and some energy.

Step 2: A newly released neutron fails to cause non-fissile uranium-238 atoms to release energy or neutrons.

Step 3: The newly released neutrons bombard other uranium-235 atoms, creating a chain reaction.

7. Radiation protection

Many people work in uranium mining, and there are others who handle radioactive substances in laboratories or nuclear power stations. Others handle machinery like x-ray machines. These nuclear workers need to be protected from exposure to the ionizing radiation, otherwise the risk of contracting a radiation-related disease is very high.
Working in a radioactive environment means that you should constantly be measuring the amount of radiation coming into your body. This is often done through the wearing of a film badge by exposed workers. The badge measures the dose of radiation which the workers are receiving. It should be measured each month. Should it exceed the officially allowed dose, the worker should be removed from any exposure for some time.
The official dose is set by the International Committee on Radiation Protection. Most radioactive workplaces accept the doses set by the ICRC. The dose for workers exposed to radioactivity is set at 10 milli-Sieverts per year (10 mSv/a^-1). Ordinary members of the public should not be exposed to more than 1 milli-Sieverts per year (1 mSv/a^-1).
These doses have sometimes been questioned. Are they safe enough? Over the years, the ICRP has reduced the allowable dose. Some argue, therefore, that the set doses are somewhat arbitrary, and that even low exposure to ionizing radiation might trigger off a disease in some people. The optimal situation is no exposure, although there will always be something of a risk because of background radiation.