There are 92 naturally occurring elements but only one, uranium, has become the key to the operation of the nuclear fuel cycle. This singular use of uranium stems from its unstable, radioactive atomic structure. The safety problems arising from the use of uranium as an energy source stem from this highly radioactive property of uranium and the wastes it produces. Uranium - The Key Element
An atom can be pictured as a small universe with a nucleus at its centre and electrons orbiting around it. The nucleus contains protons and neutrons. Each electron has a negative charge and each proton a positive charge; since there are an equal number of protons and electrons the atom the atom is neutral. The Uranium Atom
Atoms of the same element have the same number of protons (the atomic number). However, the same element can have atoms with varying numbers of neutrons in their nucleus giving atomic species of different atomic weights known as nuclides. Thus for the key element uranium, the nucleus of uranium-235 has 143 neutrons and 92 protons and uranium-238 has 146 neutrons and 92 protons.
Some nuclides are unstable and decay spontaneously into other nuclides. Unstable nuclides are called radionuclides and their decay is called radioactivity. Radioactivity of the Wastes
Some radionuclides are more unstable than others and decay at a faster rate. Each radionuclide decays at its own characteristic rate. This is known as its half-life. One half-life is the time taken for half the atoms in a quantity of a radionuclide to decay.
The half-life of uranium-235 is 713,000,000 years and for uranium-238 it is 4500,000,000 years.
The unit of radioactivity is a becquerel which is 1 disintegration each second. A terebecquerel is 1012 disintegrations a second.
As radioactive atoms decay, alpha, beta and gamma rays are emitted. Ionising Radiations
Alpha rays are heavy positively charged particles travelling at high speed (several kilometres a second). These rays emanate from heavy elements such as uranium, plutonium and americium.Beta rays are negatively charged electrons seven thousand times lighter than alpha particles.
Gamma rays are electromagnetic radiation which emanates from most though not all radionuclides.
Because radioactive decay of uranium atoms is over a great length of time the energy released cannot be readily harnessed. Atomic Fission
The energy in atomic nuclei is only unleashed through a more drastic rupture of the atom. This became practical when it was discovered in 1938 that when uranium-235 is bombarded by neutrons it is unstable enough to split or fission into two small atoms at the same time releasing more neutrons.
As uranium-235 atoms split more and more neutrons are released until a chain reaction starts. Chain Reaction
Some fission products and their half-life are:
Radionuclide Half-life Radon-222 4 days Iodine-131 8 days Krypton-85 10 years Tritium (Hydrogen-3) 12 years Strontium-90 29 years Cesium-137 30 years Americium-241 433 years Radium-226 1,622 years Plutonium-239 24,000 years An atomic explosion is an uncontrolled chain reaction. The first 'practical' application of the chain reaction was the explosion of A-bombs using uranium fuel over Hiroshima and Nagasaki. Controlled and Uncontrolled Chain Reactions
In a nuclear reactor the chain reaction is controlled by using rods containing boron, an element with the capacity to 'mop up' excess neutrons. The safe operation of a reactor depends on keeping the number of neutrons being released evenly balanced with the number absorbed by uranium-235 atoms in the splitting process.
When the number of neutrons around the uranium fuel gets out of control then an accident such as those at Windscale, Three Mile Island, Chernobyl will occur.
Uranium-238, unlike uranium-235, rarely fissions. But a uranium-238 atom can capture a neutron to produce a plutonium-239 atom although plutonium-238, -240 and -242 are formed in lesser amounts. Plutonium
Plutonium-239 fissions faster than uranium-235 but still at a rate suitable to use in nuclear explosions. Plutonium is used as fuel in the breeder reactor which is specially designed to generate more plutonium-239 than it 'burns'. So far it has not functioned successfully in practice.
Plutonium is being mixed with uranium (MOX - mixed oxide fuel ) for use in thermal reactors.
Information from the MAUM public education sheet on Uranium : The Key Element.
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