NUCLEAR CHEMISTRY

• The phenomenon of radioactivity was discovered by the French scientist Henri Becquerel, in 1896. He noted that crystals of potassium uranyl sulphate, placed over a wrapped photographic plate even in total darkness, could produce background marks on the plate after its development. Marie and Plerre Curie in 1898 isolated the elements radium and polonium. Other such elements are uranium and thorium. Such elements are known as naturally occurring radioactive elements. The man-made elements, which are radioactive and exhibit artificial radioactivity. These are technetium, neptunium, curium and californium. The phenomenon of radioactivity is due to decay of unstable nuclei Naturally occurring radioactive elements undergo decay by emitting - alpha, beta, and gamma rays, thus producing stable, non-radioactive daughter element. When a radioactive nucleus emits an alpha particle, its atomic number is reduced by 2 and its mass number is reduced by four. When uranium 238 nucleus is bombarded with alpha particle, it produces throrium-234 nucleus. The activity of a radioactive element is measured by the rate at which it changes into its daughter element.

Half-Life Period

• Half-life period is the time required for disintegration of one half of atoms of radioactive species initially present. The decay of some substances, such as uranium-238 and thorium-232, appears to continue indefinitely without detectable diminution of the decay rate per unit mass of the isotope (specific-decay rate). Other radioactive substances show a marked decrease in specific-decay rate with time. Among these is the isotope thorium-234 (originally called uranium X), which, after isolation from uranium, decays to half its original radioactive intensity within 25 days. Each individual radioactive substance has a characteristic decay period or half-life; because their half-lives are so long that decay is not appreciable within the observation period, the diminution of the specific-decay rate of some isotopes is not observable under present methods. Thorium-232, for example, has a half-life of 14 billion years.

Nuclear reactor

• A nuclear reactor consists of

  (a) Fissionable material, (uranium enriched in U-235 (2-3%)

  (b)   Moderator (graphite or heavy water) to slow down the neutrons, thereby increasing the efficiency of their capture, and to bring about a fission reaction.

  (c)   Control rods made up of boron steel or cadmium is used to capture the neutrons, and to restrict the reaction and thus it remains a controlled one.

• The large amount of energy released in the form of heat is converted into electrical energy.
• Breeder reactors, is a facility in which fissionable isotope, which is enriched and used as a fuel in a nuclear reactor.
• Ex:

  (i)    Uranium-238 is bombarded with fast neutrons to produce uranium-239, and later plutonium-239.

  (ii)   Naturally more abundant thorium-232 breeds a fissionable isotope uranium-233

• Nuclear fusion- two or more light nuclei combine to form a heavy nucleus, of helium. On the basis of fusion reaction, hydrogen or thermonuclear bomb is designed. The energy of the sun or other stars is believed to arise from the fusion of hydrogen nuclei to form helium.
• A thermonuclear fusion reactor to generate a vast source of energy is still a technological challenge. The reaction is considered possible by the fusion of Deutrium nuclei into helium. Deutrium is available in plenty.