NUCLEAR REACTION

THE NUCLEAR REACTION The reactions on which the nuclear industry is based involve the nucleus of the atom. These nuclear reactions result in the conversion of mass, m, to energy, E, as defined by Einstein’s equation; E = mc2 Where, c is the velocity of light There are two types of nuclear reactions that lead to the production of energy, fission of heavy nuclei and fusion of light nuclei. In addition, nuclear reactions involving the capture of neutrons are important in the production of additional fission and fusion fuels. The energy evolved from grams of nuclear fuels is equivalent to that evolved from tons of chemical fuels. Fission Reaction Nuclear fission is the splitting of heavy nuclei into smaller one with the release of energy. The fission reaction depends mostly on Uranium – 235, the only naturally occurring nuclide that is fissionable with thermal neutrons. However, other fissionable nuclides are readily produced in the nuclear reactor, the most important being Plutonium –239 and Uranium – 233. A typical fission reaction is as follows 235 92U + 1 0 n→ 236 92U → 89 35Br + 145 57La + 2.3n + 192 MeV Neutrons are produced in the fission and the nuclear reaction can be made self-sustaining. Similarly, 238U undergoes thermal neutron (n) capture with subsequent Beta decay to 239Np and then to 239Pu. 238 92U + 1 0 n→ 239 92U → 239 93Np → 239 94Pu 235U or Plutonium used for atomic bombing in the World War II, which exerted initially to destructive force of 20,000 tons of TNT by converting only small fraction of the mass of 235U or Plutonium into energy. Fusion Reaction Lead to production of energy by the conversion of Hydrogen to Helium. Indeed maintenance of heat of the Sun and the Stars depends on energy released during the fusion of light atoms. 4 1H → 4He + 2.67 MeV The fuels for the fusion reactions are Hydrogen – 2 and Hydrogen – 3, more commonly called deuterium and tritium. Deuterium is a naturally occurring isotope of Hydrogen. Tritium is the neutron – capture product of Lithium – 6. A third possible fuel for fusion is Helium – 3, the decay product of radioactive Tritium. The fusion reaction practical interest are Reaction 1. 2 1H + 2 1H → 3 1H + 1 1H + 4MeV 2 1H + 2 1H→ 3 2He + 1 0 n + 3.25MeV 2. 2 1H + 3 1H→ 4 2He + 1 0 n + + 17.6MeV NUCLEAR FUELS They are fissionable isotopes used as sources of energy in nuclear reactors. Three isotopes that have a higher probability of fission than capture are 233U, 235U and 239Pu. These sustain the fission reactions and therefore called nuclear fuels. Of these isotope only 235U occurs in nature. The other two are produced artificially. UO2 is enriched to varying percentages is widely preferred because of its melting point, thermal conductivity, high density and resistance to the effects of irradiation. The starting material is UF6 enriched to the desired percentage. Feed Material Production or nuclear Fuel Cycle Flow chart representing the main steps from ore to usable nuclear materials. STEPS: 1. Mining of Ore: Involves extraction and concentration to improve U3 O8, yellow cake. Mined ores carry only low % of uranium. These are generally leached with H2SO4 or HNO3, the leached solution are concentrated by solvent extraction or by ion exchange and finally precipitated by caustic soda to form yellow cake. 2. Refining for Feed Material: It involves the extraction of pure Uranium from nitrate or sulphate solutions by an organic solvent, at first diethyl ether and later tributyl phosphate, diluted with kerosine or hexane. 3. Enrichment: The only procedure used to separate significant quantities of 235U from 238U is the gaseous diffusion method. UF6 is a gas and 235UF6 can be separated from 238UF6 by a method based on the difference in the diffusion rate through porous barriers. It means fluorination of UO3 will give isotopes 235UF6 and 238UF6 which are then separated using gaseous diffusion. 4. Nuclear Fuels: Nuclear fuels are produced specially for reactor use. 5. Mechanical Fabrication: Mechanical fabrication of Uranium and Plutonium fuel elements with or without enrichment. This involves mechanical shaping into plates, rods or tubes. NUCLEAR REACTORS It is defined as devices containing fissionable material in sufficient quantity and so arranged as to be capable of maintaining a controlled, self – sustaining nuclear fission chain reaction. NUCLEAR HAZARD The hazards involved in nuclear industries are by far greater than those in other industries. Radiation is greatly feared because it cannot be sensed with the ordinary human senses. The contamination of gases, solids and liquids by substances emitting alpha, beta and gamma radiation are all possible and must be guarded against. NUCLEAR WASTE DISPOSAL The disposal of nuclear waste is a difficult problem. The radioactive materials must be stored for many years until they decay to harmless materials. Disposal by burning in deep wells, the ocean or in underground tanks has been considered. One of the difficulties is that such wastes are often liquid that could be easily stored in underground tanks.