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1511327847Paper8 Energyenv Paper No:7 Energy and Environment Module:18 Nuclear energy from fission Development Team Prof. R.K. Kohli Principal Investigator & Prof. V.K. Garg &Prof.AshokDhawan Co- Principal Investigator Central University of Punjab, Bathinda Dr. Dhanya M.S., Paper Coordinator Central University of Punjab, Bathinda Dr. Sandeep Kumar and Dr. S. Prasad Content Writer IARI, New Delhi & Dhanya M.S, Central University of Punjab, Bathinda Content Reviewer Prof. A.K Jain, Former Director, SSSNIRE 1 Anchor Institute Central University of Punjab Energy and Environment Environmental Sciences Nuclear energy from fission Description of Module Subject Name Environmental Sciences Paper Name Energy and Environment Module Name/Title Nuclear energy- fission Module Id EVS/EE-VIII/18 Pre-requisites This module helps to learn What is the principle of nuclear fission? How to control of nuclear fission? Energy release by nuclear fission Objectives Types of nuclear reactors Components of a nuclear power plant Advantages and disadvantages of nuclear power plants Nuclear power plants in India Keywords Nuclear fission, nuclear power plant, energy 2 Energy and Environment Environmental Sciences Nuclear energy from fission Learning Objectives What is the principle of nuclear fission? How to control of nuclear fission? Energy Release by Nuclear Fission Types of nuclear reactors Components Of a Nuclear Power Plant Advantages and disadvantages of nuclear power plants Nuclear power plants in India 1. Introduction The nuclear fission is one type of nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller nuclei along with release of huge amounts of energy. Fission is a form of nuclear transmutation with unsimilar resulting particles which differ from parent nuclei. The fission occurs in heavy nuclei after the capture of a neutron. The slow or thermal neutrons cause fission only in those isotopes of uranium (U) and plutonium (Pu) whose nuclei having odd numbers of neutrons (e.g. U-233, U-235, and Pu-239). The fission in nuclei with even number of neutrons occurs only when the incident neutrons have energy above around one million electron volts (MeV). The two nuclei products from common fissile isotopes are comparable differing in sizes with a mass ratio of products of nearly 3 to 2. The binary fissions that produce two charged particles are common but three positively charged particles (ternary fission) are also produced occasionally. The electricity produced by nuclear fission in a self-sustaining nuclear chain reaction that releases a significant amount of energy at a controlled rate in a nuclear reactor had wide energy potential. 3 Energy and Environment Environmental Sciences Nuclear energy from fission 1.1. Discovery of nuclear fission In 1917, Rutherford was able to perform transmutation of nitrogen into oxygen, using alpha particles directed at nitrogen 14N + α → 17O + p That was the first observation of a nuclear reaction, in which particles from one decay are used to transform another atomic nucleus. Finally, in 1932, a fully artificial nuclear reaction and nuclear transmutation were concluded by Rutherford's colleagues Ernest Walton and John Cockcroft, who applied artificially accelerated protons against lithium-7 (Li-7), to split this nucleus into two alpha particles. Fermi and his colleagues studied the results of bombarding uranium with neutrons in 1934. The nuclear fission of heavy elements was identified on December 17, 1938, by German Otto Hahn and his colleague Fritz Strassmann, and described theoretically in January 1939 by Lise Meitner and her nephew, Otto Robert Frisch. 3. Principle of nuclear fission Nuclear fission occurs when a neutron hits with a nucleus of a large atom such as U and is absorbed into it causing the nucleus to become unstable and thus break down into two smaller and more stable atoms with the release of more neutrons and an enormous amount of energy. Nuclear fission can happen naturally with the spontaneous decay of radioactive material, or it can be induced by bombarding the fuel consisting of fissionable atoms with neutrons (Fig. 1). 4 Energy and Environment Environmental Sciences Nuclear energy from fission Fig 1: Nuclear fission process a) Fission Energy Release The fission of naturally occurring U isotope induced by thermal neutrons is U-235 which splits into Ba-141 and Kr-92 and emits excess free neutrons. When nucleus fission occurs, it breaks into several smaller fragments with release of two or three neutrons. The sum of the masses of fission products is less than the original mass. On an average near about 0.1% of the original mass has been transformed into energy according to Einstein's equation. And that reduction in mass comes off in the form of energy according to the Einstein equation E = mc2. The fission of U-235 in nuclear reactors is triggered by the absorption of a low energy neutron, usually termed a "slow neutron" or a "thermal neutron." Other fissionable isotopes which can be induced to fission by slow neutrons are Pu-239, U-233, and Th-232. 5 Energy and Environment Environmental Sciences Nuclear energy from fission 1.2. Fissionable isotopes The nuclear fission reactions start when isotopes are hit by either high energy (fast moving) neutrons or thermal (slow-moving) neutrons at the optimum speed. For examples, U-235 and Pu-239 are fissionable isotopes. U-235 is the naturally occurring fissionable isotope, and Pu-239 can be produced by "breeding" from U-238. U-238 and Th-232 are the chief naturally occurring fertile isotopes. U-238, which makes up 99.3% of natural uranium, is not fissionable by slow neutrons. U-238 has a little probability for spontaneous fission and also a small chance of fission when bombarded with fast neutrons. However, it is not useful as a nuclear fuel source. Th-232 is fissionable, so could conceivably be employed as a nuclear fuel. The other isotope of natural thorium which is known to undergo fission upon slow neutron bombardment is U-233. Pu-241 and Pu-241 are fissile materials. The isotopes as mentioned earlier can be created artificially in a nuclear reactor, from the fertile nuclei of Th-232, U-238, and Pu-240. U-235 is only naturally found isotope which is thermally fissile, and present in natural uranium at a concentration of 0.7%. 1.3. Principle of Nuclear Fission When U-235 or fissionable isotopes capture a neutron, the total energy is distributed amongst the nucleons (protons & neutrons) present in the compound nucleus. These nucleuses are relatively unstable, and it is likely to break into two fragments of near about half of the mass. Production of these fission fragments is followed almost immediately by the emission of some neutrons (commonly 2 or 3, average 2.45), which facilitate the chain reaction to be sustained (Fig. 3). 6 Energy and Environment Environmental Sciences Nuclear energy from fission Fig. 2: Energy path during fission About 85% of the energy released is initially the kinetic energy of the fission fragments (Fig. 2). The balance of the power comes from gamma rays emitted during or quickly following the fission process and from the kinetic energy of the neutrons. The immediate neutrons are called prompt neutrons, and there is a small portion of delayed neutrons, as these are linkedwith the radioactive decay of certain fission products. The highest delayed neutron group has a half-life of nearly 56 seconds. Fig. 3: Nuclear fission chain reaction 7 Energy and Environment Environmental Sciences Nuclear energy from fission The number of neutrons and the particular fission products from any fission case are guided by statistical probability, in that the precise break up of a single nucleus cannot be predicted. Nonetheless, conservation laws require the total number of nucleons and the total energy to be conserved during the reaction. The fission reaction in U-235 creates fission products like Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed nearly 95 and 135. The following reactions are representing typical fissionable products: U-235 + n Ba-144 + Kr-90 + 2n + 200 MeV U-235 + n Ba-141 + Kr-92 + 3n + 170 MeV U-235 + n Zr-94 + Te-139 + 3n + 197 MeV In above reactions, the number of nucleons (protons + neutrons) is balanced. The total binding energy released in nuclear fission reaction of an atomic nucleus varies with the precise break up (near about 200 MeV for U-235). Near about 6% of the heat produced in the reactor core originates from radioactive decay of fission products and transuranic elements composed by neutron capture. That must be allowed for when the reactor is shut down since heat generation continues after fission stops. Even after one year, commonly used fuel generates about 10 kW of decay heat per ton, decreasing to about 1 kW per ton after ten years. 1.4. Nuclear Chain Reactions Nuclear chain reactions are series of nuclear fissions, each reaction initiated by a neutron produced in a preceding fission. This process may be controlled (nuclear power) or uncontrolled (nuclear weapons). If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there is 1,024 fission and in 80 generations about 6 x 10 23 (a mole) fissions (Fig. 4). The energy released from the kinetic energy of each fission reaction is nearly 200 MeV, which includes,165 MeV from fission, 6 MeV from neutrons. 7 MeV from gamma rays, 7 MeV from energy from fission products, 6 MeV from gamma rays and 9 MeV from anti-neutrinos from fission products (1 MeV = 1.609 x 10 -13 joules). 8 Energy and Environment Environmental Sciences Nuclear energy from fission Fig 4:Nuclear Chain Reactions 1.4.1. Critical Mass (K) Although two to three neutrons are created for every fission, not all of these neutrons are available for continuing the fission reaction.
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