Nuclear Reactions

PRESENTATION ON RADOACTIVITY AND NUCLEAR REACTIONS

INTRODUCTION Radioactivity – spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles, or of electromagnetic rays called X rays and gamma rays.

DISCOVERY OF RADIOACTIVITY The phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel when he observed that the element uranium can blacken a photographic plate, although separated from it by glass or black paper. He also observed that the rays that produce the darkening are capable of discharging an electroscope, indicating that the rays possess an electric charge.

HALF-LIFE 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). Each individual radioactive substance has a characteristic decay period or half-life.

NUCLEAR REACTIONS

AND NUCLEAR ENERGY

Fission and Fusion Nuclear energy can be released in two different ways: fission, the splitting of a large nucleus, and fusion, the combining of two small nuclei. In both cases energy—measured in millions of electron volts (MeV)—is released because the products are more stable (have a higher binding energy) than the reactants. Fusion reactions are difficult to maintain because the nuclei repel each other, but fusion creates much less radioactive waste than does fission

Fission and Fusion Both nuclear fission and nuclear fusion reactions can be used to generate large amounts of energy for destructive purposes. When an atom of 235U is bombarded by a neutron, it splits into atoms of cesium and rubidium, releasing a large amount of energy and three additional neutrons. CONTINUED…

CONTINUED… These neutrons, if not controlled, can then cause more 235U atoms to split, leading rapidly to a nuclear explosion (A-bomb). Fusion reactions release energy when two light nuclei combine to make a heavier atom. Microsoft Corporation. All Rights Reserved.

CHAIN REACTION

DESCRIPTION OF CHAIN REACTION When a uranium or other suitable nucleus fissions, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes it possible to achieve a selfsustaining series of nuclear fissions; the neutrons that are emitted in fission produce a chain reaction, with continuous release of energy. Continued…

The light isotope of uranium, uranium-235, is easily split by the fission neutrons and, upon fission, emits an average of about 2.5 neutrons. One neutron per generation of nuclear fissions is necessary to sustain the chain reactions. Others may be lost by escape from the mass of chain-reacting material, or they may be absorbed in impurities or in the heavy uranium isotope, uranium-238, if it is present. Any substance capable of sustaining a fission chain reaction is known as a fissile material.

APPLICATON OF NUCLEAR ENERGY

Fission Bomb The first atomic bomb used in warfare was dropped by the United States on August 6, 1945. Called Little Boy, it produced an explosion that devastated the city of Hiroshima in Japan and killed tens of thousands of people in less than one minute. In this bomb, a mass of uranium about the size of a baseball produced an explosion as powerful as 15 kilotons of TNT.

Little Boy, which is schematized here, was a gun-type fission bomb. A small wedge of uranium was fired at a larger target piece of uranium and, upon impact, the two pieces fused together briefly, forming what is called a supercritical mass (a mass slightly greater than that necessary to sustain a chain reaction

DEVELOPED BY: VARINDERJIT KAUR (SCIENCE MISTRESS)

GOVT CO . ED. SR.SEC. SCHOOL, HIGH BRANCH RAJPURA