Ahmad Saeedi Graduate Student Department of Physics, Western Illinois University, Macomb, IL Nuclear reaction Fission and Fusion

 Outline -History First: Nuclear Fission -Spontaneous Fission -Induced Fission -Process of fission -Chin reaction Second :Nuclear Fusion -Controlling Nuclear Fusion -Conclusion

The Discovery of Fission -In 1939, Otto Hahn and Fritz Strassmann announced that uranium nuclei "burst" when they are bombarded with neutrons. - For proving that the barium resulting from his bombardment of uranium with neutrons was the product of nuclear fission, Hahn was awarded the Nobal Prize for Chemistry in 1944 (the sole recipient) "for his discovery of the fission of heavy nuclei"

A reaction in which an atomic nucleus of a radioactive element splits by bombardment from an external source (like neutrons), with simultaneous release of large amounts of energy, used for electric power generation. Nuclear reaction

Nuclear fission = nuclear reaction = radioactivity decay First: Nuclear Fission  When atoms are bombarded with neutrons, their nuclei splits into 2 parts which are roughly equal in size.  Nuclear fission in the process whereby a nucleus, with a high mass number, splits into 2 nuclei which have roughly equal smaller mass numbers.  During nuclear fission, neutrons are released

1- Spontaneous Fission  Some radioisotopes contain nuclei which are highly unstable and decay spontaneously by splitting into 2 smaller nuclei.  Such spontaneous decays are accompanied by the release of neutrons.

2- Induced Fission  Nuclear fission can be induced by bombarding atoms with neutrons.  The nuclei of the atoms then split into 2 equal parts.  Induced fission decays are also accompanied by the release of neutrons.

The Fission Process  A neutron travels at high speed towards a uranium- 235 nucleus.  The neutron strikes the nucleus which then captures the neutron.  The nucleus changes from being uranium-235 to uranium-236.  It then splits into 2 fission fragments and releases neutrons.

 In a typical example of nuclear fission, one neutron starts the fission of the uranium atom and three more neutrons are produced when the uranium fissions.

Examples: U Ba n n Kr U Cs n n Rb 96 37

Energy from Fission  Both the fission fragments and neutrons travel at high speed.  The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom. E K before fission << E K after fission  Energy is being released as a result of the fission reaction.

Energy Released  The energy released can be calculated using the equation:

The amount of energy from Fission: U Cs n n Rb ElementAtomic Mass (u) U Cs Rb n10n

Energy from Fission: Calculate the total mass before and after fission takes place: = u The total mass after fission (RHS of the equation): ( ) = u U Cs n n Rb 96 37

The total mass before fission = The total mass after fission = Total mass before fission > Total mass after fission u u

Mass difference m= Total mass before fission – Total mass after fission m= LHS - RHS m = – m = u  This reduction in mass results in the release of energy.

A chain reaction is a self-sustaining reaction. A reaction event stimulates additional reaction events to keep the process going.

 If a chain reaction occurred in a chunk of pure U-235 the size of a baseball, an enormous explosion would likely result. In a smaller chunk of pure U-235, however, no explosion would occur.  A neutron ejected by a fission event travels a certain average distance before encountering another uranium nucleus.  If the piece of uranium is too small, a neutron is likely to escape through the surface before it “finds” another nucleus.  Fewer than one neutron per fission will be available to trigger more fission, and the chain reaction will die out

Second :Nuclear Fusion  Energy is released as light nuclei fuse, or combine, rather than split apart. This process is nuclear fusion.  Energy is released when heavy nuclei split apart in the fission process.  In nuclear fusion, energy is released when light nuclei fuse together.  A proton has more mass by itself than it does inside a helium nucleus.

Nuclear Fusion  Atomic nuclei are positively charged.  For fusion to occur, they must collide at very high speeds to overcome electrical repulsion.  Fusion brought about by high temperatures is called thermonuclear fusion.

Nuclear Fusion  In the central part of the sun, about 657 million tons of hydrogen are converted into 653 million tons of helium each second.  The missing 4 million tons of mass is discharged as radiant energy.

Think! First it was stated that nuclear energy is released when atoms split apart. Now it is stated that nuclear energy is released when atoms combine. Is this a contradiction?

Think! First it was stated that nuclear energy is released when atoms split apart. Now it is stated that nuclear energy is released when atoms combine. Is this a contradiction? Answer: This is contradictory only if the same element is said to release energy by both the processes of fission and fusion. Only the fusion of light elements and the fission of heavy elements result in a decrease in nucleon mass and a release of energy.

Controlling Nuclear Fusion  In the fusion reactions of hydrogen isotopes, most of the energy released is carried by the lighter-weight particles, protons and neutrons, which fly off at high speeds.

Find the Energy released per fusion? ElementAtomic Mass (u) 21H21H H31H H42H n10n

 The development of fusion power has been slow and difficult, already extending over 50 years.  It is one of the biggest scientific and engineering challenges that we face.  Our hope is that it will be achieved and will be a primary energy source for future generations.

Nuclear FissionNuclear Fusion Definition:It is the splitting of a large atom into two or more smaller ones. It is the fusing of two or more lighter atoms into a larger one. Natural occurrence of the process: It reaction does not normally occur in nature.It occurs in stars, such as the sun. Byproducts of the reaction: It produces many highly radioactive particles.Few radioactive particles are produced by fusion reaction, but if a fission "trigger" is used, radioactive particles will result from that. Conditions:Critical mass of the substance and high-speed neutrons are required. High density, high temperature environment is required. Energy Requirement:Takes little energy to split two atoms in a fission reaction. Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion. Energy Released:It is a million times greater than that released in chemical reactions; but lower than the energy released by nuclear fusion. The energy released by fusion is three to four times greater than the energy released by fission. Difference between Fission and Fusion

Nuclear fission and nuclear fusion reactions release huge amounts of energy.

Acknowledgements:  Dr. S. Mallur physics department, Western Illinois University    vs_Nuclear_Fusion  Nuclear and Particle Physics, B.R. Martin