Nuclear Fission and Nuclear Fusion Nuclear Fusion and

Nuclear Fission and Nuclear Fusion

Nuclear Fusion and Fission Reactions l l l Protons and neutrons in a nucleus are held together by very powerful binding energy Without this energy the positively charged protons would fly apart and all atoms would decompose instantly Fusion: small atoms join together to form larger atoms Fission: large unstable atoms decay into smaller fragment Both processes release enormous amounts of energy

Nuclear Fission l Process used in nuclear reactors to produce electricity l An isotope of uranium, uranium-235 is used since it decays very quickly l The nucleus of uranium-235 undergoes fission (splits) when a slow-moving neutron strikes its nucleus making it unstable l This results in two daughter nuclei, three neutrons, and a release of energy

The Equation for Nuclear Fission (copy from p. 211)

Nuclear Fission l The three neutrons that are released then strike other uranium nuclei and these nuclei split, releasing more energy and more neutrons—one decaying atom causes other atoms to decay l If enough uranium is present a chain reaction will occur—amount of radioactive substance required for a chain reaction to occur is called the critical mass

Problems with Fission If reaction is not controlled you get a rapid release of energy like in an atomic bomb or a reactor meltdown—not good! B. The products are radioactive—how to dispose of the waste since it will be radioactive for a very long time? A.

Nuclear Power Generation l Reactions are controlled to create a selfsustaining chain reaction: the rate of neutrons being produced by fission is equal to the rate of neutrons being used in the fission process l Speed of neutrons being produced in the decay process controls speed of reaction itself l See nuclear reactor diagram

Nuclear Fusion l Sun and all stars generate energy through fusion reactions; these reactions happen in core of the sun l Scientists are trying to copy this kind of reaction to generate electricity but cannot produce the high temperature needed to have atoms collide and self-sustain the reaction l Fusion reactions do not create radioactive waste products—this is good!

The Equation for Nuclear Fusion (copy from p. 212)

Difficulties with Fusion Reactions l Scientists have been trying to copy fusion reaction for use in generating electricity l But it is difficult to sustain this type of nuclear reaction because it requires extremely high temperatures (over 100 000 ºC)