Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a large amount of energy. In nuclear physics, nuclear fission is either a nuclear reaction or a radioactive decay process. The case of decay process is called spontaneous fission and it is very rare process. In this section, the neutron-induced nuclear fission, the process of the greatest practical importance in reactor physics, will be discussed.
Nuclear Fission

Basics of Nuclear Fission – for non-physicists

There are nuclei that can undergo fission on their own spontaneously, but only certain nuclei, like uranium-235, uranium-233 and plutonium-239, can sustain a fission chain reaction. This is because these nuclei release neutrons when they break apart, and these neutrons can induce fission of other nuclei. Free neutrons released by each fission play very important role as a trigger of the reaction.

Nuclear fission

Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei). This nuclear reaction is triggered by the neutron. Source: chemwiki.ucdavis.edu

Chain reaction

A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The “one or more” is the key parameter of reactor physics. To raise or lower the power, the amount of reactions must be changed (using the control rods) so that the number of neutrons present (and hence the rate of power generation) is either reduced or increased.

Nuclear chain reaction

A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions.

  • Nuclear fission is the main process generating nuclear energy.
  • Most of the energy (~85%) is released in the form of kinetic energy of the splitted parts.
  • Neutrons trigger the nuclear fission.
  • The fission process produces free neutrons (2 or 3).
  • The chain reaction means if the reaction induces one or more reactions.
  • The probability that fission will occur depends on incident neutron energy.
  • Therefore the moderator is used to slow down neutrons (to increase the probability of fission)
  • For reactors using light water as moderator, enriched uranium fuel is required.
  • Control rods contains material which absorb neutrons (boron, cadmium, …)
  • Withdrawal of the rods increases the parameter one or more (multiplication factor), thus increase the power.
  • Insertion of the rods decreases the parameter one or more (multiplication factor), thus decrease the power.
  • The multiplication factor is influenced also by other parameters such as temperature, fuel burnup and reactor poisoning.

 

Nuclear fission of heavy elements was discovered on December 17, 1938 by Otto Hahn and his assistant Fritz Strassmann. They attempted to create transuranic elements by bombarding uranium with neutrons. Rather than the heavy elements they expected, they got several unidentified products. When they finally identified one of the products as Barium-141, they were circumspective to publish the finding because it was so unexpected.When they finally published the results in 1939, they came to the attention of Lise Meitner, an Austrian-born physicist who had worked with Hahn on his nuclear experiments. She was the first to realize that Hahn’s barium and other lighter products from the neutron bombardment experiments were coming from the fission of U-235. Meitner and Frisch carried out further experiments which showed that the U-235 fission can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). They realized that this made possible a chain reaction with an unprecedented energy yield.

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