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 reactionor a radioactive decay process. The case of the decay process is called spontaneous fission, and it is a very rare process. In this section, neutron-induced nuclear fission, the process of the greatest practical importance in reactor physics, will be discussed.
Basics of Nuclear Fission – for non-physicists
Basics of Nuclear Fission
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, which can induce the fission of other nuclei. Free neutrons released by each fission play a very important role as a trigger of the 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 number 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.
Key Features of Nuclear Fission
Nuclear fission is the main process of generating nuclear energy.
Most of the energy (~85%) is released in the form of the kinetic energy of the split parts.
Neutrons trigger 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 a moderator, enriched uranium fuel is required.
Control rods contain material that absorbs neutrons (boron, cadmium, …)
Withdrawal of the rods increases the parameter one or more (multiplication factor), thus increasing the power.
Insertion of the rods decreases the parameter one or more (multiplication factor), thus decreasing the power.
The multiplication factor is also influenced 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 of publishing the finding because it was 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 could 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.