Nuclear Fission

When a nucleus is bombarded with some subatomic particles (like a particles, neutrons, protons, etc.), then these particles are either captured by the nucleus or the nucleus disintegrates ejecting some other subatomic particles. So, the new element formed has a mass either slightly greater than or slightly smaller than that of the original element.

Basic Reactor Principles


The splitting of a heavier atom into a number of fragments of much smaller mass by suitable bombardment with subatomic particles, with the liberation of a huge amount of energy, is called nuclear fission.

When uranium-235 was hit by slow neutrons, it was split up into a number of fragments, each of mass much smaller than that of uranium. The two fragments were barium and krypton (Glasstone and Sesonke 2004).

92U235 + 0n1 ® 56Ba140 + збКг93 + 3on1 + Energy (1.15)

The neutron released from the fission of the first uranium atom can hit three other uranium atoms, each of which again releases three neutrons, each of which can further hit one uranium atom and so on. In this way, a chain reaction is set up and results into the liberation of a huge amount of energy. Another aspect which is extremely important for a chain reaction to continue is that the fissionable material must have a minimum size. If the size is smaller than the minimum size, the neutrons escape from the sample without hitting the nucleus, causing fission, and thus the chain reaction stops.

The minimum mass that the fissionable material must have so that one of the neutrons released in every fission hits another nucleus and causes fission (so that the chain reaction continues at a constant rate) is called the critical mass. If the mass is less than that of the critical mass, it is called sub-critical. In this case, many neutrons released in fission are able to hit the other nuclei and thus the numbers of fissions multiply in the chain reaction. The shape and the density of the packing of the material are also significant for the nuclear fission.

Nuclear Fission

Nuclear Fusion

It involves breaking up of a heavier nucleus into lighter nuclei.

It involves the union of two or more lighter nuclei to from a heavier nucleus.

It is a chain process.

It is initiated by neutrons of suitable energy and does not need high temperature.

It can be controlled and the energy released can be harnessed for useful purposes.

A larger number of radioactive isotopes are formed and there is nuclear waste.

It is not a chain process.

It is initiated by very high temperatures.

It is difficult to control this process.

There is no nuclear waste in this process.

It requires a minimum size fissionable material, and if the size of the material exceeds the critical size, the reaction becomes explosive.

There is no limit to the size of the fuel for the reaction to start. However, the fuel does not undergo fusion until heated to a very high temperature (a few million degrees).

< Prev   CONTENTS   Source   Next >