Nuclear Fission

Properties of Neutrons 1- Neutron is an un-charged particle and hence they are not deflected by the electric and magnetic fields 2- It has very high penetrating power and has very low ionizing power. 3- In nucleus they appear to last forever. The average life of an isolated neutron is about 1000 seconds, before they decay. A free neutron spontaneously decays into a proton, electron and an antineutrino (u). 0n' 1H' + -1e° + u

4- If fast neutrons pass through substances like heavy water, paraffin wax, graphite etc., they are slowed down. These slow neutrons are also known as thermal neutrons (energy < 1 eV).

In 1939, Strassmann and Otto Hann first discovered that when Uranium-235 was bombarded with slow neutrons, it splits up into two fragments of more or less equal mass with release of three neutrons and 200 MeV energy. This process of breaking a nucleus into two or more equal fragments is called 'fission', a term borrowed from biology indicating the division of a cell into two cells of roughly the same size. Hence the fission reaction of U-235 can be written as:

This phenomenon of fission was explained by Neils Bohr and Wheeler on the basis of liquid drop model of the nucleus. According to the liquid drop model, it was due to the property of surface tension, the nucleus behaves like a liquid drop and tries to be perfectly spherical in shape. When a nucleus absorbs the neutron a compound nucleus is formed and some excitation energy is imparted to the nucleus. While the surface tension of the nucleus tries to keep the nucleus in spherical shape, the excitation energy tries to deform the nucleus and hence strong oscillations are set up in side the compound nucleus. These oscillations will distort the shape of the compound nucleus, from sphere to ellipsoid as shown in the following Fig (1b) and breaks into fragments and neutrons.

If the excitation energy is sufficiently large the ellipsoid nucleus attains the dumb bell shape [Fig.(1c)], In this case the effect of nuclear attractive force is decreased because of much increased surface area of the nucleus. Further, the Coulomb repulsive force drives the two portions of the dumb bell still farther and the nucleus undergoes fission as shown in the Fig. (1d). The liberated neutrons are called “prompt neutrons”.

Fig. 1, Fission of U-235 after capturing neutron

Chain Reaction In each fission event approximately three neutrons are released, and each of them cause further fission in three more uranium nuclei and nine neutrons are released. These nine neutrons split nine more nuclei and release 27 neutrons. As this process continues the number of neutrons released increases in geometric progression and this process is called 'chain reaction'(Fig. 2). If this process is allowed to continue, with in a short duration a tremendous amount of energy will be released and causes disaster. To sustain the chain reaction the mass of uranium should be more than certain mass called 'critical mass.

Fig. 2, Nuclear Chain Reaction

Neutron multiplication factor 'K" and conditions for sustained chain reaction In the fission of uranium nuclei, on an average 2.5 neutrons are emitted per fission. The neutrons produced in a fission event are fast neutrons and are referred to as 'neutrons of first generation'. There is certain probability for some neutrons to escape without participating in further fission process. Therefore all emitted neutrons are not available for further fissions. The basic conditions for self sustained chain reaction is that at least one neutron should be available. The requirements are given below.

1- Fast neutrons should be changed into slow neutrons by passing through moderators. 2-  At least one thermal neutron should be available to initiate the fission reaction. 3- The state of the chain reaction depends on the neutron multiplication factor 'K' which is defined as the ratio of the number of neutrons in present generation to number of neutrons in the previous generation

When K < 1, the number of neutrons in successive generations decreases and the chain reaction cannot continue. This state is called 'sub-critical state'. If K = 1, the chain reaction will proceed at a steady rate and this state is called 'critical state'. If K > 1, the number of neutrons increases and the reaction is said to be 'supercritical'.