Nuclear Fission

One of the most practical nuclear reaction is the formation of a compound nucleus when a nucleus with A > 230 absorbs an incident neutron. Many of these compound nuclei will then split into two medium –mass nuclear fragments and additional neutrons. This type of reaction is called" nuclear fission". In a nuclear reactor , the number of fissions per unit time is controlled by the absorption of excess neutrons so that , on the average , one neutron from each fission produces a new fission. The liberated heat is used to make steam to drive turbines and generate electrical power.

If the reaction is uncontrolled , so that each fission results in more than one neutron capable of reproducing further fissions , the number of fissions will increase geometrically, resulting in all the energy of the source being released over a short time interval , producing a " nuclear bomb“. A typical fission reaction is:

The ratio of the masses of the fission fragments , M1/ M2 , is found experimentally to be roughly 3/2. the number of ε neutrons released in the fissioning at a particular element will depend upon the final fragments that are produced . For the above reaction the average number of neutrons released in a fission is found experimentally to be about 2.44, the fractional number resulting from an average taken over all reaction products .

The two decay fragments usually have a neutron –proton ratio approximately equal to that of the original nucleus. Therefore , they lie above the stability curve , in a region where nuclei are neutron–rich and undergo beta decay. Usually it will require a chain of several beta decays , each decay reducing the N/Z ratio , before a stable nucleus is reached.

A fission reaction liberates about 200Mev of energy for each fission A fission reaction liberates about 200Mev of energy for each fission.This is much greater than the few MeV released in a typical exothermic reaction where the final products include only one particle comparable in mass to the original target nucleus. This 200Mev is distributed as follows :

170 MeV is kinetic energy of fission fragments . 5 MeV is the combined kinetic energy of fission neutrons. 15 MeV is β-1 and γ-ray energy. 10 MeV is neutrino energy librated in the β-1 decays of fission fragments. In many fission reaction the formation of the compound nucleus occurs most readily with thermal neutrons of energy E= 0.04 eV. from the above it is seen that the neutrons released in atypical fission reaction have large kinetic energies of about 2 MeV.

Nuclear Fusion

The fusion reaction is one in which two nucleons or relatively light ( A < 20 ) nuclei combine to form a heavier nucleus, with a resulting release of energy . An example of a fusion reaction is the formation of a deuteron from a proton and a neutron Another fusion reaction is the formation of an α-particle by the fusion of two deuterons:

Although these energies are much smaller then the energy released in a typical fission reaction ( ≈ 200 MeV ) the energy per unit mass is larger because of the smaller masses of the participating particles. The reaction that seem most promising for use in the first practical fusion reactor are the D-D reactions:

And the D-T reaction: Reaction series known as the carbon or bethe cycle and the proton-proton or Critchfield cycle are believed to occur in the stars .