Nuclear Fission

Neutrons are considered as the best projectile particles to hit the target (uranium) nucleus. The uranium may absorb these neutrons and become ₉₂U²³⁶which later splits into two smaller nuclei. In addition two neutrons are released and energy is also released. This process is called a Nuclear fission.

Recall that nuclear forces are operative up to the distances of the order of few fermi. What happens to large nuclei like uranium? Are the forces still strong to hold the protons? The answer is, the forces tend to lose their hold on the protons as the range or distance between the protons has increased. This makes such nuclei unstable. They become more stable by losing an alpha or beta particle.

Nuclear fission is a phenomenon of splitting of a heavy nucleus into two or more light nuclei. Hahn and Strassman discovered that when ₉₂U²³⁵ is bombarded with thermal neutrons, it splits up into ₅₆Ba¹⁴¹ and ₃₆Kr⁹² with emission of three neutrons along with 200 MeV of energy per fission. This energy appears in the form of g -rays.

Sub Topics

Controlled and Uncontrolled Nuclear Reactions
(i) The secondary neutrons can be made to escape out of the system.

Controlled and Uncontrolled Nuclear Reactions

Once the nucleus has been divided by fission, the neutrons that are emitted can strike other nuclei. This energy released is extremely large.

The neutrons produced during the fission are called secondary neutrons. These neutrons may bring about the fission of three more ₉₂U²³⁵ nuclei and produce nine neutrons at the same instant. If this reaction proceeds at this rate, a huge amount of energy is released in a short time. This is the uncontrolled nuclear fission reaction, which if left free would be disastrous. This uncontrolled fission leads to an explosion. This is the principle behind the working of an atom bomb.

To control the chain reaction and make it self-sustaining the following points should be considered:

(i) The secondary neutrons can be made to escape out of the system.

(ii) The neutrons can be absorbed by substances which are not fissionable. Rods of boron or cadmium have the ability to absorb the desired number of neutrons. This ensures limited number of neutrons to be available for fission.

(iii) The speed of the neutrons can be slowed down appropriately. This is slowed down by the use of moderators like heavy water, graphite etc.

(iv) To sustain the chain reaction, the number of neutrons lost due to leakage and absorption should be smaller than the number produced. For this, the size of the uranium block should be greater than a certain critical value called critical size.