Class 10 > Unit # 20:Nuclear Structure > Nuclear Fission

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Explain Nuclear Fission Reaction and Chain Reaction

Nuclear Fission Reaction

"The process in which a heavy nucleus breaks up into two lighter nuclei of nearly equal masses after bombardment by a slow neutron is known as nuclear fission."

Explanation

When an isotope of uranium ($_{92}^{235}\text{U}$) is bombarded with slow-moving neutrons, a nuclear fission reaction takes place. During this process, two new elements, three neutrons, and a large amount of energy are released. The resulting nuclei of the new elements produced are Barium and Krypton.

The nuclear fission reaction equation is represented as:

$_{92}^{235}\text{U} + _0^1\text{n} \rightarrow _{92}^{236}\text{U}^* \rightarrow _{56}^{144}\text{Ba} + _{36}^{89}\text{Kr} + 3(_0^1\text{n}) + \text{Energy ($200\text{ MeV}$)}$

Barium and Krypton are known as fission fragments (or fission products), which are typically radioactive. A large amount of heat energy is also liberated during the reaction.

Chain Reaction

In the fission process, the heavy nucleus absorbs a neutron and forms an unstable compound nucleus ($_{92}^{236}\text{U}^*$). This compound nucleus then breaks up roughly in the middle to yield fission products.

Furthermore, the neutrons released (typically three) from the fission of the first uranium atom can strike three other uranium atoms. In this manner, a chain reaction is established, resulting in the liberation of an enormous amount of energy. Uranium-236 formed during the process can break up in several different ways.

Because this fission process is self-multiplying, a tremendous amount of energy is released in an extremely short interval of time, leading to an explosion if uncontrolled. Thus, unmitigated nuclear fission is driven by an uncontrolled chain reaction.

Fig: Uncontrolled nuclear fission chain reaction showing neutron multiplication and release of fission fragments (Barium and Krypton).

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