Unizor - Physics4Teens - Energy - Energy of a Nucleus - Fission

Notes to a video lecture on http://www.unizor.com

Nucleus Fission

Fission, first of all, is a nuclear reaction, when heavier nuclei are split into lighter ones.
The reason for this reaction to release the energy is the difference between amount of energy needed to break strong forces that hold the nucleus together (this energy is consumed by fission) and amount of potential energy in positively charged and repelling protons inside nucleus (this energy is released by fission).
The former is less than the latter.

When the heavy nucleus is broken into parts, the excess of potential energy of squeezed together protons against their repelling force over the energy of strong forces that keep nucleus together is converted into thermal and electromagnetic field energy.

Analogy of this is a spring squeezed tightly and held in this position by a thread. A thread plays the role of strong forces, while a potential energy of a squeezed spring plays the role of protons kept close to each other by a this force. When you cut a thread, the spring will release the potential energy, similarly to protons repelling from each other.

Electrically positively charged protons repel each other and, at the same time, are bonded together by strong forces inside a nucleus. At the same time neutrons are also bonded by strong forces among themselves and with protons without any repulsion.
So, the more neutrons the nucleus has - the stronger it is. Neutrons only add "bonding material" to a nucleus without adding any repelling forces that work against the nucleus' stability.

Uranium-238 with 92 protons and 146 neutrons (⁹²U²³⁸) naturally occurs on Earth and is relatively stable.
Uranium-235 with the same 92 protons and 143 neutrons (⁹²U²³⁵) has less "bonding material" (less neutrons) and is more susceptible to fission.

All it takes to break the nucleus of ⁹²U²³⁵ is a little "push" from outside, which can be accomplished by bombarding it with neutrons. In the process of fission, caused by hitting a nucleus of ⁹²U²³⁵ with a neutron, it can transforms into Barium-141 with 56 protons and 85 neutrons ⁵⁶Ba¹⁴¹, Krypton-92 with 36 protons and 56 neutrons ³⁶Kr⁹² and 3 free neutrons.
As we see, the numbers of protons is balanced (input: 92, output: 56 and 36), as well as a number of neutrons (input: 1 free hitting neutron and 143 in a nucleus of ⁹²U²³⁵ total 144, output: 85 in a nucleus ⁵⁶Ba¹⁴¹, 56 in a nucleus of ³⁶Kr⁹² and 3 new free neutrons total 144).

Let's express this reaction in a formula (letter n denotes a neutron):
⁰n¹ + ⁹²U²³⁵ =
⁵⁶Ba¹⁴¹ + ³⁶Kr⁹² + 3·⁰n¹

What's interesting in this reaction is that it not only produces energy because we break a heavy nucleus into lighter ones, but also that it produces 2 new neutrons that can bombard other atoms, causing a chain reaction and, potentially, an explosion (atomic bomb). However, if we absorb extra neutrons, it will allow to slowly release of nuclear energy (nuclear power stations).