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A chain reaction is a fascinating process where one event sets off a series of subsequent events. In nuclear physics, this involves a sequence of nuclear fissions, or splits of atomic nuclei, which release energy. These fissions occur when a neutron collides with a nucleus of a fissile material such as uranium-235 or plutonium-239.
The fascinating part is when this collision causes the nucleus to split, additional neutrons are released. These newly freed neutrons may collide with other fissile nuclei, causing them to split as well.
To put it simply, a single reaction can spark off many more, much like a row of falling dominos:

• A neutron hits a fissile nucleus.
• The nucleus splits, releasing energy and more neutrons.
• These neutrons hit other fissile nuclei, continuing the process.

This explains why controlling a chain reaction is crucial in nuclear reactors to avoid potentially dangerous, uncontrolled energy release.

A nuclear reaction involves a change in an atom's nucleus and is a process that results in the transformation of atomic species into another. This is not to be confused with chemical reactions which only involve electrons around the nucleus.
Nuclear reactions can occur naturally, such as in the sun where hydrogen nuclei fuse to form helium. In laboratories or reactors, they can be induced artificially.
Here are some distinctive features of nuclear reactions:

• They typically result in a large energy release.
• They involve exchange or interactions of nucleons (protons and neutrons).
• They can involve nuclear fission (splitting nuclei) or fusion (combining lighter nuclei).

By understanding nuclear reactions, we harness nuclear power for electricity and explore future possibilities like fusion energy, which powers stars, including our sun.

Fissile materials are the key ingredients to sustaining a nuclear chain reaction. They are unique because their atomic nuclei can be split, or 'fissioned,' when impacted by low-energy neutrons.
Examples of common fissile materials include uranium-235 and plutonium-239. These materials are vital in both nuclear reactors and nuclear weapons due to their ability to maintain a chain reaction once initiated.
Some critical characteristics of fissile materials:

• They must have the ability to sustain a chain reaction (critical mass).
• They are generally isotopes of elements, having the capacity to undergo nuclear fission.
• They require precise management to ensure safety and control.

Understanding the nature of fissile materials helps scientists and engineers safely design reactors that provide low-carbon energy and secure environments to deter weapon proliferation.