(NCM) Fission vs. Fusion Lesson

Fission vs. Fusion

Nuclear Fission

Nuclear fission is the splitting of the nucleus of a radioactive atom into two smaller nuclei. This type of reaction releases a great deal of energy from a very small amount of matter. Fission occurs so that large nuclei can become more stable by splitting into smaller fragments.

Watch the video below for an explanation of fission and chain reactions.

To summarize the information from the video, here are some of the main ideas you should have taken away:

All elements beyond bismuth in the periodic table are radioactive.
Bombarding an atom with neutrons can alter its nuclear and atomic structure.
In the process of nuclear fission, an unstable nucleus splits into two smaller atoms in a reaction that releases energy.
A nuclear chain reaction occurs when one reaction causes additional reactions. For example, a neutron can cause a uranium atom to undergo fission, which releases energy and several more neutrons, which then split more uranium atoms and release more energy and neutrons. The process continues until all the uranium is spent.

The picture below shows how nuclear fission of uranium-235 occurs. It begins when a uranium nucleus gains a neutron. This can happen naturally when a free neutron strikes it, or it can occur deliberately when a neutron is crashed into it in a nuclear power plant. In either case, the nucleus of uranium-235 becomes extremely unstable with the extra neutron. As a result, it splits into two smaller nuclei, krypton-92 and barium-142. The reaction also releases two neutrons and a great deal of energy. It can be represented by this nuclear equation:

$LaTeX: ^{235}_{92}$ $^{235}_{92}$ U + LaTeX: ^1_0 $^1_0$ n $LaTeX: \rightarrow ^{92}_{36}$ $\rightarrow ^{92}_{36}$ Kr + $LaTeX: ^{142}_{56}$ $^{142}_{56}$ Ba + 2 $^1_0$ n + energy

Fission Example image

Typical nuclear fission reactions balance in terms of mass. Looking at the equation above, you can see that the total mass of the reactants is equal to the total mass of the products. There are a total of 236 mass units on the left of the equation and 236 mass units on the right. In the same manner, we see 92 protons on the left and 92 on the right. The energy that is released is the binding energy that holds the nucleus together.

Because the fission process produces more neutrons, a chain reaction can result. A chain reaction is a reaction in which the material that starts the reaction is also one of the products and can start another reaction. Illustrated below is a nuclear chain reaction for the fission of uranium-235.

Nuclear Chain Reaction of uranium breaking into two other elements and then multiplying

Atomic Bomb

In an atomic bomb, or fission bomb, the nuclear chain reaction is designed to be uncontrolled, releasing huge amounts of energy in a short amount of time. A schematic for one type of fission bomb is shown below. A critical mass of fissionable plutonium is contained within the bomb, but not at a sufficient density. Conventional explosives are used to compress the plutonium, causing it to go critical and trigger a nuclear explosion.

Atomic Bomb: aluminum pusher; plutonium core, polonium-berylium initiator, conventional explosives, uranium-238 tamper

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