Nuclear Fusion Worksheet - What elements in the Sun are fused ... - Free Printable

Nuclear Fusion Worksheet Solutions

#### 1. How is nuclear fusion different to nuclear fission?
- Nuclear Fusion: Fusion is the process where two light atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy. This process occurs at extremely high temperatures and pressures, such as in the core of the Sun.
- Nuclear Fission: Fission is the process where a heavy atomic nucleus splits into two or more lighter nuclei, also releasing a significant amount of energy. This process typically occurs when a neutron collides with a heavy nucleus like uranium or plutonium.

Key Differences:
- Energy Source: Fusion uses light elements (e.g., hydrogen isotopes), while fission uses heavy elements (e.g., uranium).
- Temperature and Pressure: Fusion requires extremely high temperatures and pressures, whereas fission can occur at lower temperatures.
- Byproducts: Fusion produces fewer radioactive byproducts compared to fission, which generates long-lived radioactive waste.

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#### 2. Nuclear fusion powers the Sun. What elements in the Sun are fused together in the Sun?
In the Sun, the primary fusion process involves the fusion of hydrogen nuclei (protons) into helium nuclei. Specifically, four hydrogen nuclei (hydrogen-1, or protium) fuse to form one helium-4 nucleus.

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#### 3. What is produced when these elements fuse?
When four hydrogen nuclei fuse together in the Sun, the following products are formed:
- Helium-4 (He): The primary product of the fusion reaction.
- Energy: A significant amount of energy is released in the form of gamma rays and other particles due to the conversion of a small amount of mass into energy (as described by Einstein's equation \( E = mc^2 \)).
- Neutrinos: High-energy neutrinos are also produced during the fusion process.
- Positrons: In intermediate steps, positrons (anti-electrons) are created, which annihilate with electrons to produce additional energy.

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#### 4. How is the Sun able to produce nuclear fusion?
The Sun is able to sustain nuclear fusion due to the extreme conditions in its core:
- High Temperature: The core temperature of the Sun is approximately 15 million Kelvin. At such high temperatures, hydrogen nuclei gain enough kinetic energy to overcome the electrostatic repulsion between their positive charges and come close enough to fuse.
- High Pressure: The immense gravitational pressure from the Sun's mass compresses the core, bringing hydrogen nuclei extremely close together.
- Density: The density in the Sun's core is very high, ensuring that there are sufficient hydrogen nuclei for fusion reactions to occur frequently.
- Stability: The balance between the inward gravitational force and the outward pressure from fusion reactions maintains the Sun's stability over billions of years.

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#### 5. JET is an experimental fusion reactor near Oxford. It has not been able to produce large amounts of nuclear fusion. Why is it so difficult to get fusion reactions to work?
Fusion reactions are challenging to achieve on Earth for several reasons:
- High Temperatures: Fusion requires temperatures of tens of millions of degrees Celsius, similar to those in the Sun's core. Achieving and maintaining such temperatures is technically difficult.
- Containment: Fusion fuel must be contained for a sufficient duration to allow fusion reactions to occur. On Earth, this is typically done using magnetic confinement (e.g., tokamaks like JET) or inertial confinement (e.g., laser-driven fusion). Containing plasma (ionized gas) at such high temperatures without it touching the reactor walls is a major challenge.
- Fuel Stability: Sustaining a stable plasma state is difficult because the plasma tends to cool down or become unstable, disrupting the fusion process.
- Net Energy Output: Current fusion reactors, like JET, have not yet achieved "breakeven," where the energy output from fusion exceeds the energy input required to maintain the reaction. This is a critical milestone for practical fusion power generation.

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#### 6. The main fusion reaction that could be used here on Earth is:
\[ {}_1^2H + {}_1^3H \rightarrow {}_2^4He + {}_0^1n \]
Explain what this reaction means.

This reaction is known as the deuterium-tritium (D-T) fusion reaction. Here's a breakdown:
- Reactants:
- \( {}_1^2H \): Deuterium (a stable isotope of hydrogen with one proton and one neutron).
- \( {}_1^3H \): Tritium (an unstable isotope of hydrogen with one proton and two neutrons).
- Products:
- \( {}_2^4He \): Helium-4 (a stable nucleus with two protons and two neutrons).
- \( {}_0^1n \): A neutron, which carries away a significant portion of the energy released in the reaction.
- Energy Release: This reaction releases a large amount of energy due to the conversion of mass into energy, as described by \( E = mc^2 \). The mass of the reactants is slightly greater than the mass of the products, and the difference is converted into energy.

This reaction is favored for terrestrial fusion because:
- Deuterium is abundant in seawater.
- Tritium can be bred from lithium in the presence of neutrons.
- The reaction has a relatively low threshold energy compared to other fusion reactions.

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#### 7. What would be the source of the fuel required by a fusion reactor?
The primary fuels for a fusion reactor would be:
- Deuterium: Deuterium is a naturally occurring isotope of hydrogen found in water. Since water is abundant on Earth, deuterium is a virtually inexhaustible resource.
- Tritium: Tritium is not naturally abundant but can be produced artificially. It can be generated by bombarding lithium with neutrons (from the D-T fusion reaction itself) in a breeder blanket surrounding the fusion reactor. Lithium is also abundant on Earth.

Thus, the main sources of fuel for a fusion reactor would be:
- Deuterium: Extracted from seawater.
- Lithium: Used to breed tritium within the reactor.

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Final Answer:

\[
\boxed{
\begin{array}{l}
\text{1. Fusion combines light nuclei; fission splits heavy nuclei.} \\
\text{2. Hydrogen nuclei (protons) are fused in the Sun.} \\
\text{3. Helium, energy, neutrinos, and positrons are produced.} \\
\text{4. High temperature, pressure, and density in the Sun's core enable fusion.} \\
\text{5. Difficulty arises from achieving and sustaining high temperatures and containment.} \\
\text{6. Deuterium and tritium fuse to form helium and a neutron, releasing energy.} \\
\text{7. Deuterium from seawater and lithium for breeding tritium.}
\end{array}
}
\]