Section 1 of 5What Fusion Is
Fusion releases energy when light nuclei join, and it powers the Sun and other stars.
Definition
- Two light nuclei join together to form a heavier nucleus, and energy is released in the process.
Fusion in stars
- Hydrogen nuclei fuse to form helium in the core of a star such as the Sun, supplying the energy that the star radiates.
Deuterium and tritium
- These are isotopes of hydrogen with one and two neutrons respectively;
- their fusion produces helium-4 and a neutron.
Mass into energy
- A very small amount of the mass of the reacting nuclei disappears and reappears as energy.
Energy per kilogram
- Each kilogram of fusion fuel releases far more energy than each kilogram of uranium undergoing fission.
Section 2 of 5Fusion Compared with Fission
Process
- Fusion joins two light nuclei, whereas fission splits one large nucleus into two smaller daughter nuclei.
Typical fuel
- Fusion uses hydrogen isotopes, while fission uses uranium-235 or plutonium-239.
Conditions needed
- Fusion needs extremely high temperature and pressure, while fission needs a neutron to be absorbed by a fissile nucleus.
Waste products
- Fusion produces much less long-lived radioactive waste than fission, which leaves unstable daughter nuclei.
| Feature | Fusion | Fission |
|---|---|---|
| Process | Light nuclei join | Heavy nucleus splits |
| Typical fuel | Hydrogen isotopes | Uranium-235, plutonium-239 |
| Conditions | Very high temperature and pressure | A neutron is absorbed by a fissile nucleus |
| Where found | Stars; experimental reactors on Earth | Nuclear power stations |
Section 3 of 5Advantages and Challenges
Advantages and Challenges
Fuel supply
- Deuterium can be obtained from water, and tritium can be produced from lithium, so the fuel is plentiful.
Energy output
- Fusion releases more energy per kilogram of fuel than fission does, which makes it attractive for future power generation.
Waste
- Fusion produces much less long-lived radioactive waste than fission reactors do.
Engineering challenge
- Maintaining the extreme temperatures and pressures needed on Earth is very difficult,
- so fusion power is not yet commercially established.
Section 4 of 5Worked Example
Question
- Complete the fusion equation: 2H + 2H gives 3He + ?.
- Use the mass numbers and atomic numbers to find the missing particle (deuterium has atomic number 1, helium-3 has atomic number 2).
- Step 1: Balance the mass numbers. Left side: 2 + 2 = 4. Right side so far: 3.
- Step 2: Missing mass number = 4 - 3 = 1.
- Step 3: Balance the atomic numbers. Left side: 1 + 1 = 2. Right side so far: 2.
- Step 4: Missing atomic number = 2 - 2 = 0, so the particle has mass number 1 and atomic number 0.
Answer A neutron (1n), so the full equation is 2H + 2H gives 3He + 1n.
Section 5 of 5Exam Checklist & Tips
7.3.4 – Key Points for the Exam
- Nuclear fusion The joining of two light nuclei to form a heavier nucleus, with energy released.
- Nuclear fission The splitting of a large, unstable nucleus into two smaller nuclei with energy released.
- Deuterium An isotope of hydrogen with one neutron in its nucleus.
- Tritium An isotope of hydrogen with two neutrons in its nucleus.
- Isotope Atoms of the same element with the same number of protons but different numbers of neutrons.
Common Mistakes
- Describing fusion as splitting nuclei; fusion joins two light nuclei together.
- Saying that fusion in the Sun needs no special conditions;
- it needs very high temperature and pressure in the core.
- Claiming that fusion reactors already generate electricity commercially on Earth.
- Finding the missing particle using only mass numbers and forgetting to balance the atomic numbers as well.