Question

Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

Hint:

To remember the distinction: "Fission" sounds like "fissure" (a split or crack), meaning breaking apart. "Fusion" means "fusing" things together into a single whole. Both processes release energy because the final products have a higher binding energy per nucleon than the initial reactants.

Answer 1

Step 1: Meaning of the two processes:
Both processes release energy because the binding energy per nucleon increases.
Fission splits one heavy nucleus. Fusion joins two light nuclei.

Step 2: Differences between nuclear fission and nuclear fusion:
1. Process: In fission, a heavy nucleus splits into two (or more) lighter nuclei of comparable mass. In fusion, two light nuclei combine to form one heavier nucleus.
2. Starting material: Fission uses heavy nuclei such as \( {}^{235}_{92}\text{U} \) or \( {}^{239}_{94}\text{Pu} \). Fusion uses light nuclei such as isotopes of hydrogen.
3. Trigger: Fission starts when a slow (thermal) neutron hits the nucleus. Fusion needs a temperature of about \( 10^{7} \) K or more so that the nuclei overcome their electrostatic repulsion.
4. Chain reaction: Fission can be a self-sustained chain reaction, which must be controlled in a reactor. Fusion is not a chain reaction.
5. Energy per unit mass: Fusion releases more energy per unit mass of fuel than fission.
6. Waste: Fission leaves radioactive fragments. Fusion gives mostly stable products.

Step 3: Example of nuclear fission:
A slow neutron is absorbed by uranium-235. The nucleus splits into barium and krypton and releases three neutrons.
\[ {}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \longrightarrow {}^{144}_{56}\text{Ba} + {}^{89}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} + \text{energy} \]
Check: mass number \( 235 + 1 = 144 + 89 + 3 = 236 \). Charge number \( 92 = 56 + 36 \).

Step 4: Example of nuclear fusion:
Two deuterium nuclei fuse to form helium-3 and a neutron.
\[ {}^{2}_{1}\text{H} + {}^{2}_{1}\text{H} \longrightarrow {}^{3}_{2}\text{He} + {}^{1}_{0}\text{n} + 3.27\ \text{MeV} \]
Check: mass number \( 2 + 2 = 3 + 1 \). Charge number \( 1 + 1 = 2 + 0 \).

Final Answer:
Fission splits a heavy nucleus (example: \( {}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \to {}^{144}_{56}\text{Ba} + {}^{89}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} \)). Fusion joins light nuclei (example: \( {}^{2}_{1}\text{H} + {}^{2}_{1}\text{H} \to {}^{3}_{2}\text{He} + {}^{1}_{0}\text{n} + 3.27 \) MeV).