Nuclear Fission and Fusion — Important Questions

To absorb excess neutrons and control the rate of chain reaction

Step 1: In a nuclear reactor, the chain reaction must be carefully controlled to avoid a runaway reaction. Step 2: Control rods are made of neutron-absorbing materials such as cadmium or boron. Step 3: By inserting or withdrawing these rods, the number of neutrons available to cause further fission can be regulated. Step 4: When rods are pushed in more, they absorb more neutrons → reaction rate decreases. When pulled out, fewer neutrons are absorbed → reaction rate increases. Step 5: The moderator (heavy water or ordinary water) slows neutrons – this is a different function. The reflector refl

To give nuclei enough kinetic energy to overcome the Coulomb repulsion barrier

Step 1: In nuclear fusion, two positively charged light nuclei (e.g., two deuterons) must come close enough for the strong nuclear force to take over. Step 2: Since both nuclei are positively charged, they repel each other strongly – this is the Coulomb barrier. Step 3: At room temperature, nuclei do not have enough kinetic energy to overcome this repulsion. Step 4: At temperatures of ~10–20 million kelvin, the average kinetic energy of particles becomes high enough to allow nuclei to approach each other closely and fuse. Step 5: This is why fusion is called a thermonuclear reaction – 'thermo'

23.855 MeV

Step 1: The energy released Q in a nuclear reaction equals the difference in binding energies of products and reactants. Step 2: Q = BE(products) – BE(reactants). Step 3: Q = 28.295 MeV – 4.44 MeV = 23.855 MeV. Step 4: This energy is released because ⁴He is much more tightly bound than the two deuterons. The extra binding energy is released as kinetic energy of products (and gamma rays). Step 5: Note: This is approximately 24 MeV, giving ~6 MeV per nucleon – which is about 7 times higher than the energy per nucleon from fission of ²³⁵U (~0.83 MeV per nucleon).

Nuclear fusion of hydrogen nuclei into helium at very high temperatures

Step 1: The Sun mainly consists of hydrogen and helium gases, so fission of heavy elements cannot be the source of its energy. Step 2: The enormous gravitational mass of the Sun compresses its core to produce temperatures of ~20 million kelvin. Step 3: At such extreme temperatures, hydrogen nuclei (protons) gain enough kinetic energy to overcome Coulomb repulsion and fuse. Step 4: The overall reaction is: 4¹₁H → ⁴₂He + 2 positrons + 26.8 MeV. Step 5: This thermonuclear fusion process releases tremendous energy. The Sun has enough hydrogen to continue shining for about 8 billion more years.