Fission & Fusion

Definition

Nuclear fission is the splitting of a large unstable nucleus into two smaller nuclei

• The large nucleus that breaks apart is called the parent nucleus
• The two smaller nuclei produced are the daughter nuclei
• The reaction also produces 2 or 3 free neutrons and a burst of gamma radiation
• The daughter nuclei are themselves unstable and usually radioactive, decaying further over time

Fissile materials

• Only certain isotopes will undergo fission. These are called fissile materials. The two used in practice are:
  • Uranium-235 (²³⁵₉₂U), the fuel in almost every commercial fission reactor
  • Plutonium-239 (²³⁹₉₄Pu), used in some reactors and in most nuclear weapons; produced inside reactors from non-fissile uranium-238

Spontaneous vs induced fission

• A small number of nuclei split all on their own, without anything hitting them. This is spontaneous fission. It is rare and far too slow to be useful for generating electricity
• The fission used in reactors is induced fission, where a free neutron is fired at the nucleus and triggers the split:
  1. A slow-moving (thermal) neutron is absorbed by a uranium-235 nucleus
  2. The nucleus becomes uranium-236, which is highly unstable
  3. Within a tiny fraction of a second, uranium-236 splits into two daughter nuclei and emits 2 or 3 fresh neutrons plus gamma rays

Why uranium-235 needs help to fission

• Uranium-235 has a half-life of about 700 million years, so on its own it decays incredibly slowly. The amount of energy released by spontaneous fission is far too small to power a reactor
• Adding the extra neutron pushes uranium-235 over the edge into uranium-236, which has no stable existence and falls apart almost immediately. Inducing fission speeds up the energy release by many orders of magnitude

A typical fission equation

• One of many possible fission outcomes for uranium-235 is:

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy

• Check the totals:
  • Mass numbers: 235 + 1 = 236 on the left; 141 + 92 + (3 × 1) = 236 on the right ✓
  • Atomic numbers: 92 + 0 = 92 on the left; 56 + 36 + (3 × 0) = 92 on the right ✓
• The daughter nuclei vary from one fission event to the next. Barium-141 and krypton-92 are one possibility; strontium-90 and xenon-143 another. The reactor does not produce a single pair of products every time

Where the energy goes

• The products of fission fly apart at very high speed. The energy that was locked up inside the parent nucleus reappears as the kinetic energy of the fast-moving daughter nuclei and neutrons, plus the energy carried away by gamma rays
• When the fast-moving daughter nuclei collide with surrounding atoms in the fuel, their kinetic energy is transferred as heat. That heat is what is harvested to make electricity in a power station

Example — During an induced fission reaction, a uranium-235 nucleus splits to form one strontium-90 nucleus and one other daughter nucleus, releasing 3 neutrons:

²³⁵₉₂U + ¹₀n → ⁹⁰₃₈Sr + ?? + 3 ¹₀n

What is the missing daughter nucleus?

• Step 1 — Balance the mass numbers: 235 + 1 = 90 + A + 3, so A = 143
• Step 2 — Balance the atomic numbers: 92 + 0 = 38 + Z + 0, so Z = 54
• Step 3 — Element with Z = 54 is xenon (Xe)
• The missing daughter nucleus is ¹⁴³₅₄Xe