Nuclear fission and chain reactions

What fission is

Nuclear fission is the splitting of one large, unstable nucleus into two smaller nuclei, releasing energy and free neutrons.

• Fission is usually induced: a slow-moving neutron is absorbed by a heavy nucleus such as uranium-235, making it so unstable that it splits almost immediately.
• The two smaller nuclei produced are called daughter nuclei or fission fragments. They are not always the same two — uranium-235 can split in many different ways.
• Two or three fast neutrons are released each time.
• A large amount of energy is released, mostly as the kinetic energy of the fast-moving fragments.

Writing a fission equation

A typical induced fission of uranium-235:

$$^{1}_{0}\text{n} + {}^{235}_{92}\text{U} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n}$$

• Check the mass numbers (top): $1 + 235 = 236$ on the left; $141 + 92 + 3(1) = 236$ on the right. ✓
• Check the atomic numbers (bottom): $0 + 92 = 92$ on the left; $56 + 36 + 3(0) = 92$ on the right. ✓
• Exactly the same two conservation rules you used for alpha and beta decay apply here — mass number and atomic number are both conserved.

Chain reactions

Because each fission releases neutrons and is triggered by a neutron, one fission can cause several more.

• A chain reaction is a self-sustaining sequence of fissions, where the neutrons from one fission go on to cause further fissions.
• If more than one neutron per fission goes on to cause another fission, the reaction grows — the number of fissions, and so the power output, increases rapidly. This is what happens in a nuclear weapon.
• If exactly one neutron per fission goes on to cause another, the reaction is steady and the power output is constant. This is what a nuclear reactor is designed to maintain.
• If fewer than one does, the reaction dies out.

Controlling a chain reaction in a reactor

• Control rods (boron or cadmium) absorb neutrons without undergoing fission. Pushing them further in absorbs more neutrons and slows the reaction; pulling them out speeds it up.
• A moderator (often water or graphite) slows the fast neutrons down, because slow neutrons are much more readily absorbed by uranium-235 — without it, most neutrons would escape without causing fission.
• Shielding (thick concrete and steel) absorbs the radiation released so it does not escape into the surroundings.