21.2 - Nuclear Fission

What nuclear fission is

• Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium) into two smaller nuclei, releasing energy.
• Spontaneous fission - Where a nucleus splits on its own - is rare. Usually the nucleus has to absorb a neutron first.

How fission is triggered

• A slow-moving neutron is absorbed by the nucleus, making it even more unstable and causing it to split.
• A neutron is absorbed easily because it has no charge, so it isn't repelled by the positive nucleus.
• Nuclei like uranium-235 that can absorb slow neutrons this way are described as fissile.

What fission produces

• The heavy nucleus splits into two lighter daughter nuclei, roughly the same size as each other. These are usually radioactive.
• Two or three neutrons are also released, along with a large amount of energy.
• Most of the energy ends up as kinetic energy of the fast-moving fission products; some is carried away as gamma radiation.

$${}_{92}^{235}\text{U}+{}_0^1\text{n}\to {}_{56}^{141}\text{Ba}+{}_{36}^{92}\text{Kr}+3{}_0^1\text{n}+\text{energy}$$

Mass and energy in a fission reaction

• A fission equation must balance: total mass number on the left = total mass number on the right, and total proton number on the left = total proton number on the right.
• Counting the nucleons on each side lets you work out the number of neutrons released.

Balancing an equation

$${}_{92}^{235}\text{U}+{}_0^1\text{n}\to {}_{36}^{91}\text{Kr}+{}_{56}^{143}\text{Ba}+2{}_0^1\text{n}+{}_0^0\gamma$$

• Mass numbers balance: 235 + 1 = 91 + 143 + 2 + 0
• Proton numbers balance: 92 + 0 = 36 + 56 + 0 + 0

Where the energy comes from

• The total mass after fission is slightly lower than before, because some mass is converted into energy.
• Some of this energy becomes kinetic energy of the products (the lighter nuclei and the neutrons); some is carried away by gamma rays.

Generating electricity from fission

1. The energy released by fission heats water into steam.
2. The steam drives a turbine, which turns a generator to produce electricity.

Using a coolant

• In many reactors the energy isn't passed straight to the water.
• A coolant (such as carbon dioxide) is pumped around the reactor core to carry heat to a heat exchanger, where it boils the water into steam.
• This keeps the water supply separate from the reactor core.