Nuclear Fission and Nuclear Fusion

The same energy source, two ways

About half of Ontario's electricity comes from nuclear fission: splitting uranium nuclei in reactors at Pickering, Darlington and Bruce. The sunlight that grows the maple trees and melts the snow in spring comes from nuclear fusion: joining hydrogen nuclei deep inside the Sun.

One process splits big nuclei apart; the other squeezes small nuclei together. Yet both release energy. How can that be? This lesson explains it, and compares the two.

Why both can release energy

In The Nucleus and Isotopes you learned that the strong nuclear force holds nucleons together. The binding energy of a nucleus is the energy you would need to pull it completely apart into separate protons and neutrons. The larger the binding energy per nucleon, the more tightly bound and stable the nucleus.

Binding energy per nucleon is highest for medium-sized nuclei around iron-56. It is lower for very light nuclei and lower again for very heavy ones.

• If a heavy nucleus like uranium splits into two medium nuclei, the products are more tightly bound. The difference is released as energy: fission.
• If two very light nuclei like hydrogen isotopes join to form helium, the product is more tightly bound. Again energy is released: fusion.

In both cases the products have slightly less mass than the starting particles. The missing mass has become energy, as described by Einstein's E = mc², which you work with in the sibling unit Work, Energy and Power.

Nuclear fission

Uranium-235 can be split by a slow-moving neutron. The neutron is absorbed, making uranium-236, which is so unstable that it wobbles and splits, usually within a tiny fraction of a second, into two medium-sized nuclei called fission fragments. It also releases 2 or 3 new neutrons and a large amount of energy (about 200 MeV per fission), mostly as kinetic energy of the fragments. One typical fission is:

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

Check: mass numbers 1 + 235 = 141 + 92 + 3 = 236 ✓; atomic numbers 0 + 92 = 56 + 36 + 0 = 92 ✓.

The fragments are not always barium and krypton; many different pairs are possible. The fragments are usually radioactive (they have too many neutrons), which is why used reactor fuel is highly radioactive.

Chain reactions

Each fission releases 2 or 3 neutrons. If, on average, at least one of them causes another fission, the process keeps itself going: a chain reaction.

• If each fission causes exactly one more fission on average, the reaction runs at a steady rate. This is what a nuclear reactor does.
• If each fission causes more than one, the number of fissions grows extremely fast. This is uncontrolled.
• If each causes fewer than one, the reaction dies out.

A chain reaction needs enough fissionable material packed closely together, the critical mass; otherwise too many neutrons escape. Reactors control the reaction with control rods or other devices made of materials that absorb neutrons (such as cadmium or boron), and with a moderator that slows neutrons down so U-235 can capture them. You will see how a CANDU reactor does this in The CANDU Reactor: Inside a Nuclear Power Plant.

Nuclear fusion

In fusion, two light nuclei join to form a heavier one. Both nuclei are positively charged, so they repel strongly. To get close enough for the strong force to grab them, they must collide at enormous speeds, which means extremely high temperatures (millions of degrees) and high density. At these temperatures matter is a plasma.

In the Sun: gravity squeezes the core to very high density, and the temperature is about 15 million °C. In a series of steps, hydrogen nuclei (protons) fuse to make helium-4. Overall, four hydrogen nuclei become one helium nucleus, and the lost mass becomes the energy that eventually reaches Earth as sunlight.

In fusion research on Earth, the easiest reaction is deuterium-tritium (D-T) fusion:

²₁H + ³₁H → ⁴₂He + ¹₀n + energy (17.6 MeV)

Check: 2 + 3 = 4 + 1 ✓ and 1 + 1 = 2 + 0 ✓.

Scientists in many countries, including the large international ITER project being built in France, are working to hold a plasma hot enough, dense enough and long enough to produce more energy than it takes to run. This has not yet been achieved in a power plant. Hydrogen bombs use uncontrolled fusion; controlled fusion for electricity is still in the research stage.