Nuclear fission and nuclear fusion

Fission

Process of splitting the atom

1. Make the nucleus absorb a neutron.
2. It becomes unstable, and splits.
3. Neutrons are released, causing a chain reaction.

How is energy released?

• The kinetic energy of the fission products and neutrons,
• the energy released when the fission products decay in the future,
• neutrinos produced (this energy is lost),
• γ-rays produced at time of fission.

Parts of a fission reactor

Fuel rod	Enriched uranium which gets hot as it reacts
Coolant	Carbon dioxide gas which passes the energy on. Heated by the fuel rods while being pumped about by the circulators, it super-heats the water passing through pipes near the reactor into steam (which goes on to drive the turbines).
Control rods	Keeps the chain-reaction critical (under control). For a good reactor, we want decay after decay, not an exponentially increasing number of decays. About 1.4 neutrons must be lost each decay for this. Made of neutron-absoring material such as boron. Lowered into the chamber to absorb neutrons and slow reactions (and vice versa).
Moderator	Slows down fast neutrons to increase the likelihood of a reaction. Done by allowing neutrons to collide with with similar-sized atoms, the hydrogen in usually heavy water or graphite. Hydrogen gas is no good, as a head-on elastic collision would rob the neutron of all KE.

Fusion

1. Heat the gas to about 10⁸ K.
2. Electrons break away from their atoms, forming a plasma and ionising the atoms.
3. A neutron will collide with the atom in the plasma. It must be going fast as it has to overcome the repulsive ion and get within 1 fm of the nucleus, ∴ initial KE ≥ PE at 1 fm from the atom.