A-level Physics (Advancing Physics)/Fission

Nuclear fission is the splitting of the nucleus of a massive atom into smaller nuclei. This is used to produce energy in power stations and nuclear bombs.

Chain Reaction
In order to start nuclear fission, one nucleus must be made to split apart. This is achieved by getting the nucleus to absorb a slow-moving neutron. When the nucleus splits, it releases energy, two components, and possibly some more neutrons. If at least one neutron is released, then a chain reaction occurs. This neutron goes on to make another nucleus unstable, which splits, and produces more neutrons, and so on.

If this chain reaction is uncontrolled, a massive amount of energy is released very fast. This is an atomic explosion, which is used in nuclear bombs. In order to use nuclear fission in a power station, the number of neutrons released must be controlled by inserting a substance such as boron into the reactor, which absorbs the neutrons, preventing them from going on to make more nuclei split.

Binding Energy
The reason nuclear fission produces energy is that the binding energy of the original nucleus is greater than the binding energy of the products of the fission reaction. This difference in binding energy is the amount of energy released as photons (some of which are infra-red). This energy is used to heat up steam, pressurizing it, and enabling it to turn a turbine, producing electricity.

Neutron Moderator
Neutrons have to be moving slowly in order to cause a nucleus to become unstable and split. If they are moving too fast, then they simply bounce off. A neutron moderator (such as graphite or heavy water) is used to slow them down.

Questions
1. A neutron is fired at some Uranium-235. Barium-141 and Krypton-92 are produced:

$$_0^1n + _{92}^{235}U \to _{56}^{141}Ba + _{36}^{92}Kr + N_0^1n$$

How many neutrons are produced (i.e. what is the value of N)?

2. What proportion of the neutrons produced must be absorbed in order to make the reaction stable?

3. What would happen if too many neutrons were absorbed?

4. Alternatively, Uranium-235 can split into Xenon-140, two neutrons and another element. What is this element? (You will need to use a periodic table.)

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