Episode 528: Controlling fission

In this episode, you can look at the different features of the core of a nuclear reactor, and explain its operation using your students’ knowledge of nuclear physics.

Cut away of a Magnox nuclear reactor


  • Discussion: The construction of a nuclear reactor (10 minutes)
  • Discussion: Moderation (10 minutes)
  • Discussion: Enrichment (10 minutes)
  • Discussion: Critical mass (10 minutes)
  • Discussion: Control rods and coolant (10 minutes)
  • Student questions: Power reactors (30 minutes)
  • Discussion: Nuclear fusion (15 minutes)
  • Student questions: Fusion calculations (30 minutes)

Discussion: The construction of a nuclear reactor
Look at a diagram or animation of a nuclear reactor. Check what your students already know about the reactor’s construction.

Episode 528-1: Nuclear fission reactor (Word, 61 KB)

Discussion: Moderation
How can we make it more likely that a neutron will collide with a 235U nucleus? There are two ways, both used in nuclear power reactors:

Slow down the fast neutrons to increase their chance of being captured by a fissile 235U nucleus. This process is called moderation.

Concentrate the 235U compared to the 238U. This process is called enrichment.

The speed of the fast fission neutrons is slowed down (‘moderated’) by allowing them to collide with a suitable moderator nucleus. Conservation of momentum tells us that the speed of a light neutron colliding with a massive nucleus will be little affected. We need a material with relatively light nuclei to absorb momentum and energy from the neutron.

Look at the periodic table for some ideas:

  • Hydrogen – i.e. protons. Virtually the same mass (great), but gaseous (not very dense) and explosive. Hydrogen in water maybe? Yes, pressurised water reactors use water as the moderator (as well as the coolant), but the protons are attached to the rest of the water molecule and have an effective mass of 18 times that of a free proton
  • Helium – inert (good) but gaseous, so not dense enough
  • Lithium – too rare (expensive), melting point too low anyway
  • Beryllium – possible but expensive
  • Boron absorbs neutrons
  • Carbon – mass equivalent to 12 protons, solid (good), flammable (bad). Used in the first generation of UK ‘Magnox’ reactors

So there are a number of possibilities, each with a balance of advantages and disadvantages.

Discussion: Enrichment
Nuclear power stations use uranium enriched to typically 2.5% - a factor of 2.5/0.7 = 3.6 times the proportion found in natural uranium. Ask your students how much 238U must be discarded to produce 1 tonne of enriched uranium, i.e. with the fraction of 235U increased from 0.7% to 2.5%. (You need 3.6 tonnes of natural uranium, so you discard 2.6 tonnes of 238U.)

Bombs require 90% enrichment. Power station enrichment can be easily extended to get pure fissile 235U. Herein lies an easy route to the proliferation of nuclear weapons by countries that have nuclear power programs.

Discussion: Critical mass
Extend your earlier discussion of chain reactions to introduce the idea of critical mass. At least one of the fission neutrons must induce a further fission to allow for a chain reaction. Some may simply escape from the fuel assembly; others may be absorbed by the 238U, by structural materials used in the construction, by the coolant, by the fission fragments etc. Fewer will escape if there is a smaller surface area to volume ratio.

For enriched uranium, the critical mass is roughly the size of a grapefruit. Picture bringing two half-grapefruit together to cause an explosion. Why would the critical mass be different for shapes other than a sphere? (A sphere has the lowest area to volume ratio. Other shapes with the same mass would have greater areas, so more neutrons would escape, making a chain reaction less likely.)

Discussion: Control rods and coolant
The chain reaction in a nuclear power stations must be controlled, which means that the number of neutrons must be continuously regulated to stop the chain reaction diverging or closing down. To do this control rods are moved into or out of the reactor core. They are made from a substance that absorbs neutrons (e.g. boron).

A coolant carries energy away from the core. What are the desirable properties of the coolant? (It must not absorb neutrons; it must have high thermal conductivity, high specific heat capacity and high boiling point.)

Student questions
These questions compare Magnox and PWR reactors.

Binding energy per nucleon curve

Episode 528-2: Fission in a nuclear reactor – how the mass changes (Word, 42 KB)

Discussion: Nuclear fusion
You can now look at the process of nuclear fusion. (This will have been touched on when considering the graph of binding energy per nucleon.) Students should be able to calculate the energy changes from values of nuclide mass. Emphasise that the energy released per nucleon in fusion is larger than for fission.

Episode 528-3: Fusion (Word, 40 KB)

Student questions: Fusion calculations
Calculating the energy released in fusion reactions.

Episode 528-4: Fusion questions (Word, 27 KB)

Episode 525-3: Fusion in a kettle? (Word, 35 KB)

Download this episode
Episode 528: Controlling fission (Word, 178 KB)