1. Mass Defect Check these figures: proton mass, mp = 1.673 ´ 10-27 kg
neutron mass, mn = 1.675 ´ 10-27 kg
mass of a 
nucleus = 6.643 ´ 10-27 kg

2. the atomic mass unit amu, or u is a convenient unit of nuclear mass.
1 amu or 1 u = 1/12 the mass of a neutral12C atom (i.e. including its six electrons) =  ´ 10-27 kg.
Thus:
mp = u
mn = u
me = u
mass of a neutral atom = u

3. The mass of a nucleus is less than the sum of the masses of its parts; this is true for all nuclides. So much for conservation of mass!
When nucleons are bound in a nucleus they have less potential energy than when they are separated, so as a nucleus they have less mass – the mass defect.
Rather than saying that mass is ‘converted to energy’ it is better to say that, by measuring an object’s mass, we are determining its energy.
A helium nucleus has less mass than its constituent nucleons; in pulling them apart, we do work and increase their potential energy; hence their mass is greater.

4. Mass defect and binding energy
What has happened to the missing mass – or mass defect – between the whole and the sum of the parts?
To separate the particles, they must be pulled apart against the attractive strong force. They therefore have more potential energy when they are separated.
So energy must be put in to separate the nucleons of a nucleus. This energy is known as the binding energy
This can be a confusing term because you may think that this means that energy is required to bind nucleons together. As with chemical bonds, this is the opposite of the truth. Energy is needed to break bonds.
Einstein’s Special Theory of Relativity (1905) relates mass and energy via the equation E = mc2 (where c is the speed of light in a vacuum). In this case, we have:
binding energy = mass defect x c2 or ΔE =  Δm x c2

5. Nuclear Power

6. Fission
The fragments lose mass because of their greater binding energy per nucleon. ie, they are more stable and therefore more tightly bound. Each nucleon gains about 1MeV, so the whole nucleus releases about 200MeV.
The mass deficit (equal to the change in binding energy) is released as kinetic energy which heats up the surroundings.
See Kerboodle animation
Greater bepn = less mass!

7. Fission: the nucleus as a liquid drop
In many ways, nuclei behave like a drop of liquid. A water filled balloon is a good model for a nucleus. After the absorption of the neutron, the nucleus wobbles. As soon as the electric charge distribution departs from the spherical (pinch the balloon into a dumbbell like shape) the mutual coulomb repulsion between the two ends drives the fission process.
An alternative is to grease a plate and put a large drop of water on it. Wobble the plate about and watch the drop split.

8. Fission is spontaneous in neutron rich nuclei
It can be induced when a large nucleus absorbs a slow moving (thermal) neutron.
It was first observed when attempts were made to make a bigger nucleus than the biggest naturally occurring nucleus, Uranium. It failed!
When U-235 fissions, the fission products are not always the same.

9. Fission products and radioactive waste
Most of the energy released is in the form of the kinetic energy of the fission fragments. Because they have a relatively high fraction of neutrons, they are unstable, and decay with short half-lives. They form the ‘high-level’ radioactive waste that cannot be simply disposed of; it has to be stored somewhere for a minimum of 20 half lives, approx 600 yrs.

10. A fission reaction Here is a typical fission process:
An average 2 neutrons and 200MeV of energy

12. Controlled chain reactions If at least one surplus neutron can induce fission in another 235U nucleus and so on, then a self sustaining release of nuclear energy is possible. For a power station a controlled chain reaction is needed. Should each fission result in more than one further fission, then the chain reaction is said to diverge. In a bomb the aim is to get the chain reaction to diverge as fast as possible.

13. The possibility of fission
What are the chances that a neutron will strike another nucleus?
The nuclei of two adjacent uranium atoms are typically 10,000 nuclear diameters apart - If a student was a nucleus, the next student nucleus would be 3 km away.
99% of natural uranium is non-fissile U-238, so most neutrons are absorbed by 238U nuclei, which are also quite good at absorbing fast neutrons.
Instead of fissioning 238U nuclei transmute into 239Pu which is fissile, the favourite explosive material for making nuclear bombs. Pure natural uranium is incapable of sustaining a fission reaction – less than one fission neutron succeeds in inducing a further fission.

14. 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.

15. Moderators
A material with relatively light nuclei to absorb momentum and energy from the neutron is needed.
Hydrogen – i.e. protons. Virtually the same mass as the neutron (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.

16. Commonly used moderators
Graphite
Heavy water (deuterium in place of the H). In normal water, H atoms absorb neutrons.

17. Enrichment
Natural uranium is more than 99% U-238 which doesn’t fission easily.
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.
To produce 1 tonne of enriched uranium, you need 3.6 tonnes of natural uranium, so you must 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.

18. Critical mass
At least one of the fission neutrons must induce a further fission to allow for a chain reaction.
Some neutrons may 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 larger mass and/or a smaller surface area to volume ratio.
For enriched uranium, the critical mass is roughly the size of a grapefruit, approx 15kg. Picture bringing two half-grapefruit together to cause an explosion. Why would the critical mass be different for shapes other than a sphere?

19. Control rods and coolant
The chain reaction must be controlled, to stop the chain reaction diverging or closing down. To do this control rods are moved into or out of the reactor core by (failsafe) electromagnets. They are made from a substance that absorbs neutrons readily without becoming unstable themselves, e.g. boron.
Cadmium and boron can capture neutrons over much of their nuclear diameter.
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.
EG. Water, CO2, liquid sodium

20. Fast Breeder Reactors
Use plutonium Pu-239 (94) This is formed when natural U-238 captures fast moving neutrons, then decays twice by beta emission. When the Pu fissions the fast neutrons released generates (breeds) more plutonium fuel.

21. (Fission ships)

22. You need to know:
Design features of nuclear power stations - ie. purpose and materials they are made from
Moderators
Control rods
Coolant
Shielding and other safety features
Dealing with high, medium and low level waste

23. Fusion
(the holy grail?)

24. Fusion

25. Greater bepn = less mass!
In fusion, small nuclei join to make a larger one with greater binding energy per nucleon. The nucleons are more tightly bound, even more trapped in the nucleus. Energy is released equal to the total increase in binding energy, so the combined nucleus must have lost mass. This mass deficit is released as energy – heat. Very high speeds (ie temperatures) are needed to overcome the strong repulsive forces to drive the nuclei together.

26. Fusion Power
Read about the reaction used in the JET project and note how lithium is used as a source of tritium to feed the fusion reaction.