1. Topic 7.3 Continued, 8.4 – Nuclear Power
Nuclear Reactions
Topic 7.3 Continued, 8.4 – Nuclear Power

2. Review: Properties of Radiation
Create a chart to summarize for the 3 types of Radiation:
Relative Charge (Compared to a single proton)
Mass
Speed
Ionizing effect
Penetrating affect
Effects of Fields
The beta particles are much lighter than the alpha particles and have a negative (‑) charge, so they are deflected more, and in the opposite direction.
Being uncharged, the gamma rays are not deflected by the field.
Alpha and beta particles are also affected by an electric field ‑ in other words, there is a force on them if they pass between oppositely charged plates.

4. Stability
What is the main difference between smaller nuclei (lower left) and the bigger ones (upper right)?
Why do heavier nuclei need more neutrons to be stable?
If you plot the neutron number N against the proton number Z for all the known nuclides, you get the diagram shown here
More neutrons are needed to hold the nucleus together (although adding too many neutrons can also cause instability).
There is an upper limit to the size of a stable nucleus, because all the nuclides with Z higher than 83 are unstable.

5. Decay Chains
A radio‑nuclide often produces an unstable daughter nuclide.
The daughter will also decay, and the process will continue until finally a stable nuclide is formed.
This is called a decay chain or a decay series.
Part of one decay chain is shown below

6. When determining the products of decay series, the same rules apply as in determining the products of alpha and beta, or artificial transmutation.
The only difference is several steps are involved instead of just one.

7. Unified Atomic Mass Unit (u)
Defined as 1/12 of the mass of an atom of Carbon-12
u = x kg
How much energy is in 1u?

8. Mass Energy Equivalence
If the principle of conservation of energy is to hold for nuclear reactions it is clear that mass and energy must be regarded as equivalent.
The implication of E = mc2 is that any reaction producing an appreciable mass decrease is a possible source of energy.
Using the principles of conservation, what is the implication of this equation?

9. Energy of 1 u E = mc2 E = 1.6605402 x 10-27 kg x (2.9979 x 108 ms-1)2
E = x J
Remembering 1 eV = x J
1 u = MeV

10. Mass Energy Equivalence
If mass and energy are considered to be equal, could there be reactions in which mass is not conserved?

11. Mass Defect

12. Mass Defect
Where is the missing mass?

13. Mass Defect
The missing mass (mass defect) has been stored as energy in the nucleus. It is called the binding energy of the nucleus.
Energy stored in the nucleus:
E = mc2
(ΔE = Δmc2)

14. Binding Energy Released
The binding energies for the fragments, as compared to the whole, is higher

15. Binding Energy per Nucleon
Nuclides in the middle of the graph have the highest binding energy per nucleon and are thus the most stable since they need most energy to disintegrate.
The smaller values for higher and lower mass numbers imply that potential sources of nuclear energy are reactions involving the disintegration of a heavy nucleus or the fusing of particles to form a nucleus of high nucleon number.
In both cases nuclei are produced having a greater binding energy per nucleon and there is consequently a mass transfer during their formation.

16. Mass defect calculation
Find the mass defect of the nucleus of gold, – Au
From data booklet:

17. Mass defect calculation
The mass of this isotope is u
Since it has 79 electrons its nuclear mass is u – 79x u = u
This nucleus has 79 protons and 118 neutrons, individually these have a mass of 79x u x u = u
The difference in mass (mass defect) is therefore u

18. Mass Defect
Where is the missing mass?
1 u = MeV

19. Your Turn
Use the following information to calculate the mass defect of Lithium-7
Proton: u
Neutron: u
Periodic table is on MyClass

20. Your Turn

22. Fission In a fission reaction a large nucleus splits in two.
What happens to the binding energy per nucleon of U- 235 when it undergoes fission?

23. Fission
What does it mean to have a higher binding energy per nucleon ?
What happens to the excess energy?
If a nucleus with A > 200 splits in half, the two fragments have a higher binding energy per nucleon than the parent.
This means that the fragments are more stable than the parent.
The excess energy is released by the reaction.

24. Uranium
Uranium 235 has a large unstable nucleus.

25. Capture
A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236.
What happens next?
The strong forces that hold the nucleus together only act over a very short distance.
When a uranium nucleus absorbs a neutron it knocks the nucleus out of shape.
If the nucleus deforms enough, the electrostatic repulsion between the protons in each half becomes greater than the strong force.
It then splits in two.

26. Fission
The Uranium 236 is very unstable and splits into two smaller nuclei (this is called nuclear fission)
The nucleus splits randomly.
In the diagram, the fission fragments are shown as isotopes of barium and krypton.
This is just one of the many possible combinations.
Fission of a uranium nucleus gives out about 200 MeV of energy.

27. Free neutrons
As well as the two smaller nuclei (called daughter nuclei), three neutrons are released (with lots of kinetic energy)
What happens next?

28. Fission
These free neutrons can strike more uranium nuclei, causing them to split.

29. Fission Example

30. Chain Reaction
If there is enough uranium (critical mass) a chain reaction occurs. Huge amounts of energy are released very quickly.

31. Chain Reaction

32. Chain Reaction

33. Simulation Videos Tank Video 500 of them!
There’s TOO MUCH energy! Moderators & Control rods slow them down.

34. Bang!
This can result in a nuclear explosion! Video:

36. Fusion What is it? Which nuclei are involved in fusion reactions?
Which part of the binding energy curve does fusion fit it?
How does the energy you get from fusion compare with fission?
Fusion means joining together.
In a fusion reaction two light nuclei join together to make a heavier nucleus.
Fusion gives out more energy per kilogram of fuel than fission.
Can you see why from the graph?

37. The binding energy curve
 The increases in binding energy per nucleon are much larger for fusion than for fission reactions, because the graph increases more steeply for light nuclei.
So fusion gives out more energy per nucleon involved in the reaction than fission.

39. Nuclear fusion – Star power!
Aims
Interpret the BE curve to understand why a large nucleus will release energy if it splits into two small ones.
Understand that two pull a nucleus apart requires energy to be put in and this can be achieved by adding a neutron to 235U.
Calculate the energy released in a simple fission reaction.
Interpret the isotope chart to understand why fission fragments are neutron rich.
Interpret the BE curve to understand why small nuclei joining together will release energy.

40. Nuclear fusion – Star power!
Each second, in our Sun, more than 560 million tonnes of hydrogen fuse together to make helium.
One series of reactions for this is shown here:

41. Star power… on Earth?
One possible reaction is the fusion of deuterium and tritium.
These are isotopes of hydrogen
What are some advantages of fusion over fission?
Fusion has a number of advantages over fission:
greater power output per kilogram,
the raw materials are cheap and readily available,
no radioactive elements are produced directly,
irradiation by the neutrons leads to radioactivity in the reactor materials but these have relatively short half lives and only need to be stored safely for a short time.
While a 1,000 MW coal-fired power plant requires 2.7 million tons of coal per year, a fusion plant of the kind envisioned for the second half of this century will only require 250 kilos of fuel per year, half of it Deuterium, half of it Tritium.

42. The Fusion Challenge
We have yet to produce usable energy through fusion
What are some of the challenges?
The JET (Joint European Torus) project was set up to carry out research into fusion power.
It has yet to generate a self‑sustaining fusion reaction.
The main problem is getting two nuclei close enough for long enough for them to fuse
The enormous temperatures and pressures in the Sun's core provide the right conditions.
On Earth temperatures of over 100 million kelvin are needed.
At this temperature all matter exists as an ionised gas or plasma.
Another problem is containment.
What can you use to hold something this hot?
JET uses magnetic fields in a doughnutshaped chamber called a torus to keep the plasma away from the container walls.
Unfortunately generating high temperatures and strong magnetic fields uses up more energy than the fusion reaction produces!
We are still some years off a fusion power station.

43. ITER Project