Nuclear Reactions

E = mc2

This states that energy and mass are the same ‘thing’

E = energy, Joules,J

m = mass, kilogrammes, kg

c = speed of light through air, 3.0 x108 ms-

1

Example

Calculate the energy released if you could be turned into energy.

m = 75 kg, c = 3.0 x108 ms-1

E = m.c2

E = 75 x(3.0x108)2

E = 6.75x1018 J

This would keep a 40% efficient 750MW power station going for 114 years !

The Atom

p1

1n1

1e1

1

Particle

Mass number

Charge

Symbol

Proton 1 +1

Neutron 1 0

Electron 0* -1

p11

n10e01

e01

Atomic and Mass Numbers

The total number of protons and neutrons in the nucleus is called the mass number, A.

The number of protons in the nucleus is called the atomic number, Z.

In a neutral atom the number of protons equals the number of electrons.

CAZ

Alpha Decay

An unstable nucleus can emit an alpha or beta particle or a gamma ray and become more stable.

An alpha particle, α,is a Helium nucleus, 2 protons and 2 neutrons.

The mass and atomic no are conserved.

HeUPu 42

23692

24094

Beta Decay

A neutron in the nucleus can decay into a proton, ejecting an electron and an antineutrino.

eAcRa 01

22889

22888

Gamma Decay

There is NO change to the mass or atomic no. The nucleus does become more stable as excess energy is ejected.

Nuclear Fission and FusionFission is the break up of a large nucleus into smaller fragments. It can be spontaneous or induced.

Fusion is when 2 small nuclei join together to form a larger nucleus.

nHeHH 10

42

31

21

nRbCsnU 10

9637

13855

10

23592 2

In both cases there is a mass difference , this is turned into kinetic energy of the products ( E = m.c2 )

Energy From Fission and Fusion ExampleCalculate the energy released in this induced fission reaction;

Given the following mass information:

U235

923.9014 x 10-

25

particle mass (kg)

Cs138

552.2895 x 10-25

Rb96

371.5925 x 10-

25

n1

01.6750 x 10-

27

nRbCsnU 10

9637

13855

10

23592 2

Energy From Fission and Fusion Example

Mass Before Reaction (Left Hand Side) 

E = 2.65 x 10-28 x (3 x 108)2 = 2.385 x 10-11J

(3.9014 x 10-25 + 1.6750 x 10-27) = 3.91815 x 10-25kgMass After Reaction (Right Hand Side)

(2.2895 x 10-25 + 1.5925 x 10-25 +

(2 x 1.6750 x 10-27)) = 3.9155 x 10-

25kgMass Difference = Mass Before – Mass After= 3.91815 x 10-25 – 3.9155 x 10-25 = 2.65 x 10-

28kgEnergy Released (E) = mc2

Energy From Fission and Fusion Example

This may seem very small but when we consider that 1kg of uranium contains about 2.56 x 1024 atoms, then it has the potential to release as much energy as 2 million kilograms of coal.

Fusion

Hydrogen nuclei fuse together to form Helium in the sun, the same reaction takes place in a Hydrogen bomb.

The hope for the future is that Fusion reactors can be used to produce electrical energy.

RequirementsHigh temperatures of 100 million Kelvin are needed to ‘rip’ the electrons from atoms and form a plasma. Inducing an electrical current in the plasma is one way of achieving this.

The plasma needs to be contained by a magnetic field. If it touches the walls of the container it cools down.

The plasma needs to be confined for a long enough time for the particle density to reach a sufficiently high level so that more energy is given out than absorbed.