Nuclear Reactions

Fission and Fusion

CS 4.4

CS 4.5

State that in fission a nucleus of large mass splits into 2 nuclei of smaller mass numbers, usually with the release of neutrons.

State that fission may be spontaneous or induced by neutron bombardment.

CS 4.6

CS 4.7

CS 4.8

State that in fusion, 2 nuclei combine to form a nucleus of larger mass number.

Explain, using E = mc2, how the products of fission and fusion acquire large amounts of kinetic energy.

Carry out calculations using E = mc2 for fission and fusion reactions.

Fission

When atoms are bombarded with neutrons, their nuclei splits into 2 parts which are roughly equal in size.

Nuclear fission in the process whereby a nucleus, with a high mass number, splits into 2 nuclei which have roughly equal smaller mass numbers.

During nuclear fission, neutrons are released.

Nuclear Fission

There are 2 types of fission that exist:

1. Spontaneous Fission

2. Induced Fission

Spontaneous Fission

Some radioisotopes contain nuclei which are highly unstable and decay spontaneously by splitting into 2 smaller nuclei.

Such spontaneous decays are accompanied by the release of neutrons.

Induced Fission

Nuclear fission can be induced by bombarding atoms with neutrons.

Induced fission decays are also accompanied by the release of neutrons.

The nuclei of the atoms then split into 2 equal parts.

U23592n

1 0

The Fission Process

A neutron travels at high speed towards a uranium-235 nucleus.

U23592n

1 0

The Fission Process

A neutron travels at high speed towards a uranium-235 nucleus.

U23592n

1 0

The Fission Process

A neutron travels at high speed towards a uranium-235 nucleus.

U23592n

1 0

The neutron strikes the nucleus which then captures the neutron.

The Fission Process

U23692

The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron.

The Fission Process

The uranium-236 nucleus formed is very unstable.

The Fission Process

It transforms into an elongated shape for a short time.

The uranium-236 nucleus formed is very unstable.

The Fission Process

It transforms into an elongated shape for a short time.

The uranium-236 nucleus formed is very unstable.

The Fission Process

It transforms into an elongated shape for a short time.

It then splits into 2 fission fragments and releases neutrons.

The Fission Process

14156Ba

9236Kr

n 1 0

n 1 0

n 1 0

It then splits into 2 fission fragments and releases neutrons.

The Fission Process

14156Ba

9236Kr

n 1 0

n 1 0

n 1 0

It then splits into 2 fission fragments and releases neutrons.

The Fission Process

14156Ba

9236Kr

n 1 0

n 1 0

n 1 0

It then splits into 2 fission fragments and releases neutrons.

The Fission Process

14156Ba

9236Kr

n 1 0

n 1 0

n 1 0

Nuclear Fission Examples

U235

92 +Ba141

56+ n1

03n

1

0 +Kr 92

36

U235

92 +Cs138

55+ n1

02n

1

0 +Rb 96

37

Energy from Fission

Both the fission fragments and neutrons travel at high speed.

The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom.

EK before fission << EK after fission

Energy is being released as a result of the fission reaction.

Energy from Fission

U235

92 +Cs138

55+ n1

02n

1

0 +Rb 96

37

Element Atomic Mass (kg)

23592U 3.9014 x 10-25

13855Cs 2.2895 x 10-25

9637Rb 1.5925 x 10-25

10n 1.6750 x 10-27

Energy from Fission

Calculate the total mass before and after fission takes place.

The total mass before fission (LHS of the equation):

The total mass after fission (RHS of the equation):

3.9014 x 10-25 + 1.6750 x 10-27 = 3.91815 x 10-25 kg

2.2895 x 10-25 + 1.5925 x 10-25 + (2 x 1.6750 x 10-27) = 3.9155 x 10-25 kg

Energy from Fission

The total mass before fission =

The total mass after fission =

3.91815 x 10-25 kg

3.91550 x 10-25 kg

total mass before fission > total mass after fission

Energy from Fission

mass difference, m = total mass before fission – total mass after fission

m = 3.91815 x 10-25 – 3.91550 x 10-25

m = 2.65 x 10-28 kg

This reduction in mass results in the release of energy.

Energy Released

The energy released can be calculated using the equation:

E = mc2

Where:

E = energy released (J)

m = mass difference (kg)

c = speed of light in a vacuum (3 x 108 ms-1)

E

m c2

Energy from Fission

E = mc2

U235

92 +Cs138

55+ n1

02n1

0 +Rb 96

37

Calculate the energy released from the following fission reaction:

m = 2.65 x 10-28 kg c = 3 x 108 ms-1

E = E

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

E = 2.385 x 10-11 J

Energy from Fission

The energy released from this fission reaction does not seem a lot.

This is because it is produced from the fission of a single nucleus.

Large amounts of energy are released when a large number of nuclei undergo fission reactions.

Energy from Fission

Each uranium-235 atom has a mass of 3.9014 x 10-25 kg.

The total number of atoms in 1 kg of uranium-235 can be found as follows:

No. of atoms in 1 kg of uranium-235 = 1/3.9014 x 10-25

No. of atoms in 1 kg of uranium-235 = 2.56 x 1024 atoms

Energy from Fission

If one uranium-235 atom undergoes a fission reaction and releases 2.385 x 10-11 J of energy, then the amount of energy released by 1 kg of uranium-235 can be calculated as follows:

total energy = energy per fission x number of atoms

total energy = 2.385 x 10-11 x 2.56 x 1024

total energy = 6.1056 x 1013 J

Nuclear Fusion

In nuclear fusion, two nuclei with low mass numbers combine to produce a single nucleus with a higher mass number.

H 2

1 +He 4

2+ n1

0H

3

1 +Energy

The Fusion Process

H 2 1

H 3 1

The Fusion Process

H 2 1

H 3 1

The Fusion Process

H 2 1

H 3 1

The Fusion Process

H 2 1

H 3 1

The Fusion Process

The Fusion Process

The Fusion Process

The Fusion Process

The Fusion Process

He 4 2

n 1 0

ENERGY

The Fusion Process

He 4 2

n 1 0

ENERGY

The Fusion Process

He 4 2

n 1 0

ENERGY

The Fusion Process

He 4 2

n 1 0

ENERGY

Energy from Fusion

Element Atomic Mass (kg)

21H 3.345 x 10-27

31H 5.008 x 10-27

42He 6.647 x 10-27

10n 1.6750 x 10-27

H 2

1 +He 4

2+ n1

0H

3

1 +Energy

Energy from Fusion

Calculate the following:

• The mass difference.

• The energy released per fusion.

Energy from Fusion

The total mass before fusion (LHS of the equation):

The total mass after fission (RHS of the equation):

3.345 x 10-27 + 5.008 x 10-27 = 8.353 x 10-27 kg

6.647 x 10-27 + 1.675 x 10-27 = 8.322 x 10-27 kg

H 2

1 +He 4

2+ n1

0H

3

1 +Energy

Energy from Fusion

m = total mass before fission – total mass after fission

m = 8.353 x 10-27 – 8.322 x 10-27

m = 3.1 x 10-29 kg

Energy from Fusion

E = mc2m = 3.1 x 10-29 kg c = 3 x 108 ms-1

E = E

E = 3.1 x 10-29 x (3 x 108)2

E = 2.79 x 10-12 J

H 2

1 +He 4

2+ n1

0H

3

1 +Energy

The energy released per fusion is 2.79 x 10-12 J.