Nuclear PhysicsNuclear Physics

Selected TopicsSelected Topics

5 –Fission and Fusion5 –Fission and Fusion

Fission

A nucleus of large mass number splits into two smaller nuclei.

The total mass of the products is less than the original mass of the heavy nucleus.

The mass difference appears as the kinetic energy of the fission products.

Fission of 235U by a slow (low energy) neutron:

1 235 2360 92 92n U U* X Y neutrons

Fission of 235U by a slow (low energy) neutron:1 235 2360 92 92n U U* X Y neutrons

236U* is an intermediate, short-lived state that lasts about 10-12 s.

X and Y are the fission fragments.

Many combinations of X and Y satisfy the requirements of conservation of energy and charge.

Fission of 235U by a slow (low energy) neutron:

Several neutrons are also produced in each fission event.

Example:1 235 141 92 10 92 56 36 0n U Ba Kr 3 n

Sequence of Events in Fission

The 235U nucleus captures a thermal (slow-moving) neutron.

Sequence of Events in Fission

The 235U nucleus captures a thermal (slow-moving) neutron.This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations.

Sequence of Events in Fission

The 235U nucleus captures a thermal (slow-moving) neutron.This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations.The 236U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion.

Sequence of Events in Fission

The 235U nucleus captures a thermal (slow-moving) neutron.This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations.The 236U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion.The nucleus splits into two fragments, emitting several neutrons in the process.

Sequence of Events in Fission – Diagram(The “liquid drop” model)

Energy in a Fission Process

Binding energy for heavy nuclei is about 7.2 MeV per nucleon.Binding energy for intermediate nuclei is about 8.2 MeV per nucleon.

Therefore, the fission fragments have less mass than the nucleons in the original nuclei.This decrease in mass per nucleon appears as released energy in the fission event, about 240 MeV per fission event..

Chain Reaction

Neutrons are emitted when 235U undergoes fission. These neutrons are then available to trigger fission in other nuclei.

This process is called a chain reaction.

If uncontrolled, a violent explosion can occur.This is the principle behind the nuclear bomb, where 1 kg of U can release energy equal to about 20 000 tons of TNT.

Chain Reaction – Diagram

Basic Reactor Design

Fuel elements consist of enriched uranium.

The moderator material helps to slow down the neutrons.

The control rods absorb neutrons.

Pressurized Water Reactor

Text Problem Example – Chapter 30, # 2

Find the energy released in the fission reaction

1 235 98 135 10 92 40 52 0n U Zr Te 3 n

The atomic masses of the fission products are 97.912 0 u for 98Zr and 134.908 7 u for 135Te.

Text Problem Example – Chapter 30, # 2

Find the energy released in the fission reaction

1 235 98 135 10 92 40 52 0n U Zr Te 3 n

The atomic masses of the fission products are 97.912 0 u for 98Zr and 134.908 7 u for 135Te.

235.043924 2 1.008665 97.9120 134.9087 u 931.5 MeV u

192 MeV

Nuclear Fusion

Nuclear fusion occurs when two light nuclei combine to form a more massive nucleus.

The mass of the final nucleus is less than the masses of the original nuclei.

This loss of mass is accompanied by a release of energy.

Considerations for a Fusion Reactor

The most promising reactions involve deuterium (D = 2H) and tritium (T = 3H).

2 2 3 11 1 2 0

2 2 3 11 1 1 1

2 3 11 1 0

D D He n 3.27 MeV

D D T H 4.03 MeV

D T 4He n 17.59 MeV

Q

Q

Q

Text Problem Example – Chapter 30, # 12

Find the energy released in the fusion reaction

1 2 31 1 2H H He

Text Problem Example – Chapter 30, # 12

Find the energy released in the fusion reaction

1 2 31 1 2H H He

1.007825 2.014102 3.016029 u 931.5 MeV u

=5.49 MeV