Nuclear Fission

Nuclear Fission is the process by which the nucleus of an atom splits into two or more nuclei and some by-products.

Steps of Nuclear FissionA slow moving neutron is captured by the

nucleus

The nucleus becomes unstable and splits.

Two or more fast moving nuclei form and are released

A large amount of energy is released.

A neutron can be captured by the nucleus of some heavy atoms. The nucleus then becomes unstable and splits. Other neutrons are released when the nucleus splits.

In order for fission to take place the neutrons must be slow moving. A moderator is a medium, such as graphite, heavy water, and beryllium, that causes the neutrons to travel more slowly.

A typical fission releases 200 MeV of energy. This energy is composed of the following: 170 MeV of kinetic energy of fission fragments. 5 MeV of kinetic energy of neutrons. 15 MeV of energy beta particles and gamma

rays.10 MeV as energy of antineutrinos.

1 millielectron volt = 1.60217646 × 10-22 joules

One electron volt is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt.

NoteNeutrinos are created as a result of certain

types of radioactive decay or nuclear reactions such as those that take place in the Sun.

A neutrino is an elementary particle that usually travels close to the speed of light, is electrically neutral, and is able to pass through ordinary matter almost undisturbed.

Antineutrinos are the anti particles of neutrinos.

Example

Mass is not conserved in a nuclear reaction. The products have a slightly lower mass due to the nuclear mass defect.

Definition of mass defect:The amount by which the mass of an atomic

nucleus differs from the sum of the masses of its constituent particles, being the mass equivalent of the energy released in the formation of the nucleus

This nuclear mass defect can be used to determine the nuclear binding energy which held the heavier nucleus together and was released during fission.

where E = energy releasedc = speed of light = 3.0 x 108 m/s m= nuclear mass defect

You can calculate mass defect by:

m = masses of parent atom and neutron - masses of daughter atoms and neutrons emitted.

Sample Problem In a fission reaction, the loss in mass is 0.025 g. How much energy is produced?

Solution: E = energy released = ?

c = speed of light = 3.0 x 108 m/s m= nuclear mass defect = 0.025 g = 2.5 x 10-

5 kg

The fast moving neutrons, released during fission, can cause other nuclei to undergo fission if they are slowed down by a moderator.

A sustained fission reaction caused in this way is called a chain reaction.

EnrichmentRaw Uranium Ore, Uranium dug out of the

ground that is, contains 99.284% Uranium 238 and only 0.711% Uranium 235. U-235 is known as fissile Uranium. Fissile Uranium is the type required for Nuclear Fission (the splitting of an atom).

To increase the likelihood of sustaining a chain reaction the concentration of U-235 must be increased through a process called enrichment.

Uranium enrichment is grossly complicated but in a nutshell we put the stuff in a centrifuge and spin substance until it separates into two different materials:1. Enriched Uranium (higher concentration

of U-235) which can be used as Nuclear Fuel, or if enriched far beyond that point can be used for Nuclear Weapons.

2. Depleted Uranium (lower concentration of U-235) which is often used by the military for armor piercing projectiles.

Nuclear Reactor Produces a chain reaction at a controlled rate. The rate is controlled by control rods, often

made of cadmium, which absorb neutrons. The energy produced is used to drive turbines

which generate electricity.

Critical mass is the minimum mass of fissionable material that will produce a nuclear explosion. A mass greater than the critical mass is needed

to produce a sustainable nuclear chain reaction.

Atomic Bomb An explosion results when two or more sub-

critical masses (less than critical mass) of fissionable material are brought together rapidly.

Produces devastating destruction. Produces radioactive contamination of the

environment. The first atomic bombs were developed by the

United States during the Manhattan Project in WW II.