Nuclear Power Plant Isar, Germany

Spent nuclear fuel pool

Turbine of a nuclear plant

Nuclear fission

Nuclear fission

We can obtain energy manipulating one or several nuclei of atoms throught two different methods: attaching the cores from different atoms (the nuclear fusion) or splitting a given atom nuclei (the nuclear fission).

In nuclear energy, nuclear fission is the action through which the nucleus of an atom is divided. The core forms different fragments with a mass equivalent to half of the original mass and two or three neutrons.

The total mass of the fragments is smaller than the original mass. The mass that's "missing" (about 0.1 percent from the original mass) has been converted into energy according to Einstein's equation (E = mc2). In this equation E is the energy obtained, m the referred mass and c is a constant, the light's speed: 299,792,458 m/s2.

Nuclear fission occurs when the nucleus of a heavy atom captures a neutron (induced fission), or it can occur spontaneously due to the instability of the isotope (spontaneous fission).

Nuclear chain reactions

Schematic of a chain of nuclear fission reactions

The nuclear chain reaction is the process of releasing neutrons in a first nuclear fission and, then, produce an additional fission in at least one more core. This nucleus also produces neutrons, and the process continues.

These chain reactions can be controlled or uncontrolled. Controlled reactions would be produced in nuclear reactions in nuclear power plants that generate electricity and their objective is steadily. Uncontrolled nuclear reactions occur in nuclear weapons.

If two neutrons are released in each fission caused by a neutron, the number of fissions doubles on each generation. In this case, there are 10 generations in 1,024 fissions and 80 generations in approximately 6 x 10 23 fissions.

Critical mass

The critical mass is the smallest amount of fissile material necessary to maintain a nuclear chain reaction.

Although each nuclear fission is produced from two to three neutrons, not all the neutrons are valid to continue the fission reaction, as some of them are lost. If neutrons released by each nuclear reaction are lost faster than they are formed by the fission rate, the chain reaction will not be self-sustaining and will stop.

The amount of critical mass of fissile material depends on several factors: physical properties and nuclear properties, the geometry and the purity.

A sphere has the smallest surface area for a given mass, so it minimizes the neutron leakage. If you also bordered the fissile material with a neutron reflector, we lose less neutrons and the critical mass is reduced.

Controlled nuclear fission

Scheme for as the neutrons released are controlled to control the fission chain reaction

To maintain a sustained nuclear reaction control every 2 or 3 neutrons released, only one can give another uranium nucleus. If this ratio is under one, then the reaction will die. If it's more than one, it will grow uncontrollably (an atomic explosion). An absorber of neutrons must be present to control the amount of free neutrons in the reaction space. Most of reactors are controlled by control rods made of a strong material that absorbs neutrons (i.e boron or cadmium).

In addition to the need to capture neutrons, the neutrons often have a high kinetic energy (moving at high speed). These fast neutrons are reduced through the use of a moderator, such as heavy water and tap water. Some reactors use graphite as a moderator, but this design presents some problems. Once the fast neutrons are slowed they are more likely to produce more nuclear fissions or they can be absorbed by the control bar.

Spontaneous nuclear fission

spontaneous fission

In these reactions the outer neutron absorption is not necessary. In certain isotopes of uranium and plutonium, they have an unstable atomic structure and they fissiona spontaneously.

The rate of spontaneous fission is the probability per second for a given atom to fission spontaneously - that is, without any external intervention. Plutonium 239 has a high spontaneous fission rate compared to the rate of spontaneous fission of uranium-235.

Nuclear fission illustrative 

Video

This video shows how a neutron released from a nucleus generates nuclear fission generates another nuclear fission chain.

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References

Last review: April 21, 2014