8.6.1.1. Fission Process

Table of Contents

What Happens in Nuclear Fission

Nuclear fission is the process in which a heavy nucleus splits into two smaller nuclei, usually after absorbing a neutron. This splitting releases a large amount of energy and often produces additional neutrons. The process is most important for very heavy nuclei such as uranium-235 and plutonium-239.

A heavy nucleus is held together by the strong nuclear force, but it also contains many positively charged protons that repel each other electrically. In very large nuclei, this electrical repulsion becomes more significant. As a result, some heavy nuclei are only marginally stable. If such a nucleus is disturbed, for example by capturing a neutron, it can deform and break apart into two medium-sized nuclei.

Neutron Absorption and Excitation

A common way fission begins is through neutron capture. A neutron enters the nucleus and is absorbed, forming a compound nucleus in an excited state. For example,

$$
{}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n} \rightarrow {}^{236}_{92}\mathrm{U}^{*}
$$

The star indicates that the uranium-236 nucleus is excited. This excitation energy can cause the nucleus to vibrate and deform. If the deformation becomes large enough, the nucleus may split.

Neutrons are especially effective at triggering fission because they carry no electric charge. Since they are not repelled by the positive nucleus, they can approach and enter it more easily than charged particles.

A typical induced fission process starts with neutron absorption:
$$
\text{heavy nucleus} + n \rightarrow \text{excited compound nucleus} \rightarrow \text{fission fragments} + \text{neutrons} + \text{energy}
$$

Deformation of the Nucleus

Before splitting, the nucleus does not break instantly. It first changes shape. A nearly spherical nucleus can become elongated, somewhat like a droplet being stretched. This picture is useful because the nucleus behaves in some ways like a liquid drop.

As the nucleus elongates, two effects compete. The strong nuclear force tries to keep the nucleons together, while the electric repulsion between protons pushes parts of the nucleus apart. If the stretching passes a critical point, the nucleus develops a narrow neck. This neck becomes thinner until the nucleus separates into two fragments.

Rendering scientific sketch…

Schematic stages of nuclear fission

Splitting into Fragments

When fission occurs, the heavy nucleus usually splits into two fragments of unequal size, not two exactly equal halves. These products are called fission fragments. They are generally radioactive and often emit gamma rays and beta particles later as they move toward greater stability.

One possible fission reaction is

$$
{}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n}
\rightarrow
{}^{141}_{56}\mathrm{Ba} + {}^{92}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n} + \text{energy}
$$

This is only one example. Many different fragment pairs are possible. The exact products vary from event to event, but conservation laws must always be satisfied. The total number of nucleons and the total electric charge remain the same before and after the reaction.

In every fission reaction, conservation laws apply:
$$
A_{\text{initial}} = A_{\text{final}}, \qquad Z_{\text{initial}} = Z_{\text{final}}
$$
where $A$ is mass number and $Z$ is atomic number.

Why Energy Is Released

The fission fragments are more tightly bound per nucleon than the original very heavy nucleus. Because of this, the total binding energy of the products is greater. The difference appears as released energy.

Most of this energy becomes kinetic energy of the two fission fragments. Smaller portions appear as kinetic energy of emitted neutrons and as electromagnetic radiation, especially gamma rays. The fragments fly apart at high speed because they strongly repel each other electrically once the nucleus has split.

A simple energy balance can be written as

$$
Q = \left(m_{\text{initial}} - m_{\text{final}}\right)c^2
$$

where $Q$ is the released energy. The mass difference between the initial and final states is converted into energy.

Typical Features of the Fission Process

Although each fission event is individual, several features are common.

Feature	Typical behavior
Starting nucleus	Very heavy nucleus
Trigger	Often absorption of a neutron
Intermediate stage	Excited, deformed compound nucleus
Main products	Two medium-mass nuclei
Additional products	Usually 2 or 3 neutrons
Energy output	Large, mainly fragment kinetic energy

The emitted neutrons are very important because they can go on to cause fission in other nuclei. This makes a chain reaction possible, which is treated separately in another chapter.

Spontaneous and Induced Fission

Fission can occur in two basic ways. In spontaneous fission, a heavy nucleus splits on its own without first absorbing a neutron. This is relatively rare for most nuclei used in reactors. In induced fission, an incoming neutron triggers the process. Induced fission is the main mechanism in practical nuclear energy systems.

For some nuclei, slow neutrons are especially effective. Uranium-235 is a classic example. After absorbing even a low-energy neutron, it can fission readily. Other nuclei may require faster neutrons or may be less likely to fission.

A heavy nucleus does not always split immediately on its own. In many important cases, fission is much more likely after neutron absorption.

A Simple Picture of the Process

It is helpful to imagine the fission process in stages. First, a neutron is captured. Second, the nucleus becomes excited and deforms. Third, the deformation grows until the nucleus passes the point where splitting is favorable. Fourth, the nucleus separates into two fragments, and neutrons and energy are released.

Rendering scientific sketch…

Basic sequence of induced fission

Final View

The fission process is the splitting of a heavy nucleus into smaller nuclei after the balance between nuclear attraction and proton repulsion is upset, often by neutron absorption. The nucleus first becomes an excited compound system, then deforms, then breaks into fragments. Several neutrons and a large amount of energy are released. This single nuclear event is the fundamental step behind both chain reactions and nuclear power.

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