Nuclear Fission

Nuclear Fission

Nuclear fission occurs when an atom splits into two or more smaller atoms, most often the as the result of neutron bombardment.

Nuclear fission is a process by which the nucleus of an atom is split into two or more smaller nuclei, known as fission products. The fission of heavy elements is an exothermic reaction, and huge amounts of energy are released in the process. The nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3:2 for common fissile isotopes. Most fissions are binary fissions that produce two charged fragments. Occasionally, about 2 to 4 times per 1000 events, three positively charged fragments are produced, which indicates a ternary fission. The smallest of these fragments in ternary processes ranges from the size of a proton to the size of an argon nucleus.

Origin of Nuclear Instability

Within the nucleus, there are different forces that act between the particles. The strong nuclear force is the force between two or more nucleons. This force binds protons and neutrons together inside the nucleus, and it is most powerful when the nucleus is small and the nucleons are close together. The electromagnetic force causes the repulsion between like-charged protons. These two forces produce opposite effects in the nucleus. The strong nuclear force acts to hold all the protons and neutrons close together, while the electromagnetic force acts to push protons further apart.

In atoms with small nuclei, the strong nuclear force overpowers the electromagnetic force. As the nucleus gets bigger, the electromagnetic force becomes greater than the strong nuclear force. In these nuclei, it’s possible for particles and energy to be ejected from the nucleus. These nuclei are called unstable, and this instability can result in radiation and fission.

Neutron Bombardment

In order to initiate fission, a high-energy neutron is directed towards a nucleus, such as ²³⁵U. The combination of these two produces ²³⁶U, which is an unstable element that undergoes fission. The resulting fission process often releases additional neutrons, which can go on to initiate other ²³⁵U atoms, forming a chain reaction. While nuclear fission can occur without this neutron bombardment, in what would be termed spontaneous fission, this is a rare occurrence; most fission reactions, especially those utilized for energy and weaponry, occur via neutron bombardment. If an element can be induced to undergo fission via neutron bombardment, it is said to be fissile.

The Atomic Bomb

Atomic bombs are nuclear weapons that use the energetic output of nuclear fission to produce massive explosions.

Atomic Bombs

Atomic bombs are nuclear weapons that use the energetic output of nuclear fission to produce massive explosions. These bombs are in contrast to hydrogen bombs, which use both fission and fusion to power their greater explosive potential.

History

Only two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6th, 1945, a uranium gun-type fission bomb code-named “Little Boy” was detonated over the Japanese city of Hiroshima. Three days later, on August 9th, a plutonium implosion-type fission bomb code-named “Fat Man” was exploded over Nagasaki, Japan. These two bombings resulted in the deaths of approximately 200,000 Japanese people—mostly civilians. The role of the bombings in Japan’s surrender, and their ethical status, remain the subject of scholarly and popular debate.

Nuclear Chemistry Behind the Explosion

Atomic bombs are made up of a fissile element, such as uranium, that is enriched in the isotope that can sustain a fission nuclear chain reaction. When a free neutron hits the nucleus of a fissile atom like uranium-235 (²³⁵U), the uranium splits into two smaller atoms called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions. This creates the chain reaction.

The uranium-235 content of “weapons-grade” uranium is generally greater than 85 percent, though inefficient weapons, deemed “weapons-usable,” can be made of 20 percent enriched uranium. The very first uranium bomb, Little Boy, dropped on Hiroshima in 1945, used 64 kilograms of 80 percent enriched uranium.

In fission weapons, a mass of fissile material, either enriched uranium or plutonium, is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction. This is accomplished either by shooting one piece of sub-critical material into another, termed the “gun” method, or by compressing a sub-critical sphere of material using chemical explosives to many times its original density, called the “implosion” method.

The implosion method is considered more sophisticated than the gun method and only can be used if the fissile material is plutonium. The inherent radioactivity of uranium will then release a neutron, which will bombard another atom of ²³⁵U to produce the unstable uranium-236, which undergoes fission, releases further neutrons, and continues the process.The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to 236 (uranium plus the extra neutron). The following equation shows one possible split, namely into strontium-95 (⁹⁵Sr), xenon-139 (¹³⁹Xe), and two neutrons (n), plus energy:

[latex]^{235}\text{U}+ ^1_0\text{n}\rightarrow \ \ ^{95}\text{Sr}+^{139}\text{Xe} +2\ ^1_0\text{n}+180 \text{MeV}[/latex]

The immediate energy release per atom is about 180 million electron volts (Me). Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons. This initial kinetic energy imparts an initial speed of about 12,000 kilometers per second.

However, the charged fragments’ high electric charge causes many inelastic collisions with nearby nuclei, and thus these fragments remain trapped inside the bomb’s uranium pit. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second. By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees. This X-ray energy produces the blast and fire which are normally the purpose of a nuclear explosion.

Nuclear Reactors

A nuclear reactor is a piece of equipment in which nuclear chain reactions can be harnessed to produce energy in a controlled way.

Reactors and Fission

The energy released from nuclear fission can be harnessed to make electricity with a nuclear reactor. A nuclear reactor is a piece of equipment where nuclear chain reactions can be controlled and sustained. The reactors use nuclear fuel, most commonly uranium-235 and plutonium-239. The amount of free energy in nuclear fuels is far greater than the energy in a similar amount of other fuels such as gasoline. In many countries, nuclear power is seen as an environmentally friendly alternative to fossil fuels, which are non-renewable and release large amounts of greenhouse gases. However, nuclear reactors produce nuclear waste containing radioactive elements.

The Chain Reaction

When a large, fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.

This chain reaction can be controlled using neutron poisons and neutron moderators to change the portion of neutrons that can cause more fissions. A neutron moderator works to reduce a newly produced neutron’s kinetic energy from several MeV to thermal energies of less than one eV, making them more likely to induce further fission.

Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected. The amount and nature of neutron moderation affects reactor controllability and safety. Since moderators both slow and absorb neutrons, there is an optimum amount of moderator to include in a given geometry of reactor core.

Criticality

In a nuclear reactor, the neutron population at any instant is a function of the rate of neutron production and the rate of neutron loss. When a reactor’s neutron population remains steady from one generation to the next by creating as many new neutrons as are lost, the fission chain reaction is self-sustaining and the reactor’s condition is referred to as ” critical.” When the reactor’s neutron production exceeds losses, characterized by increasing power level, it is considered “supercritical.” When losses dominate, it is considered “subcritical” and exhibits decreasing power.

The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to “strike” a chain reaction, and if the spontaneous fission rate is sufficiently low, it may take a long time before a chance neutron encounter starts a chain reaction—even if the reactor is supercritical. In ²³⁵U reactors, this time might be a long as many minutes. Most nuclear reactors include a “starter” neutron source that ensures a few free neutrons in the reactor core, so a chain reaction will occur immediately when the core is made critical. A common type of startup neutron source is a mixture of an alpha particle emitter such as ²⁴¹Am (americium-241) with a lightweight isotope such as ⁹Be (beryllium-9).

Generating Electricity

Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from nuclear fission. The heat is removed from the reactor core by a cooling system that generates steam. The steam drives a turbine which runs a generator to produce electricity.