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24.3: Fission and Fusion

Created by: CK-12

Lesson Objectives

  • Define fission and explain why energy is released during the fission process.
  • Describe a nuclear chain reaction and how it is applied in both a fission bomb and in a nuclear power plant.
  • Define fusion and explain the difficulty in using fusion as a controlled energy source.
  • Explain how ionizing radiation is measured and how it is detected.
  • Describe some uses of radiation in medicine and agriculture.

Vocabulary

  • nuclear fission
  • chain reaction
  • critical mass
  • nuclear fusion
  • control rod
  • moderator
  • ionizing radiation
  • roentgen
  • rem
  • Geiger counter
  • scintillation counter
  • film badge
  • radioactive tracer

Check Your Understanding

Recalling Prior Knowledge

  • What is nuclear binding energy and how does it relate to the stability of nuclei?
  • Why do alpha particles need to be accelerated in order to collide with other nuclei, but neutrons do not?

Common methods of energy production include the burning of fossil fuels, solar power, wind, and hydroelectric. Each has advantages and disadvantages. In this lesson, you will learn about nuclear fission and nuclear fusion, two processes that release tremendous amounts of energy but also present unique challenges.

Nuclear Fission

According to the graph of nuclear binding energy per nucleon in the lesson “Nuclear Radiation,” the most stable nuclei are of intermediate mass. To become more stable, the heaviest nuclei are capable of splitting into smaller fragments. Nuclear fission is a process in which a very heavy nucleus (mass > 200) splits into smaller nuclei of intermediate mass. Because the smaller nuclei are more stable, the fission process releases tremendous amounts of energy. Nuclear fission may occur spontaneously or may occur as result of bombardment. When uranium-235 is hit with a slow-moving neutron, it absorbs it and temporarily becomes the very unstable uranium-236. This nucleus splits into two medium-mass nuclei while also emitting more neutrons. The mass of the products is less than the mass of the reactants, with the lost mass being converted to energy.

Nuclear Chain Reaction

Because the fission process produces more neutrons, a chain reaction can result. A chain reaction is a reaction in which the material that starts the reaction is also one of the products and can start another reaction. Illustrated below (Figure below) is a nuclear chain reaction for the fission of uranium-235.

The nuclear chain reaction is a series of fission processes that sustains itself due to the continuous production of neutrons in each reaction.

The original uranium-235 nucleus absorbs a neutron, splits into a krypton-92 nucleus and a barium-141 nucleus, and releases three more neutrons upon splitting.

\mathrm{^{235}_{92}U +^1_0n \rightarrow ^{92}_{36}Kr + ^{141}_{56}Ba + 3 \: ^1_0n}

Those three neutrons are then able to cause the fission of three more uranium-235 nuclei, each of which release more neutrons, and so on. The chain reaction continues until all of the uranium-235 nuclei have been split, or until the released neutrons escape the sample without striking any more nuclei. If the size of the original sample of uranium-235 is sufficiently small, too many neutrons escape without striking other nuclei, and the chain reaction quickly ceases. The critical mass is the minimum amount of fissionable material needed to sustain a chain reaction. Atomic bombs and nuclear reactors are two ways to harness the large energy released during nuclear fission.

Atomic Bomb

In an atomic bomb, or fission bomb, the nuclear chain reaction is designed to be uncontrolled, releasing huge amounts of energy in a short amount of time. A schematic for one type of fission bomb is shown below (Figure below). A critical mass of fissionable plutonium is contained within the bomb, but not at a sufficient density. Conventional explosives are used to compress the plutonium, causing it to go critical and trigger a nuclear explosion.

An atomic bomb uses a conventional explosive to bring together a critical mass of fissionable material, which then explodes because of the chain reaction and releases a large amount of energy.

Nuclear Power Plant

A nuclear power plant (Figure below) uses a controlled fission reaction to produce large amounts of heat. The heat is then used to generate electrical energy.

A nuclear reactor harnesses the energy of nuclear fission to generate electricity.

Uranium-235, the usual fissionable material in a nuclear reactor, is first packaged into fuel rods. In order to keep the chain reaction from proceeding unchecked, moveable control rods are placed in between the fuel rods. Control rods, limit the amount of available neutrons by absorbing some of them and preventing the reaction from proceeding too rapidly. Common control rod materials include alloys with various amounts of silver, indium, cadmium, or boron. A moderator is a material that slows down high-speed neutrons. This is beneficial because slow-moving neutrons are more efficient at splitting nuclei. Water is often used as a moderator. The heat released by the fission reaction is absorbed by constantly circulating coolant water. The coolant water releases its heat to a steam generator, which turns a turbine and generates electricity. The core of the reactor is surrounded by a containment structure that absorbs radiation.

Controversy abounds over the use of nuclear power. An advantage of nuclear power over the burning of fossil fuels is that it does not emit carbon dioxide or various other conventional pollutants. However, the smaller nuclei produced in the fission process are themselves radioactive and must be disposed of or contained in a safe manner. Containment of this nuclear waste is a challenging problem because the half-lives of the waste products can often be thousands of years. Spent fuel rods are typically stored temporarily on site in large pools of water to cool them before being transported to permanent storage facilities where the waste will be kept forever.

Another risk of nuclear power is that an accident at a nuclear power plant is life-threatening and very harmful to the environment. On April 26, 1986, an accident occurred at the Chernobyl Nuclear Power Plant in Ukraine. Thousands were killed either from the initial effects of the explosion or from radiation-induced cancers in subsequent years. Note that this was not a nuclear explosion, simply a conventional explosion that unfortunately spread radioactive materials into the surrounding environment. Because the fuel rods used in nuclear reactors are not as enriched with radioactive nuclei as the materials used in an atomic bomb, an accidental release of the massive destruction associated with a runaway nuclear reaction is not a major risk factor at nuclear power plants.

Nuclear Fusion

The lightest nuclei are also not as stable as nuclei of intermediate mass. Nuclear fusion is a process in which light-mass nuclei combine to form a heavier and more stable nucleus. Fusion produces even more energy than fission. In the sun and other stars, four hydrogen nuclei combine at extremely high temperatures and pressures to produce a helium nucleus. The concurrent loss of mass is converted into extraordinary amounts of energy (Figure below).

Nuclear fusion takes place when small nuclei combine to make larger ones. The enormous amounts of energy produced by fusion, powers our sun and other stars.

Fusion is even more appealing than fission as an energy source because no radioactive waste is produced and the only reactant needed is hydrogen. However, fusion reactions only occur at very high temperatures—in excess of 40,000,000°C. No known materials can withstand such temperatures, so there is currently no feasible way to harness nuclear fusion for energy production, although research is ongoing.

Uses of Radiation

As we saw earlier, different types of radiation vary in their abilities to penetrate through matter. Alpha particles have very low penetrating ability and are stopped by skin and clothing. Beta particles have a penetrating ability that is about 100 times that of alpha particles. Gamma rays have very high penetrating ability, and great care must be taken to avoid overexposure to gamma rays.

Exposure and Detection

Radiation emitted by radioisotopes is called ionizing radiation. Ionizing radiation is radiation that has enough energy to knock electrons off the atoms of a bombarded substance and produce ions. The roentgen is a unit that measures nuclear radiation and is equal to the amount of radiation that produces 2 × 109 ion pairs when it passes through 1 cm3 of air. The primary concern is that ionizing radiation can do damage to living tissues. Radiation damage is measured in rems, which stands for roentgen equivalent man. A rem is the amount of ionizing radiation that does as much damage to human tissue as is done by 1 roentgen of high-voltage x-rays. Tissue damage from ionizing radiation can cause genetic mutations due to interactions between the radiation and DNA, which can lead to cancer.

You are constantly being bombarded with background radiation from space and from geologic sources that vary depending on where you live. Average exposure is estimated to be about 0.1 rem per year. The maximum permissible dose of radiation exposure for people in the general population is 0.5 rem per year. Some people are naturally at higher risk because of their occupations, so reliable instruments to detect radiation exposure have been developed. A Geiger counter is a device that uses a gas-filled metal tube to detect radiation (Figure below). When the gas is exposed to ionizing radiation, it conducts a current, and the Geiger counter registers this as audible clicks. The frequency of the clicks corresponds to the intensity of the radiation.

A Geiger counter is used to detect radiation.

A scintillation counter is a device that uses a phosphor-coated surface to detect radiation by the emission of bright bursts of light. Workers who are at risk of exposure to radiation wear small portable film badges. A film badge consists of several layers of photographic film that can measure the amount of radiation to which the wearer has been exposed. Film badges are removed and analyzed at periodic intervals to ensure that the person does not become overexposed to radiation on a cumulative basis.

Medicine and Agriculture

Radioactive nuclides, such as cobalt-60, are frequently used in medicine to treat certain types of cancers. The faster growing cancer cells are exposed to the radiation and are more susceptible to damage than healthy cells. Thus, the cells in the cancerous area are killed by the exposure to high-energy radiation. Radiation treatment is risky because some healthy cells are also killed, and cells at the center of a cancerous tumor can become resistant to the radiation.

Radioactive tracers are radioactive atoms that are incorporated into substances so that the movement of these substances can be tracked by a radiation detector. Tracers are used in the diagnosis of cancer and other diseases. For example, iodine-131 is used to detect problems with a person’s thyroid. A patient first ingests a small amount of iodine-131. About two hours later, the iodine uptake by the thyroid is determined by a radiation scan of the patient’s throat. In a similar way, technetium-99 is used to detect brain tumors and liver disorders, and phosphorus-32 is used to detect skin cancer.

Radioactive tracers can be used in agriculture to test the effectiveness of various fertilizers. The fertilizer is enriched with a radioisotope, and the uptake of the fertilizer by the plant can be monitored by measuring the emitted radiation levels. Nuclear radiation is also used to prolong the shelf life of produce by killing bacteria and insects that would otherwise cause the food to spoil faster.

Lesson Summary

  • Nuclear fission involves the splitting of large nuclei into nuclei of intermediate size. A chain reaction is self-sustaining and is used in an uncontrolled fashion in an atomic bomb.
  • Nuclear power plants use controlled fission to generate electricity from the large amounts of heat produced.
  • Nuclear fusion involves combining of small nuclei into larger ones, a process that releases considerably more energy than fission. Fusion powers our sun and other stars but cannot currently be harnessed directly to generate electricity.
  • Ionizing radiation is capable of doing cellular damage and is detected by a Geiger counter, scintillation counter, or film badge.
  • Radiation is used for various cancer treatments, as a tracer to study other processes, and as a way to prolong the shelf life of food.

Lesson Review Questions

Reviewing Concepts

  1. Explain the difference between nuclear fission and nuclear fusion.
  2. What is a nuclear chain reaction, and how is it related to the concept of a critical mass?
  3. What is the function of the conventional explosive in an atomic bomb?
  4. Describe the purposes of control rods and moderators in a nuclear power plant.
  5. Why is fusion not used to generate electrical power?
  6. Why are x-rays and the radiation emitted by radioisotopes called ionizing radiation?
  7. Describe two applications of radioisotopes in medicine.
  8. Explain why irradiation of food helps it to last longer.

Problems

  1. Explain why fusion reactions release more energy than fission reactions. (Hint: Use the nuclear binding energy per nucleon graph from the lesson “Nuclear Radiation.”)
  2. Fill in the unknown nuclides for the fission and fusion reactions shown below.
    1. \mathrm{ ^{235}_{92}U + ^1_0n \rightarrow ^{144}_{55}Cs+ \underline{\hspace{1cm}} +2^1_0n}
    2. \mathrm{ ^{2}_{1}H + ^3_1H \rightarrow + \underline{\hspace{1cm}} +^1_0n}

Further Reading / Supplemental Links

Points to Consider

Fission and fusion are promising as energy sources, but they are not without difficulties and controversy.

  • Do the advantages of nuclear power justify the risk of accidents and the problems associated with the disposal of nuclear waste?
  • What methods are being investigated as a way to use controlled nuclear fusion as an energy source?

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CK.SCI.ENG.SE.1.Chemistry-Intermediate.24.3

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