The Handy Chemistry Answer Book (2014)

NUCLEAR CHEMISTRY

NUCLEAR CHEMISTRY AT WORK

What is nuclear fusion?

Nuclear fusion is the process by which two nuclei combine to form a single, heavier nucleus. Energy is usually released when two lightweight nuclei fuse, though for heavier nuclei, fusion generally requires an input of energy. Nuclear fusion can be used in bombs to cause a massive and rapid release of energy. Fusion is also responsible for the fact that stars burn bright and give off light and heat.

What is cold fusion?

A cold fusion reaction is one that takes place under ambient conditions using simple equipment. Such a fusion reaction would be extremely desirable, since it could allow for a simple and efficient means of energy production.

What is nuclear fission?

Nuclear fission is essentially the opposite of nuclear fusion. Here, a single nucleus divides into two smaller nuclei. In the case of heavy atoms, this is often accompanied by the release of heat. For example, the radioactive decay of uranium-235 can be used to generate the heat used to drive turbines to generate electricity in nuclear power plants. The use of nuclear fission to harness energy for use by humans is typically considered the much more viable choice (as opposed to nuclear fusion).

Is cold fusion really possible?

In the late 1980s, reports surfaced of experimentally realized cold fusion, exciting the scientific community. It turned out, however, that these reports were false, and nobody was able to reproduce the results of what were initially reported as relatively simple experiments. Since these experiments were disproved, other credible reports of cold fusion experiments have indeed surfaced, and thus it does appear that cold fusion is possible in principle. Unfortunately, the energy released from the few successful experiments has been much smaller than the amount of energy needed to actually run the experiments, making the feasibility of cold fusion as a source of energy production unlikely. Compared to the initial burst of interest, mainstream scientists have generally lost interest in the topic, though there remains a group of fringe experimentalists who still seek to make cold fusion for energy production a reality. If such experiments could work, they would certainly be of great interest to the scientific community, but today most believe that it just isn’t possible to generate enough energy from cold fusion sources to make it a viable source of energy production.

Is mass conserved during a fission process?

Almost, but not quite. A small amount of mass is given off in the form of energy. Specifically, the relationship between the amount of energy, E, released and the amount of mass, m, that becomes energy is given by the famous relationship E = mc2, where c is the speed of light.

How can radioactivity be measured?

Radioactivity is measured by detecting the products of radioactive decay processes. The most well-known instrument used for this purpose is the Geiger counter. A Geiger counter is sensitive to the products of nuclear decay, including alpha and beta particles and gamma rays. The units used to quantify radiation are the Curie or the Becquerel, which describe the number of nuclear decays a substance undergoes per unit of time.

In many cases, it may not be necessary to directly detect the radiation being given off at this instant, but rather to just determine the isotopic ratio of an element present in a sample. This can be done using techniques borrowed from analytical chemistry, such as mass spectrometry. Information on the isotopic ratio present, along with knowledge of the half-life of the isotope in question, can be related to the age of the sample being studied.

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A Geiger counter is a useful tool for measuring radioactivity in almost anything. It can detect alpha and beta particles, as well as gamma rays.

How does radioactive dating work?

Radioactive dating (also called radiometric dating) is a technique used to determine the age of a sample based on the ratio of isotopes of an element present in the sample. Using the known half-life of the isotope being studied, along with knowledge of the natural abundance of the isotopes present at the time the sample was formed, the age of the sample can be determined. To obtain an accurate age for a sample, it is required that none of the isotopes being measured have been able to escape or re-enter the sample over the course of its lifetime. Otherwise this could serve to establish a ratio of isotopic abundances that is not representative of that based purely on the half-life of the isotope whose decay is being measured.

What is a nuclear chain reaction?

A nuclear chain reaction is a string of reactions that occurs when a given nuclear reaction causes, on average, at least one more nuclear reaction to take place. Such chain reactions are important for the generation of nuclear power and also for nuclear weapons. Uranium-235 is responsible for the chain reaction that generates power in nuclear reactors and in some bombs as well. Uranium-238 is the more common isotope, so it is typically necessary to first enrich the uranium to be used in the 235 isotope. When a neutron collides with uranium-235 it generates uranium-236, which then undergoes fission to release energy and further neutrons that can collide with other uranium-235 atoms, causing the chain reaction to continue.

How does an atomic bomb work?

Atomic bombs (A-bombs) are based on nuclear chain reactions that occur very rapidly, causing a huge release of energy in a very short amount of time. In early designs, two pieces of uranium would be fired at one another in the core of the bomb, initiating the fission chain reaction responsible for the explosion of the bomb. As the bomb starts to detonate the core of the bomb expands, and it is necessary that pressure be applied against the expanding core while the fission process takes place. Within a fraction of a second after detonation, the explosion takes place. These are the type of bombs that were used at Hiroshima and Nagasaki in World War II and are the only nuclear weapons that have been used in war to this day.

What’s the difference between an H-bomb and an A-bomb?

The hydrogen bomb (H-bomb) is actually significantly more destructive than even an A-bomb. While A-bombs release energy via chain fission reactions (breaking apart heavy nuclei), H-bombs release energy through fusion of light nuclei. This energy comes from an overall increase in stability due to the strong force that holds nuclei together as the light nuclei fuse to create heavier ones. To give an idea of the relative powers of these two weapons of mass destruction, consider that the A-bomb dropped on Hiroshima had a force on the order of 10 kilotons (explosive force equivalent to 10,000 tons of TNT), while a common H-bomb has a force on the order of 10 megatons, or 1,000 times the explosive force of the A-bomb used at Hiroshima.

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Uranium-235 chain reactions start because the radioactive substance naturally emits neutrons that then collide with other atoms. In a nuclear bomb, the goal is to let the reaction reach a critical point where there is an explosion, but with a nuclear reactor the tricky part is controlling the reaction.

How is radiation used in medicine?

We should begin by pointing out the distinction between radiation used in nuclear medicine/radiopharmaceuticals (more akin to the other topics of this chapter) and electromagnetic radiation (light of different wavelengths).

Nuclear medicine is the branch of medicine most closely tied to the concepts of nuclear chemistry discussed in this chapter. Diagnosis via nuclear medicine typically involves the injection of a radiopharmaceutical into the body, and the radiation released by this drug can then be monitored to gain information about organ function, blood flow, the location of a tumor, or to locate a fractured bone. In some cases, the use of nuclear medicine can allow for earlier diagnosis than with other imaging techniques.

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An atomic bomb blast (illustration shown here) releases huge amounts of energy by creating a fision chain reaction within the bomb.

In terms of using electromagnetic radiation for medical applications, perhaps one of the first treatments that come to mind is radiation therapy, which is used to fight against a broad range of cancers. This involves using focused electromagnetic radiation to damage the DNA in the tissue of a tumor while hopefully not causing too much damage to the surrounding healthy tissue. The goal is to damage the DNA of cancerous cells so that they are unable to reproduce, hopefully killing the tumor with time. Beams of radiation are focused onto the tumor from different angles to minimize the effect on any one area of healthy tissue.

X-rays and CT scans are two commonly used, noninvasive medical techniques that make use of electromagnetic radiation to take pictures of what’s going on inside the human body. It should be noted that prolonged exposure to the X-rays used in these procedures can be harmful and are capable of causing cancer themselves over long periods of time.

How are isotopes made?

Specific isotopes of an element can be obtained in one of two ways: either by separation of the desired isotope from a naturally occurring sample or by synthesis of the desired isotope.

Since the different isotopes of an element all have the same chemical properties, they can be quite difficult to separate. The separation techniques used to separate different isotopes are thus based on their differences in mass, rather than on differences in chemical properties. Some of the methods used include separation by diffusion in the gas or liquid phases, centrifugation, ionization and mass spectrometry, or chemical methods based on differences in reaction rates due to different atomic masses.

Different isotopes of an element can also be generated synthetically. One way to do this is to fire high-energy particles at the nucleus of an atom. Depending on the situation, this can either cause a particle to be emitted from the parent nucleus (generating a lighter nucleus) or the fired particle can be absorbed (generating a heavier nucleus). It is also possible to synthesize isotopes of some elements by making use of another naturally occurring nuclear reaction, such as when the particles released by one nuclear fission reaction are absorbed by another nucleus.

Which elements are man-made?

Actually, we can make a lot of elements synthetically. Here’s a list of all of the man-made elements:

technetium (Tc)—43 (the first man-made element)

promethium (Pm)—61

neptunium (Np)—93

plutonium (Pu)—94

americium (Am)—95

curium (Cm)—96

berkelium (Bk)—97

californium (Cf)—98

einsteinium (Es)—99

fermium (Fm)—100

mendelevium (Md)—101

nobelium (No)—102

lawrencium (Lr)—103

rutherfordium (Rf)—104

dubnium (Db)—105

seaborgium (Sg)—106

bohrium (Bh)—107

hassium (Hs)—108

meitnerium (Mt)—109

darmstadtium (Ds)—110

roentgenium (Rg)—111

copernicium (Cn)—112

ununtrium (Uut)—113

ununquadium (Uuq)—114

ununpentium (Uup)—115

ununhexium (Uuh)—116

ununseptium (Uus)—117

ununoctium (Uuo)—118

How do nuclear power reactors work?

A nuclear power reactor works by generating heat from a controlled fission reaction, which then generates steam used to drive turbines to generate electricity. The fuel for the nuclear reactor is typically uranium-235 or plutonium–239.

What is a thorium reactor?

A thorium fuel cycle is also possible for use in nuclear power reactors. This involves using thorium–232 to generate uranium-233, which is capable of undergoing fission processes to generate energy in the form of heat.

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Nuclear reactors work by generating heat from controlled fission reactions. Breeder reactors actually create more fissionable material than they use and are self-sustaining.

What is a breeder reactor?

A breeder reactor is a type of nuclear reactor that is capable of generating fissile material (material that can sustain a chain fission reaction) faster than it uses it up. This is accomplished by using the neutrons given off in the fission reaction to generate additional isotopes capable of fusion. Typically this involves the use of either thorium to generate fissile uranium or uranium to generate fissile plutonium.

What is radon?

Radon is an element that is widely known for its potential to cause cancer. It is the heaviest gas known to man, with a density roughly nine times greater than that of air. It is usually found in soil and rocks, though it can also be found in water. Fortunately, radon detectors are commonly available that allow you to test your home for elevated radon levels.

What are some of the worst nuclear disasters in history?

A few of the worst nuclear disasters in history are those which took place at Three Mile Island in the USA in 1979, at Chernobyl in the Ukraine in 1986, and more recently following an earthquake in Fukushima, Japan, in 2011. Nuclear disasters are very dangerous if they do occur, and the possibility of a nuclear disaster represents a primary reason that some people oppose the construction of new nuclear power plants.