Chemical fuels, such as oil or methane, release energy when the atoms in their molecules are rearranged into lower energy configurations. The energies involved are those of molecular binding and are of the order of tens of MJ/kg. When the components of an atom are arranged into lower energy configurations, then the energy released is orders of magnitude larger (hundreds of TJ/kg) because of the much larger intra-atomic binding energies.
The internal structure of atoms can be changed in different ways:
1. An atomic nucleus can be bombarded with a neutron, absorbing it. A different atom emerges.
2. An atom can spontaneously change by emitting either electrons (beta-rays) or helium nuclei (alpha-rays). Such radioactive decay releases energy, which can be harvested as, for instance, it is done in Radioisotope Thermal Generators (RTGs). (See Chapter 5).
3. Atoms with large atomic numbers can be made to break up into smaller atoms with the release of energy. This is called nuclear fission and requires that the atomic number, Z, be larger than 26.
4. Atoms with low atomic numbers can be assembled into a heavier one, releasing energy. This is called nuclear fusion and requires that the final product have an atomic number smaller than 26.^
Currently, only two techniques are used to produce energy from nuclear sources: the RTG mentioned above and nuclear fission reactors.^ But, nuclear energy has developed a bad reputation, especially after the Chernobyl accident in 1986. Nevertheless, it is a source of substantial amounts of energy in many countries. According to the Energy Information Administration, EIA, since 1998, the number of nuclear plants in the United States has remained unaltered at 104. Nevertheless, there has been a 2% per year secular increase in the generation of nuclear electricity owing mostly to an improvement of the plant utilization factor from 78.2% in 1998 to over 94% in 2007. It appears that after 2008, a number of new reactors may be purchased.In 2007, the United States led the world in installed capacity—104 GW—followed by France (63 GW) and Japan (47.6 GW). The utilization factor of nuclear plants that year was excellent. In the United States, it was over 94%, in France, 77.5%, and in Japan, 68.9%.
Of the total electricity generated, nuclear plants in the United States (2008) contributed a relatively modest 19.9%, while in France, heavily reliant on this form of energy, the contribution was 76.1%. In Japan, it was 34.6%. In 2000, Germany decided to phase out its 19 nuclear power plants. Each one was assigned a 32-year life after which they would be deactivated. Many plants have already operated more than half of their allotted lifetime.
The cost of nuclear electricity is high, about double that from fossil fuel. In the United States (1996), it was 7 cents/kWh, whereas that of a state of the art natural gas plant was 3 cents/kWh (Sweet, William #1). Advanced reactor designs may bring these costs down considerably while ensuring greater safety (Sweet, William #2). This promised reduced cost combined with the ecological advantage of no greenhouse gas emission—a growing concern—may lead to a renewed popularity for nuclear generators.
The major objection to fission-type reactors is not so much the danger of the operation of the power plants (the Chernobyl accident was perfectly avoidable), but rather the problem of disposing of large amounts of long-lived radioactive by-products. If the need for such disposal can be avoided, then there is good reason to reconsider fission generators as an important contributor to the energy supply system, especially if they are not restricted to the use of the rare 236U fuel the way present-day reactors are.
^ All are transmutations, the age-old dream of medieval alchemists. ^ Cannons preceded by centuries the invention of heat engines. Nuclear bombs were used before nuclear reactors—fusion has for decades been used in thermonuclear bombs, but its use in reactors still seems far into the future.
Specifications of new-generation nuclear fission reactors might include (not necessarily in order of priority), the following items:
1. Safety of operation (including resistance to terrorist attacks)
4. Absence of weaponizable subproducts
5. Absence of long-lived waste products
6. Ability to transmute long-lived radioactive waste products from old reactors into short-lived radioactive products
The U.S. Department of Energy is funding research (2004) in technologies that might realize most of these specifications. One of these is the heavy-metal fast breeder reactor technology. It appears that this type of reactor may be able not only to produce waste with relatively short half-lives (100 years contrasted with 100,000 years of the current waste), but in addition may be able to use current-type waste as fuel. Furthermore, because heavy-metal reactors operate at high temperatures (yet at low pressures), the thermolytic production of hydrogen (see Chapter 10) for use in fuel cell-driven automobiles looms as a good possibility. For further reading on this topic see Loewen (2004).
The waste disposal problem is absent in fusion devices. Unfortunately, it has been impossible to demonstrate a working prototype of a fusion machine, even after several decades of concerted research.
To do even a superficial analysis of the technical aspects of nuclear reactions, we need to know the masses of the atoms involved (see Table 1.9). Most of the mass values are from Richard B. Firestone. Those marked with a ^^ are from Audi and Wapstra (1993), and the one marked with a • is from a different source. It can be seen that the precision of the numbers is very large. This is necessary because, in calculating the energy released in a nuclear reaction, one uses the small difference between large numbers, which is, of course, extremely sensitive to uncertainties in the latter.
The listed values for the masses of the nucleons (the proton and the alpha in the table) are nearly the values of the masses of the corresponding atoms minus the mass the electron(s). On the other hand, there is a large difference between the the mass of a nucleon and the sum of the masses of the component protons and neutrons. Indeed, for the case of the alpha, the sum of the two protons and the two neutrons (4.03188278 daltons) exceeds the mass of the alpha (4.001506175 daltons) by 0.030376606 daltons— about 28MeV of mass. This is, of course, the large nuclear binding energy necessary to overcome the great electrostatic repulsion between the protons.
There are at least four fissile elements of practical importance: 233U, 235U, 239Pu, and 241 Pu. Of these, only 235U is found in nature in usable
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