Nuclear Reactions
Simple introduction
light arguably is the most celebrated formula in the modern world.
And the subject of this text, nuclear power reactors, constitutes the
most widespread economic ramification of this formula. The nuclear
fission reactions that underlie power reactors—that is, reactors built
to produce electric power, propulsion for ships, or other forms of
energy use—convert measurable amounts of mass to energy. Thus
an appropriate place to begin a study of the physics of nuclear power
is with the underlying nuclear reactions. To understand the large
amounts of energy produced by those reactions in relation to the
mass of fuel consumed it is instructive to introduce our study by
comparing the production of nuclear power with that created by
fossil fuels: coal, oil, or natural gas. Contrasting these energy sources,
which result from chemical reactions, to nuclear energy assists in
understanding the very different ratios of energy created to the
masses of fuel consumed and the profound differences in the quantities
of by-products produced.
Coal is the fossil fuel that has been most widely used for the
production of electricity. Its combustion results predominantly
from the chemical reaction: C + O2 = CO2. In contrast, energy production
from nuclear power reactors is based primarily on the
nuclear reaction neutronþuranium-235 ! fission. Energy releases
from both chemical and nuclear reactions is measured in electron
volts or eV, and it is here that the great difference between chemical
and nuclear reactions becomes obvious. For each carbon atom combusted
about 4.0 eV results, whereas for each uranium atom fissioned
approximately 200 million eV, or 200MeV is produced. Thus roughly
50 million times as much energy is released from the nuclear fission
of a uranium nucleus as from the chemical combustion of a carbon
atom.
For comparison, consider two large electrical generation plants,
each producing 1000 megawatts of electricity (i.e., 1000 MW(e)), one
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burning coal and the other fissioning uranium. Taking thermal
efficiency and other factors into account, the coal plant would
consume approximately 10,000 tons of fuel per day. The uranium
consumed by the nuclear plant producing the same amount of electrical
power, however, would amount to approximately 20 tons per
year. These large mass differences in fuel requirements account for
differences in supply patterns. The coal plant requires a train of 100
or more large coal cars arriving each day to keep it operating. The
nuclear power plant does not require a continual supply of fuel.
Instead, after its initial loading, it is shut down for refueling once
every 12 to 24 months and then only one-fifth to one-fourth of its fuel
is replaced. Similar comparisons can be made between fossil and
nuclear power plants used for naval propulsion. The cruises of oilpowered
ships must be carefully planed between ports where they
can be refueled, or tanker ships must accompany them. In contrast,
ships of the nuclear navy increasingly are designed such that one fuel
loading will last the vessel’s planned life.
The contrast in waste products from nuclear and chemical reactions
is equally as dramatic. The radioactive waste fromnuclear plants
ismuchmore toxic thanmost by-products of coal production, but that
toxicitymust be weighed against themuch smaller quantities ofwaste
produced. If reprocessing is used to separate the unused uranium from
the spent nuclear fuel, then the amount of highly radioactive waste
remaining from the 1000-MW(e) nuclear plant amounts to substantially
less than 10 tons per year. In contrast, 5% or more of the coal
burned becomes ash that must be removed and stored in a landfill or
elsewhere at the rate of more than five 100-ton-capacity railroad cars
per day. Likewise it may be necessary to prevent nearly 100 tons of
sulfur dioxide and lesser amounts of mercury, lead, and other impurities
from being released to the environment. But the largest environmental
impact from burning fossil fuels may well be the global
warming caused by the thousands of tons of CO2 released to the
atmosphere each day by a 1000-MW(e) coal-fired power plant.
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