The Promise and Problems of Nuclear Energy
CHAPTER 6
The Promise and Problems of Nuclear
Energy
6.1 Introduction
The story of nuclear energy is complex; it involves science, engineering, economics,
health and safety, psychology, and politics. It is not easy to set aside the strongly
negative image formed by the relationship of nuclear energy to nuclear weapons and
the serious accidents that have occurred with radioactive materials and nuclear
facilities. The mere terms nuclear energy, radioactivity, radiation, criticality, and
meltdown often have a frightening connotation, and they are not well understood by a
large part of the public. These words sometimes find their way into the headlines, even
though they often don't belong there.
Certainly there are problems with the nuclear power industry that do not stem
only from public apprehension. No new orders have been placed for reactors by the
public utilities in the United States since 1978, and there have been many cancellations
of orders placed before that time. This is largely because of the unfavorable economics
experienced with some power reactors. Legal liability, increasingly stringent
regulatory procedures, radioactive waste storage, and siting problems have added to
the negative side of the ledger, therefore no new orders are presently being realistically
-1-
discussed by the public utilities. This is not true in some other countries; both Japan
and France, for example, are moving ahead with nuclear electric power programs.
There are certain positive features associated with nuclear power reactors that
should not be overlooked in considering the possible future of nuclear power. Reactors
are not affected by the looming shortage of fossil fuels, and they emit no CO2, SO2,
CO, or particulates into the atmosphere. Uranium reactor fuel is now reasonably
abundant and inexpensive, and a breeder reactor technology could extend the resource
to many thousands of years. There now are 109 power reactors operating in the United
States with a total capacity of 99,000 MWe. They produce about 22% of the nation's
electricity. In France, 79% of the electricity comes from nuclear reactors. The reactor
safety record is as good or better than that of any other electricity-producing
technology. The Three Mile Island incident was costly, but endangered few people.
The reactor industry is now studying ways to produce more reliable, less expensive,
and safer reactors, making full use of the experience gained during the first 25 year
period of development from 1953 to 1978.
A second path to utilizing nuclear energy, by way of nuclear fusion reactions,
has problems and benefits quite distinct from those of nuclear fission reactors. Nuclear
fusion, the joining together of two light nuclei to form a heavier nucleus, releases
energy, but it is yet to be shown that nuclear fusion can be a practical source of
electrical energy. Intense efforts to develop fusion reactors have been underway for
more than 40 years, and progress is being reported, but it is not seen as likely that
commercial electricity generation from fusion reactors will be a reality within the next
few decades. The technical challenges are daunting, and it is possible that fusion
power will never make its way to the power grid. By contrast, it may be noted that the
time from the first laboratory demonstration of nuclear fission in 1938 to the first
demonstration of a simple reactor in 1942 was only four years, and fifteen years later,
in 1957, a power reactor at Shippingport, Pennsylvania, was producing commercial
electricity.
6.2 A Short History of Nuclear Energy
In the early 1930s, laboratory experiments established that the nucleus of the atom was
made up of neutrons and protons. The neutron and the proton have very nearly the
same mass, (1.67 x l0-27 kg), about 1840 times heavier than the electron. The neutron
has no electric charge, and the proton is positively charged. In the shorthand of
designating nuclear properties, the atomic number, Z, is the number of protons in the
nucleus, and the atomic mass number. A, is the sum of Z and N, the number of
neutrons. In addition to merely investigating nuclear masses and atomic numbers,
scientists in Europe, the United States, and a few other countries conducted intensive
research to determine more detailed properties of atomic nuclei. The scientists Hahn,
Strassman, Meitner, and Frisch discovered in 1938 and 1939 that the nuclei of uranium
atoms did a remarkable thing. When bombarded with neutrons, these heavy nuclei
fissioned, that is, they often split into two fragments, at the same time emitting more
neutrons. The two fragments are lighter nuclei, each having a mass roughly half that of
the uranium nucleus. These fragments are called fission products, and they carry off
about 160 MeV of kinetic energy, divided between the two fragments, from the fission
reaction.
The energy release in fission can be explained in terms of nuclear mass energy.
If one adds together the masses of the two fission fragments and the emitted neutrons,
the total will be appreciably less than the mass of the initial uranium nucleus, 235U,
along with that of the neutron which initiated the fission. The difference in mass, ¦¤ m
is related to the energy release, ¦¤ E, by the Einstein equation, ¦¤ E = ¦¤ mc2. The
magnitude of the energy that can be obtained from direct conversion of nuclear mass
to energy is impressively large compared to the energy released from burning a fuel
such as coal. Chemical reactions involve a few eV of energy per atom; nuclear fission
reactions hundreds of MeV per atom¡ª a factor of about 108 times larger. The energetic
-2-
fission fragments move away from the site of the fission reaction, and come to stop in
the surrounding material. In this way their kinetic energy very quickly becomes heat
energy in the material. The fission process is shown in Figure 6.1.
After the first reports of the laboratory findings concerning fission, it didn't take
long for physicists to realize that all the ingredients of an exponentially-increasing
chain reaction were at hand. Because each fission releases more than one neutron, the
neutrons from one fission reaction could induce further fission reactions in more than
one neighboring uranium nucleus. One becomes two, two becomes four, four becomes
eight, and so forth, in a simple model. The process would then repeat itself at a
growing rate and release a large amount of energy in a short time. The potential for
enormous explosive power was obviously there. It was also apparent that if the rate of
the chain reaction could be controlled by some means so that it was steady, rather than
increasing, then one would have a source of heat energy appropriate to drive an
electric generator. Because of the onset of World War II with the Nazi military
occupation of much of Europe in the years immediately following the discovery of
fission, the initial efforts to exploit this new phenomenon centered on producing a
nuclear weapon rather than a nuclear power reactor. The first nuclear reactor was
assembled at the University of Chicago in 1942 by Enrico Fermi and his coworkers,
and they successfully demonstrated a chain reaction, even though it was at a very low
power level, about 200 watts.
The fear that Nazi Germany would develop a nuclear bomb spurred on intense
efforts in the United States to get there first. The American project was given the code
name The Manhattan Project; it was centered at large laboratories in Los Alamos, New
Mexico; Oak Ridge, Tennessee; and elsewhere. The project faced the difficult task of
accumulating enough of the uranium isotope, 235U, to form the critical mass1 needed
for a workable bomb. Only two isotopes of uranium are found in nature. They are
235
238
92U and
92U. The left superscript gives the atomic mass, either 235 or 238, the left
subscript gives the atomic number, 92, and the right subscript gives the number of
neutrons, either 143 or 145, in the nucleus. The difficulty comes about because 235U is
the only isotope found in nature that is able to sustain a chain of fission reactions, and
235
U is only 0.7% abundant. That is, only 0.7% of uranium as it is mined from the earth
is 235U; the remaining 99.3% is 238U, which is not useful for a nuclear fission weapon.
Enormous engineering efforts were undertaken to process natural uranium so that it
was enriched to the 90% 235U necessary for a nuclear explosive. After some years of
effort, the Manhattan Project did produce three nuclear bombs. Two of these were
fueled with 239Pu,2 the other with 235U. The first weapon was exploded as a test in July
1945 in New Mexico, the other two were dropped on Hiroshima and Nagasaki in
August of 1945 at the end of the war with Japan. It turned out that neither Germany
nor Japan were close to having nuclear weapons during World War II, although their
scientists were certainly aware of the fundamental principles of these weapons.
Following the war, efforts were made to utilize nuclear reactors for the generation of commercial electricity and also for the propulsion of submarines and other
vessels for the U.S. Navy. The first production of electricity for the civilian market by
a reactor was at Shippingport, Pennsylvania in 1957. That was a small power reactor
largely financed by the federal government. Following this demonstration, several
much larger reactors having capacities of more than 100 MWe3 were ordered by
various public utilities. The Dresden I reactor station in Illinois was the first of these to
go on-line in 1962. From 1953 to 1978, 253 nuclear power reactors were ordered in the
United States, but 118 of these orders were cancelled. The generating capacity of the
135 that were not cancelled was 114 GWe. In the United States, there have been no
new orders placed for power reactors since 1978.
1
A critical mass is the minimum amount of a fissionable isotope needed to sustain a nuclear chain reaction.
It is often reported as about 15 kilograms for 235U, and about 5 kilograms for 239Pu.
2
The origin and uses of 239Pu are discussed in Section 6.4 of this chapter.
3
This means 100 megawatts of electrical output. The thermal power of the reactor is about three times
larger.
-3-
Up to about 1978 there was great optimism that an inexpensive, abundant,
environmentally acceptable source of energy had at last been found. In order to
understand some of the reasons why this optimism has not been fulfilled, a few more
details on nuclear physics and engineering are needed.
Krypton
Neutron
Figure 6.1 Three steps in the neutron-induced fission of 235U. The combination of a neutron and
235
U forms 236U in a highly excited state, that promptly fissions into two lighter nuclei, emitting
neutrons and gamma rays in the process.
6.3 Radioactivity
The word radioactivity has become a part of everyday conversation, but it has different
meanings for different people, and understanding is often vague and limited.
Radioactivity is an important part of any serious discussion of nuclear power because
almost all of the fission products are radioactive, as are the nuclear fuels uranium and
plutonium. It is also true, to some degree, that our food, the air, our natural
surroundings, and even our bodies are radioactive.
For our purpose, radioactive refers to an atomic nucleus that is unstable. It can
spontaneously decay, most commonly becoming a nucleus of another element, and
emit an energetic electron or alpha particle in the process. Some types of radioactive
nuclei decay very soon after they are formed; others have a low probability for decay
and are likely to survive for billions of years after their formation.
An example of the decay of a radioactive nucleus is the beta decay of 137Cs, a
common fission product of exceptional concern because of its thirty-year half-life and
likelihood of being taken up by living things.
137
55Cs 82
¨¤
137
56Ba 81 +
¦Â - + ?, T1/2 = 30 yr
Here we see that one of the neutrons in 13755Cs 82 has become a proton in
55Cs81 emitting an electron (also known as a beta particle), and an antineutrino in the
process. These light particles carry off the majority of the decay energy. The energetic
electron is responsible for much of the damage to living things that can be done by this
radioactivity. Some beta-decaying radioactive nuclei emit positive rather than negative
beta particles.
137
Another type of radioactive decay, important to nuclear reactors and their fuel,
is alpha particle emission. The alpha particle is the nucleus of the ordinary helium
atom, and it is stable against radioactive decay. It is common for the heaviest
radioactive nuclei to decay by emitting alpha particles. The decay of plutomum-239 is
an example of alpha decay.
239
94Pu 145
¨¤ 23592U143 + 42He2, T1/2 = 24,000 yr
-4-
In this process, two of the protons and two of the neutrons originally in the plutonium
combine to form the alpha particle. When heavy nuclei undergo alpha decay, most of
the decay energy appears as kinetic energy of the alpha particle. In this typical
example the alpha particle kinetic energy is about 5.2 MeV.
The half-life for radioactive decay is that characteristic time during which half
of the nuclei in any given sample will undergo decay. This time is different from one
nuclear species to another. For example, it is 1.3 billion years for potassium-40, 24,000
years for plutonium-239, 30 years for cesium-137, 12 seconds for oxygen-15, and
5730 years for carbon-14. The half-life is also, correspondingly, that time during
which the rate of emission of radiation from a radioactive sample will decline by a
factor of two. The process is continuous. The radiation or number of radioactive nuclei
in a sample will decline by a factor of two in one half-life, a factor of four in two halflives, a factor of eight in three half-lives, and so on.
There is a large variety of radioactive nuclei present in a collection of fission
products. Their half-lives range from very short, a fraction of a second, to very long, in
excess of a million years. Some of the fission product nuclei are not radioactive. Once
formed, they remain as they are. All radioactive nuclei found naturally on earth must
have half-lives at least comparable to the age of the earth or be formed by the decay of
these long-lived radioactivities, except for a very few such as 14C, with a half-life of
5730 years, that is formed continuously in our atmosphere by cosmic rays arriving
from outer space.
Example 6.1
Calculate the number of years needed for the following radioisotopes to decay to one
thousandth of their original activity. (Note that 1/1000 ¡Ö1/210 = 1/1024.)
239
Pu (T1/2 = 24,000 yr)
137
Cs (T1/2 = 30 yr)
3
H (T1/2 = 12.4 yr)
89
Sr (T1/2 = 50.5 days)
Solution
Each radioisotope will decay to one thousandth of its initial activity in about 10 halflives. For the listed radioisotopes this will require:
239
Pu : 240.000 yr
137
Cs : 300 yr
3
H : 124 yr
89
Sr : 1.38 yr
As a general rule the emissions from radioactive nuclei are classified as
ionizing radiation, that is, they are capable of removing a bound electron from an atom
or molecule by impact. If the atom or molecule happens to be a part of a biological
system, the ionizing event may lead to the breaking of a molecular bond, with severe
consequences, such as the disordering of genetic information, or the changing of
normal cells into cells that eventually become cancerous.
The human race has evolved in the presence of ionizing radiation that comes
continuously from cosmic rays and from the natural radioactivity in the earth.
Although such radiation certainly has the potential to be harmful, the effects have not
been of sufficient magnitude to thwart the development of the human race or that of
innumerable other animal and plant species. In fact, radiation certainly has caused
some of the mutations needed for the evolutionary process to proceed as it has.
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