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

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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

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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.

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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

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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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