Energy Resources

197

13

Nuclear Energy

Introduction

Nuclear energy is energy that is obtained from the nucleus of an atom. The

energy is released from an atom through one of two processes: (1) nuclear

fusion or (2) nuclear ssion.

Energy is released in nuclear fusion when atoms are combined or fused

together (fusion). This is how energy is generated in the sun. In nuclear s-

sion, energy is released when atoms are split apart. Nuclear ssion is the only

method currently used by nuclear plants to generate electricity. However, the

terms nuclear fusion and nuclear ssion are referred to synonymously as

nuclear energy. Both involve changes in (nuclear) mass.

Regarding ssion, uranium is the heaviest of the 92 naturally occurring

elements. Since it is also one of the few elements that is easily ssioned, it

is the fuel of choice used by today’s nuclear power plants. This element was

formed when the Earth was created and is usually found in rocks. Rocks that

contain signicant quantities of uranium are referred to as uranium ore, or

pitchblende. Two forms (isotopes) of uranium are found in these rocks: ura-

nium-235 and uranium-238. These three-digit numbers refer to the number of

neutrons and protons in each atom. Uranium-235 is the form normally used

for energy production because, unlike uranium-238, the nucleus splits more

easily when bombarded by a neutron. During ssion, the uranium-235 atom

absorbs a bombarding neutron, causing its nucleus to split apart into two

atoms of lighter weight. (See Figure13.1.) The ssion reaction simultaneously

releases energy as both heat and radiation. It also releases more neutrons.

These released neutrons go on to bombard other uranium atoms, and the

process repeats itself. This repetitive process is referred to as a chain reaction.

When a uranium atom is split apart (ssioned), the mass of the fragments

is less than the mass of the original atom. The energy corresponding to this

loss of mass is dened as ssion energy. It is represented in equation form by

Einstein’s equation

[1]:

=E mc

2

(13.1)

198 Energy Resources: Availability, Management, and Environmental Impacts

where (in SI units)

E = joules (J)

m = kilograms (kg)

c = meters/second (m/s); the velocity of light, 3 × 10

8

m/s

One can show that each kilogram of mass converted liberates approxi-

mately 10

17

J. This thermal energy is enough to run a large turbine generator

(a 1-GW unit) for a year at 35 percent thermal efciency and is equivalent to

the energy possessed by 2.5 million tons of coal.

Similarly, if two or more light elements (e.g., hydrogen) are combined

(fused) to form a heavier one, the resulting mass of the atom is less than the

sum of the two original atoms. The difference (or loss) in mass is converted

to energy, as with the fusion process. This energy is dened as fusion energy.

The peaceful use of nuclear energy involves the following process. Nuclear

energy is generated by the aforementioned splitting of uranium atoms. The

energy in the form of heat from this ssion process is used to drive a turbine

to generate electricity. The operation of a nuclear reactor and the related elec-

tric generating equipment is only one part of an interconnected set of indi-

vidual processes. These processes include mining, milling, and transporting

uranium; enriching uranium and packing it in appropriate form; designing

and constructing the reactor and auxiliary equipment; and treating and dis-

posing of spent fuel. All these activities thus require several sophisticated

and interactive industrial processes that involve many separate pieces of

equipment requiring many specialized skills. Additional details are pro-

vided in the next section.

When commercial use of nuclear power was rst developed (in the late

1950s—see also the next section), many thought it would become the great

savior of human civilization, providing a source of clean, limitless electric-

ity that would replace fossil fuels in the future. Unfortunately, that has not

come to fruition; simply put, nuclear power has yet to achieve that promise,

and concerns to be discussed later have severely hampered the development

Neutron

Neutron

Energy

Atom Splits

Uranium

235

Lighter

Element

Lighter

Element

Neutron

Fission

FIGURE 13.1

The ssion process.