Environmental Engineering Reference
In-Depth Information
energy use, but it is doubtful that renewable energy will replace large-scale centralized fossil fueled
or nuclear power plants supplying the base load for urban-industrial areas.
Nuclear energy resources are far more abundant than fossil fuel resources. It is estimated
that high-grade uranium ores could provide the present mix of reactors for about 50 years, but
utilization of lower-grade ores (with concomitant increase of refined uranium fuel price) would
last for many centuries. Utilization of thorium ores and fast breeder reactors could extend nuclear
energy resources to millennia.
1
Thus, it is possible that worldwide nuclear power plants will again
win public favor and become economically competitive with other energy sources.
In this chapter we describe the fundamentals of nuclear energy, its application for electricity
generation, the nuclear fuel cycle, and the problems associated with nuclear power plants in regard
to their safety, nuclear weapons proliferation, and radioactive waste disposal.
6.2
NUCLEAR ENERGY
Nuclear energy is derived from the binding force (the “strong” force) that holds the nucleons
2
of
the atomic nucleus together. The binding force per nucleon is greatest for elements in the middle
of the periodic table and is smallest for the lighter and heavier elements. When lighter nuclei
fuse
together, energy is released
3
; when heavier nuclei undergo
fission,
energy is also released. When
a nucleus of
235
U (an isotope of uranium) is bombarded with a neutron, it splits into many fission
products with the release of two to three times as many neutrons as were absorbed. For example,
one of the fission reactions is the splitting of
235
U into
144
Ba and
89
Kr, with the release of 3 neutrons
plus 177 MeV of energy
4
,
5
:
235
U
144
Ba
89
Kr
+
n
→
+
+
3n
+
177MeV
(6.1)
where n stands for a neutron.
1
“Nuclear Electricity,” 1999. Uranium Information Centre, Melbourne, Australia.
2
For our purposes we shall consider as nucleons only positively charged protons and chargeless neutrons.
Other particles have been observed in nuclear disintegration experiments, but they are not germane to our
discussion.
3
We shall discuss fusion at the end of this chapter.
4
Electron volt (eV)
=
energy gained by an electron when accelerated through an electrical potential difference
of 1 volt. 1 eV
=
1.602E(
−
19) J; 1 MeV
=
1.602E(
−
13) J.
5
The energy released in a fission reaction can be calculated by means of the famous Einstein equation that
ties energy to mass,
E
=
mc
2
. The masses on the left-hand side of equation (6.1) are
235
U
=
235.04394 amu
(1 atomic mass unit
=
1.66E(
−
27) kg), n
=
1.00867 amu; on the right-hand side
144
Ba
=
143.92 amu,
89
Kr
=
3.026 amu. If we subtract the sum of the masses on the right-hand side from the sum of
the masses on the left-hand side, there is a “mass deficit,”
88.9166 amu, 3n
=
m
=
0
.
19 amu. This mass deficit is converted into
energy
E
=
mc
2
=
2.84E(
−
11) J
=
177 MeV. (1 amu
deficit is equivalent to 931.5 MeV.) The fission of
235
U produces 2.84E(
−
11) J
×
6.023E(23) atoms mole
−
1
=
0
.
19 amu
×
1.66E(
−
27) kg
×
[3E(8)]
2
m
2
s
−
2
÷
0.235 kg mol
−
1
=
7.3E(13) J kg
−
1
energy. In comparison, the combustion of carbon produces 3.3E(7) J kg
−
1
,
about 2 million times less energy per unit weight than a fission reaction.
