uranium-235 atom absorbs a neutron and splits during a fission process, it
produces a cesium-140 atom, a rubidium-93 atom, 3 neutrons, and 3.2
10 ^11 J of energy. In practical terms, the complete fission of 1 kg of ura-
nium-235 releases 6.73 1010 kJ of heat, which is more than the heat
released when 3000 tons of coal are burned. Therefore, for the same amount
of fuel, a nuclear fission reaction releases several million times more energy
than a chemical reaction. The safe disposal of used nuclear fuel, however,
remains a concern.
Nuclear energy by fusion is released when two small nuclei combine into
a larger one. The huge amount of energy radiated by the sun and the other
stars originates from such a fusion process that involves the combination of
two hydrogen atoms into a helium atom. When two heavy hydrogen (deu-
terium) nuclei combine during a fusion process, they produce a helium-3
atom, a free neutron, and 5.1 10 ^13 J of energy (Fig. 2–8).
Fusion reactions are much more difficult to achieve in practice because of
the strong repulsion between the positively charged nuclei, called the
Coulomb repulsion. To overcome this repulsive force and to enable the
two nuclei to fuse together, the energy level of the nuclei must be raised by
heating them to about 100 million °C. But such high temperatures are found
only in the stars or in exploding atomic bombs (the A-bomb). In fact, the
uncontrolled fusion reaction in a hydrogen bomb (the H-bomb) is initiated
by a small atomic bomb. The uncontrolled fusion reaction was achieved in
the early 1950s, but all the efforts since then to achieve controlled fusion by
massive lasers, powerful magnetic fields, and electric currents to generate
power have failed.
EXAMPLE 2–1 A Car Powered by Nuclear FuelAn average car consumes about 5 L of gasoline a day, and the capacity of
the fuel tank of a car is about 50 L. Therefore, a car needs to be refueled
once every 10 days. Also, the density of gasoline ranges from 0.68 to 0.78
kg/L, and its lower heating value is about 44,000 kJ/kg (that is, 44,000 kJ
of heat is released when 1 kg of gasoline is completely burned). Suppose all
the problems associated with the radioactivity and waste disposal of nuclear
fuels are resolved, and a car is to be powered by U-235. If a new car comes
equipped with 0.1-kg of the nuclear fuel U-235, determine if this car will
ever need refueling under average driving conditions (Fig. 2–9).Solution A car powered by nuclear energy comes equipped with nuclear
fuel. It is to be determined if this car will ever need refueling.
Assumptions 1 Gasoline is an incompressible substance with an average den-
sity of 0.75 kg/L. 2 Nuclear fuel is completely converted to thermal energy.
Analysis The mass of gasoline used per day by the car isNoting that the heating value of gasoline is 44,000 kJ/kg, the energy sup-
plied to the car per day is 1 3.75 kg>day 21 44,000 kJ>kg 2 165,000 kJ>dayE 1 mgasoline 21 Heating value 2mgasoline 1 rV (^2) gasoline 1 0.75 kg>L 21 5 L>day 2 3.75 kg>day
Chapter 2 | 57
U-235
3.2 × 10 –11 J
3 neutrons
neutron
(a) Fission of uranium
Uranium
Ce-140
Rb-93
n
n
nn
5.1 × 10 –13 J
neutron
(b) Fusion of hydrogen
He-3
n
H-2
H-2
FIGURE 2–8
The fission of uranium and the fusion
of hydrogen during nuclear reactions,
and the release of nuclear energy.
Nuclear
fuel
FIGURE 2–9
Schematic for Example 2–1.