E
Atomic Physics
Chapter 21 Nuclear Energy
21 Nuclear Energy
Practice 21.1 (p. 288)
1
B
2
C
(c) Fission products are usually radioactive
and a lot of radiation is given off during
fission. They could be harmful to the
Mass of U-235 required =
10
11
7.2  10
10
environment when they are not handled
= 1.39 g
with great care. On the other hand, fusion
3
B
4
B
products are safe to the environment. For
example, in 21 H  31H  42 He  01n , He-4
5
Since both  particles and uranium nuclei are
and neutrons are not radioactive.
positively charged, the electrostatic repulsion
8
(a) By E = mc2,
between  particles and uranium nuclei is very
E = 1.99  1030  (3 108)2
large. Hence, it is very difficult for  particles
= 1.791  1047 J
Energy of 1.791  1047 J will be released.
E
(b) By t = ,
P
1.791  10 47
t=
= 4.59  1020 s
3.90  10 26
to get close to uranium nuclei and break them.
6
From the total number of protons,
92 + 0 = a + 37 + 3  0
a = 55
From the total mass number,
= 1.46  1013 years
235 + 1 = 143 + b + 3  1
The remaining life of the Sun is
b = 90
7
1.46  1013 years.
(a) For nuclear fission, the fuel (fissile
(c) The estimate in (b) is longer than the
materials) has a large mass number, e.g.
U-235 and Pu-239. For nuclear fusion, the
actual remaining life of the Sun. It is
fuel has a small mass number, e.g. H-1,
because only a fraction of the mass of the
H-2 and H-3.
Sun will be converted into energy when
the Sun undergoes fusion.
(b) Both nuclear fission and nuclear fusion
can give out a large amount of energy.
However, with the same mass of fuel, the
Practice 21.2 (p. 297)
energy produced by fusion is more than
1
B
that by fission.
2
D
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3
Atomic Physics
Chapter 21 Nuclear Energy
(a) Burning fossil fuels to generate electricity
Mass of uranium fuel used
produces greenhouse gases, like CO2 and
=
SO2, and ashes.
4.32  1014
8.64  1013
= 5 kg
Using nuclear power does not produce
greenhouse gases, but the nuclear waste is
radioactive and it is difficult to handle
Revision exercise 21
nuclear waste.
Multiple-choice (p. 301)
Section A
(b) Nuclear power plants generally have high
safety standards because the high radiation
1
D
level near the station is harmful to living
2
D
things when there is any serious accident.
3
C
However, burning fossil fuels has much
4
D
fewer safety issues.
Section B
(c) The reserve of nuclear fuel is huge, while
4
the reserve of fossil fuels will soon be
5
D
exhausted.
6
C
7
D
For a nuclear reactor, the chain reaction is
controlled by a moderator and control rods
inside the reactor, and the chain reaction can
Conventional (p. 301)
either speed up and cool down by the control
Section A
of the above two components.
1
(a) Radioactive decay occurs randomly (1A)
while nuclear fission should be triggered.
For an atomic bomb, nothing inside the bomb
(1A)
controls the chain reaction. Once the
5
radioactive source reaches the critical mass,
In each radioactive decay, the mother
uncontrolled chain reaction starts, releasing a
nucleus forms a single new nucleus; (1A)
huge amount of heat and nuclear radiation.
however, in each nuclear fission, the
(a) For each reactor, by E = Pt,
heavy nucleus splits to form more than
E = 5  10  8  60  60
one smaller nuclei.
9
= 1.44  10 J
(b) When a neutron collides with a uranium
The energy supplied by the power stations
= 1.44  10  3 = 4.32  10 J
14
(1A)
(Accept other reasonable answers.)
14
nucleus, the uranium nucleus splits in a
14
nuclear fission and neutrons are emitted.
(b) Energy supplied by 1 kg of uranium in the
(1A)
reactor
= 2.88  10  30% = 8.64  10 J
14
The emitted neutrons carry on splitting
13
other uranium nuclei and a chain reaction
is resulted.
New Physics at Work (Second Edition)
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(1A)
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2
Atomic Physics
(a) (i)
Chapter 21 Nuclear Energy
A gas-burning power station does not
(b) By
emit greenhouse gas, SO2, and (1A)
its efficiency is higher.
efficiency =
(1A)
energy released by the fuel
(ii) The reserve of coal is more than that
of gas. Coal-burning power stations
can operate a longer time.
=
(1A)
5
(b) It is because, for the same mass of fuel,
repel each other.
nuclei must approach each other with very
high speed to overcome the repulsion.(1A)
Tidal energy
In order to achieve this, the temperature of
Geothermal energy
the hydrogen gas should be raised to an
Bioenergy
(a) Energy output
= 2  109  60  60
(1M)
= 7.2  1012 J
(1A)
6
(b) Raising or lowering the control rods in the
(1A)
(For effective communication.)
(1C)
(d) In the Sun.
(1A)
(a) Neutrons are also emitted.
(1A)
by a concrete shield.
(1A)
(Accept other reasonable answers.)
(1A)
(c) The nuclear wastes are radioactive. (1A)
As a result, the power output of the
They are sealed in metal containers to
nuclear reactors is adjusted.
prevent leakage of radiation.
(1M)
7
E = (2  500 + 800)  106
 20  60  60
= 1.296  1014 J
extremely high value.
(b) The nuclear reactor should be surrounded
(1A)
can made the rate of chain reaction faster
(a) By E = Pt,
(1A)
For fusion to occur, the two hydrogen
Solar energy
4
(1A)
(c) Since nuclei carry positive charge, they
(1A)
Hydropower
or slower.
(1A)
not radioactive.
(1A)
Wind energy
nuclear reactors
(1A)
(b) The products formed by nuclear fusion is
Also, the reserve of nuclear fuel is more
3
(a) In nuclear fission, a heavy nucleus splits to
form some heavier nuclei.
(1A)
(c) Any one of the following:
(1A)
In nuclear fusion, light nuclei combine to
is much more than that by burning coal or
than that of coal and gas.
1.296  10 14
= 4.32  1014 J
0.3
form some smaller nuclei.
the amount of energy generated by fission
gas.
energy output
 100% (1M)
energy input
(1A)
(1A)
(a) Nuclear fission is the splitting of a heavy
nucleus
(1A)
by neutrons.
(1A)
(b) Heat / kinetic energy of fission products is
Power station can supply 1.296  1014 J
released in nuclear fission.
everyday.
(1A)
(c) Advantages of nuclear power stations, any
two of the following:
(2  1A)
The fuel transportation problem is small.
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E
Atomic Physics
Chapter 21 Nuclear Energy
(c)
The nuclear energy generated per unit
1
235
92
0 n  92 U  36 Kr

141
56 Ba
 3 01n  energy
mass of fuel is huge.
(1A)
No greenhouse gas, like CO2 and SO2, and
(d) If the outgoing neutrons collide with other
ashes is produced.
U-235 nuclides, those U-235 nuclides
The reserve of nuclear fuel is huge.
would undergo fission and produce more
Disadvantages of nuclear power stations,
neutrons that can induce fission of other
any two of the following:
(2  1A)
U-235 nuclides.
(1A)
Nuclear waste is radioactive and it is
In this way, fission reaction can carry on
difficult to handle nuclear waste.
and it forms a chain reaction.
After accidents, the rise of the radiation
Since the number of neutrons produced
level near a nuclear power station is
grows, the rate of reaction grows and more
harmful to human.
and more heat is generated.
The maintenance cost of a nuclear power
If heat is generated fast enough, explosion
plant is high.
may occur.
(1A)
(For effective communication.)
(1C)
(d) reactor, heat exchanger, steam, generator
(4  1A)
(1A)
(1A)
(e) The nuclear energy changes into heat.(1A)
(f) Advantages (any two of the following):
(2  1A)
Section B
8
(a) This nuclear power station uses nuclear
fusion.
(b) X is a neutron.
2
1H

3
1H

4
2 He

Since only small amount of nuclear fuel is
(1A)
used in nuclear power station, the fuel
(1A)
transportation problem is small.
1
0n
The nuclear energy generated per unit
(Correct symbols for the nuclides.) (1A)
mass of fuel is huge.
(Correct mass numbers and atomic
The reserve of nuclear fuel is huge.
numbers.)
No greenhouse gas, like CO2 and SO2, and
(1A)
ashes is produced by nuclear power
(Correct presentation of equation.) (1A)
station.
(c) The total mass of the nuclei before
reaction is greater than that after reaction.
Disadvantages (any two of the following):
(1A)
(2  1A)
Nuclear waste is radioactive and it is
(d) Nuclear fusion can provide a very cheap,
abundant and safer source of energy. (1A)
difficult to handle nuclear waste.
The waste product of nuclear fusion is
If serious accidents (e.g. meltdown of a
inert and non-radioactive.
reactor) happen, the high radiation level
(1A)
near the nuclear power plant is harmful to
(Accept other reasonable answers.)
9
(a) Nuclear fission
(1A)
living things.
(b) X is a neutron.
(1A)
The maintenance cost of a nuclear power
plant is high.
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Atomic Physics
10 (a) (i)
Chapter 21 Nuclear Energy
Neutrons are difficult to detect
(iv) The lost mass has been converted to
because they are electrically neutral.
energy.
(1A)
(1A)
11 (a) When those nuclei undergo fusion, a
They do not deflect in magnetic field
heavier nucleus is formed
(1A)
and electric field.
and energy is released.
(1A)
(ii) They induce fission.
(1A)
(1A)
(b) Since objects of the same charge repel,
They bombard and split the nuclei of
when nuclei approach each other, they
fissile materials.
experience repulsive force,
(1A)
(iii) No, electrons cannot replace neutrons
in fission.
(1A)
which decelerates them and may change
(1A)
their moving directions.
(1A)
Since objects of the same charge
Hence, nuclei should move at a high speed
repel,
(high kinetic energy) to get close enough
(1A)
the approaching electrons will be
for fusion to occur.
easily repelled by the electrons
(c) Since the speed of nuclei increases with
surrounding the nuclei of fissile
materials.
their temperature and
(1A)
(1A)
nuclei should have a large amount of
Thus electrons can hardly get close to
kinetic energy to get close together. (1A)
the nuclei of fissile materials and
12
(HKCEE 2003 Paper I Q9)
induce fission.
(b) (i)
Uranium-235 is the nuclear fuel used
Physics in articles (p. 305)
in this reactor.
(a) Water or welded containers can minimize the
(1A)
(ii) The mass numbers and atomic
amount of radiation releases from the nuclear
numbers on both sides of the
waste
equation should be balanced.
to the surroundings by absorbing the radiation.
(1A)
a = 1 + 235  135  5 = 96
(1A)
(1A)
b = 92  40 = 52
(1A)
(b) Leakage of nuclear waste from the containers
(iii) According to the equation, 5 neutrons
are emitted during fission.
(1A)
(1A)
causes serious environmental problems and
Some of these neutrons would collide
damages to health of living things.
with other U-235 nuclei to induce
(c) (Accept any reasonable answer.)
(1A)
(2  1A)
fission and produce more neutrons,
some of which will also collide with
other U-235 nuclei and induce fission.
(1A)
Hence, fission reaction carries on and
leads to chain reaction.
(1A)
(For effective communication.) (1C)
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