Download chapter 21 blm answer key

CHAPTER 21 BLM ANSWER KEY
BLM 21-1: Fission and Fusion/
Reinforcement
nucleus are not attracted to one another. However,
protons on either side of the nucleus repulse one
another. When they absorb a neutron, certain large
nuclei therefore react by splitting into smaller
nuclei with higher binding energies per nucleon.
Answers
1.
Both fission and fusion reactions are transmutations, in
which a nuclide or nuclides change to become a
nuclide or nuclides of different elements. Both fission
and fusion release a great deal of thermal energy.
5.
In fission reactions, one large nucleus absorbs a
neutron and splits, producing two smaller nuclei and
neutrons. An example of a fission reaction is:
1
0
n
235
92
U
92
36
Kr 
141
56
Ba  301 n  thermal energy
6.
E  mc 2
  4.32  108 kg  2.998  108 m/s 
In a fusion reaction, two nuclei join, forming a single,
larger nucleus. An example of a fusion reaction is:
2
1
2. (a)
(b)
(c)
3. (a)
(b)
4. (a)
(b)
1
0
n 
235
92
4
2
U 
90
37
1
0
Rb 
144
55
7.
Cs  2 01 n  thermal energy
The reaction is a fission reaction because one large
nucleus splits when it absorbs a neutron, releasing
two smaller nuclei, neutrons, energy.
The energy produced by the fission reaction is from
the conversion of some of the mass of the uranium
nucleus into energy. Therefore, the combined mass
of the rubidium and cesium nuclei is less than the
mass of the uranium nucleus.
2
 3.88  109 J
H  H  He  n  thermal energy
3
1
When a large nucleus splits into two smaller nuclei,
the daughter nuclei have roughly the same neutron to
proton ratio as the parent nucleus. Stable nuclei of the
same size as the daughter nuclei have a much smaller
neutron to proton ratio. As a result, the daughter nuclei
are highly unstable.
Fusion reactions require very high temperatures to
proceed. Only at high temperatures do the parent
nuclei have sufficient kinetic energy to overcome the
Coulomb repulsion between them. The nuclei need to
get very close to one another so that the attractive
strong nuclear force can attract them.
BLM 21-2: Nuclear Waste/Science Inquiry
Teaching Tips

Encourage students to find their information from a
wide variety of sources, such as the Internet,
government publications, journals, texts, and e-mail
interviews with experts.

Students should consider the source of their
information and possible bias, especially in relation to
a topic as controversial as nuclear waste. For example,
information from the Sierra Club will likely have an
anti-nuclear bias, while information from AECL
(Atomic Energy of Canada Limited) will likely have a
pro-nuclear bias.

Ensure students understand the difference between
storage of nuclear waste and disposal of nuclear
waste. Storage is a way of temporarily dealing with
waste before it can be disposed of permanently.
The equation shows a fission reaction.
235
92
U  01 n 
140
54
Xe 
94
38
Sr  2 01 n
For nuclides with atomic mass numbers over 60,
binding energy per nucleon decreases as atomic
mass number decreases. Therefore, when large
nuclei fission, they form smaller nuclei with higher
binding energies. The difference in binding energy
translates as a mass difference between the parent
nucleus (more massive) and the daughter nuclei
(less massive). As a result, fission releases thermal
energy.
Very large nuclei are unstable due to the short
range of the strong nuclear force. As the size of a
nucleus increases, nucleons on either side of the
CHAPTER 21 BLM ANSWER KEY
BLM 21-5: Chapter 21 Test/Assessment
 Calandria — the sealed calandria contains the fuel
rods and heavy water moderator. It is one of the
many levels of containment for the CANDU
reactor.
Answers
1. (a)
A chain reaction is a self-sustaining series of
reactions, in which one reaction stimulates further
reactions. An example is nuclear fission, in which
the neutrons emitted by one fission even can
stimulate further fission events.
4. (a)
a nucleus of uranium-235 absorbs a neutron,
the nucleus splits into two smaller nuclei and some
neutrons. There are a number of possible fission
products—the only restriction is that the total mass
number of the products must remain the same as
the total mass number of the reactants. For
example, the fission of uranium-235 can produce
an atom of barium-141, an atom of krypton-92, and
three neutrons; or it can produce an atom of
rubidium-90, an atom of cesium-144, and two
neutrons. There are a great number of other
possibilities.
The moderator slows down the fast neutrons
emitted by fissioning nuclei, ensuring that the
neutrons will cause further fissions, and thus
maintaining the chain reaction at a critical level.
(b) When
2. (a)
(b)
(b) The
isotopes of cadmium and boron used in control
rods have an exceptional ability to absorb neutrons,
thus slowing down the chain reaction.
(c)
The incomplete equation shows a fusion reaction.
2
1
H  31 H  42 He  01 n  thermal energy
3.
 Calandria tubes — sealed calandria tubes hold the
zirconium alloy tubes, which contain the fuel
pellets.
 Control rods — rods made of boron or cadmium
are lowered into or raised out of the reactor core,
absorbing the right number of neutrons to maintain
the chain reaction at a critical level.
When the fission process is critical, one neutron
from each fission event causes one more fission
event. At the critical level, the reaction is sustained
at a constant rate. When the process is subcritical,
fewer than one neutron from each fission event
causes a further fission event. The reaction slows
down and will eventually cease. When the process
is supercritical, more than one neutron from each
fission event causes a further fission event. As a
result, the chain reaction speeds up very quickly. It
is important to maintain the chain reaction in a
reactor at the critical level so that energy is
produced at a constant rate. The chain reaction
does not stop, nor does it escalate out of control.
5.
CANDU reactors use heavy water as a moderator.
Heavy water is an excellent moderator, meaning it
slows down enough neutrons to sustain a chain
reaction at the critical level when natural uranium is
used as a fuel. When other moderators are used,
enriched uranium (containing an increased percentage
of uranium-235) must be used as fuel to ensure the
chain reaction remains critical.
6.
In fission, a large nucleus splits into two smaller
nuclei. In fusion, two small nuclei come together to
form one larger nucleus. In both cases, the binding
energy per nucleon of the product(s) is larger than the
binding energy per nucleon of the reactant(s). This
means that the total mass of the product(s) is smaller
than the total energy of the reactant(s). The difference
in mass is converted into energy via Einstein’s
equation, ΔE = Δmc2. Because c2 is such an enormous
factor, even a small change in mass translates to a
large change in energy.
7. (a)
Compared to fission, fusion produces more energy
per gram of fuel, the fuel itself is more plentiful,
and the byproducts of the chain reaction are not
nearly as dangerous.
CHAPTER 21 BLM ANSWER KEY
(b)
To initiate and sustain a fusion chain reaction,
gases must be raised to and maintained at
extremely high temperatures. No materials can
withstand such high temperatures, so other ways
must be found to contain the gases. One potential
method is to use magnetic fields, as has been done
in the experimental Tokamak fusion reactor at
Princeton University.
8.
Accept all student answers that are well thought out
and supported by examples. Most students will
probably say that nuclear reactions are not always
harmful to humans. For example, we rely on fusion
reactions that take place in the Sun for our very
existence.