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Init 9/16/2008 by Daniel R. Barnes

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Init 9/16/2008
by Daniel R. Barnes
Section 25.3
Fission and Fusion of Atomic Nuclei
25.3 Section Assessment, page 813
15. Explain what happens in a nuclear
chain reaction.
Neutrons produced by fissioning atoms strike other fissionable
atoms, causing them to split, which produces even more
neutrons, which can then strike even more fissionable atoms.
Lather, rinse, repeat.
Ba-142
U-235
U-236
Kr-91
Ba-142
U-235
U-236
Kr-91
Ba-142
U-235
U-236
Kr-91
Ba-142
U-235
U-236
Kr-91
U-235
U-235
U-235
U-235
U-235
U-235
U-235
U-235
U-235
The energy
released by
nuclear
reactions is
much
larger, per
gram of
explosive
material,
than the
energy
relased by
chemical
explosions.
c2 = 34,500,000,000 mi2/s2
c = 186,000 mi/s
e=
2
mc
The atom bomb
dropped on
Hiroshima contained
64 kg of uranium, of
which 0.7 kg
underwent nuclear
fission, and of this
mass only 0.6 g was
transformed into
energy.
The 1.5 pounds of
uranium that split that
day yielded the same
explosive energy as
15,000 tons of TNT.
1.5 pounds of uranium = 30,000,000 pounds of TNT.
25.3 Section Assessment, page 813
16. Why are spent fuel rods from a nuclear
reaction stored in water?
Water cools spent fuel rods and provides a
radiation shield.
25.3 Section Assessment, page 813
17. How are fusion reactions different from
fission reactions?
Fission reactions involve splitting nuclei.
In fusion reactions, small nuclei combine
and release much more energy.
In fission reactions, big atoms fall apart.
In fusion reactions, little atoms come together.
Fission reactions make atoms smaller and more numerous.
Fusion reactions make atoms bigger but less numerous.
Ba-142
U-235
U-236
Kr-91
Protons don’t like
other protons.
Positives don’t like
positives.
The closer two
charged particles
get, the stronger the
force between them.
Remember how
small a nucleus is
compared to the
atom as a whole?
In a nucleus,
protons are VERY
close together.
Protons close enough
to join and form one
nucleus will repel
each other with
extreme force.
To get protons to get
close enough to fuse
together, you need to
overpower this
electrostatic repulsion.
Extreme heat and/or
extreme pressure,
both of which are
found in the centers
of stars, can make
fusion happen.
If protons are
smashed together
by exterme heat
and/or pressure,
they will get close
enough for
something magical
to happen . . .
The strong force
has a strength of
zero until protons
get VERY close
together, and
then it gets VERY
strong, very
suddenly.
The “strong force”
turns on.
The strong force
is so powerful,
that it overpowers
the electrostatic
repulsion the
protons feel for
each other . . .
(The “strong force”
is sometimes called
the “strong nuclear
force.)
. . . and it “glues”
them together.
H
H
He
Ba-142
U-235
U-236
Kr-91
H
H
25.3 Section Assessment, page 813
18. What does nuclear [sic] moderation
accomplish in a nuclear reactor?
Neutron moderation slows down neutrons
so that they can be absorbed by fissile
atoms.
If you don’t slow down the neutrons, the chain reaction stops.
If you throw the baseball too fast, the next guy can’t catch it.
Without a moderator . . .
Ba-142
U-235
U-236
Kr-91
U-235
. . . fission can not continue
But with a moderator . . .
Ba-142
U-235
U-236
Kr-91
MODERATOR
Nice throw!
Nice and
slow!
Ba-142
U-235
U-236
Kr-91
. . . the chain reaction can continue.
25.3 Section Assessment, page 813
19. What is the source of the radioactive
nuclei in spent fuel rods?
Unused nuclear fuel and fission products
are the radioactive nuclei in spent fuel
rods.
Because “spent” fuel rods still have some unsplit fissile nuclei in
them (U-235 or Pu-239), some day, when uranium and
plutonium are more expensive, we may need to recycle our
spent fuel rods to get the remaining U-235 & Pu-239 out of
them.
“Spent” fuel rods aren’t 100% spent. There’s still unused fuel in
them. Now that’s nuclear waste, isn’t it?
25.3 Section Assessment, page 813
20. Assuming technical problems could be
overcome, what are some advantages to
using a fusion reactor to produce
electricity?
Potential fuels are inexpensive and readily
available.
Fusion uses heavy hydrogen, and the world is covered with H2O.
Not every hydrogen atom is deuterium or tritium, but when you’ve
got as much water as we do in our oceans, it adds up to quite a
bit of these less abundant hydrogen isotopes.
Also, fusion produces helium instead of radioactive fission
products. Helium is a very safe by-product.
Perfecting nuclear fusion power will help us fill party balloons.
Section 25.4
Radiation in Your Life
25.4 Section Assessment, page 813
21. Describe three methods of detecting
radiation.
If you want to detect radiation, you could
use a Geiger counter, a scintillation
counter, or a film badge.
25.4 Section Assessment, page 813
22. Describe two applications of
radioisotopes in medicine.
Radioisotopes can be used to both
diagnose and cure disease.
“Diagnosis” means figuring out what’s wrong with a sick person.
For example, the radioisotope iondine-131 can be used to
detect thyroid problems in a patient.
As harmful as radiation is, radioisotopes can also be used,
ironically, to help cure a sick person. For instance, salts of
radioisotopes can be sealed in gold tubes and implanted in
tumors. With any luck, this kills tumor cells much more than it
kills healthy cells nearby, but, sometimes, the cure is worse
than the disease and the patient suffers horribly.
25.4 Section Assessment, page 813
23. If you work regularly near a radiation
source, why might your employer want to
monitor your exposure to radiation by
havnig you use a film badge rather than
a Geiger counter?
A Geiger counter only detects radiation being given off by
radioactive atoms in you or on you. It doesn’t measure how
badly you’ve been hit by radiation in the past. You can be hit by
lots of damaging radiation but not become radioactive.
Therefore, a dose of radiation might make your film badge
change color, but you wouldn’t make a Geiger counter click.
Don’t confuse radioactive atoms with radiation. Don’t confuse the
gun with the bullet. You can be full of bullet holes, but with no
guns anywhere on you.
25.4 Section Assessment, page 813
24. What is an advantage of using a
radioactive seed, consisting of a
radioisotope in a small gold tube, to treat
a cancerous tumor?
The radioisotope inside the “seed” will emit beta and gamma rays
into the surrounding tumor cells, killing them. The seed will not
move from its inserted location.
Why gold? Gold does not rust, so the tube is leakproof. You don’t
want that radioactive stuff leaking out and wandering around
the body. Keep the cobra in its cage!
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