49 fission three steps involved for any chain

49
FISSION
Three steps involved (for any chain reaction)
•
Initiation- reaction of a single atom needed to start the chain
•
Propagation- This part repeats itself over and over again
producing more product
•
Termination- The chain will end, either because the reactants
are used up or neutrons no longer available.

FIGURE 21.14
When a slow neutron hits a fissionable U-235 nucleus, it is absorbed and forms an
unstable U-236 nucleus. The U-236 nucleus then rapidly breaks apart into two smaller
nuclei (in this case, Ba-141 and Kr-92) along with several neutrons (usually two or
three), and releases a very large amount of energy.
50

FIGURE 21.16
The fission of a large nucleus, such as U-235, produces two or three neutrons, each of
which is capable of causing fission of another nucleus by the reactions shown. If this
process continues, a nuclear chain reaction occurs.
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52

FIGURE 21.18
(a) The nuclear fission bomb that destroyed Hiroshima on August 6, 1945, consisted of
two subcritical masses of U-235, where conventional explosives were used to fire
one of the subcritical masses into the other, creating the critical mass for the
nuclear explosion.
(b) The plutonium bomb that destroyed Nagasaki on August 12, 1945, consisted of a
hollow sphere of plutonium that was rapidly compressed by conventional
explosives. This led to a concentration of plutonium in the center that was greater
than the critical mass necessary for the nuclear explosion.
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54
FISSIONABLE MATERIAL
fissionable isotopes include U-235, Pu-239, and Pu-240
natural uranium is less than 1% U-235
•
Is mostly U-238
•
not enough U-235 to sustain chain reaction
to produce fissionable uranium, the natural uranium must be enriched
in U-235
•
to about 7% for “weapons grade”
•
to about 3% for reactor grade

PLUTONIUM-239
Half Life of 24,000 years
Used in weapons
Used in some reactors
Produced as by product in some
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56
NUCLEAR POWER
Nuclear reactors use fission to generate electricity
•
About 19.2% of U.S. electricity
•
36% of electricity in Virginia
•
The fission of U-235 produces heat
The heat boils water, turning it to steam
The steam turns a turbine, generating electricity

57
NUCLEAR POWER PLANTS VS.
COAL-BURNING POWER PLANTS
Nuclear Plant
Use about 50 kg of fuel to
generate enough electricity for 1
million people
No air pollution
Coal Burning Plant
Use about 2 million kg of fuel to
generate enough electricity for 1
million people
Produces NO
2
and SO
x
that add to
acid rain
Produces CO
2
that adds to the
greenhouse effect

58
NUCLEAR POWER PLANTS - CORE
the fissionable material is stored in long tubes,
called fuel rods , arranged in a matrix
•
subcritical
between the fuel rods are control rods made of
neutron absorbing material
•
B and/or Cd
•
neutrons needed to sustain the chain reaction
the rods are placed in a material to slow down the
ejected neutrons, called a moderator
•
allows chain reaction to occur below critical mass

59
Power Plant

60
REACTOR
Cold
Water
Fuel
Rods
Hot
Water
Control
Rods

61
CONCERNS ABOUT NUCLEAR POWER
core melt-down/ natural disaster
•
water loss from core, heat melts core
•
Chernobyl
•
Japan